New MineralSpecies and Their
Crystal Structures
Edited by

Irina O. Galuskina and Igor V. Pekov
Printed Edition of the Special Issue Published in Minerals

www.mdpi.com/journal/minerals

New Mineral Species and Their
Crystal Structures

New Mineral Species and Their
Crystal Structures

Special Issue Editors
Irina O. Galuskina
Igor V. Pekov

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade

Special Issue Editors
Irina O. Galuskina

Igor V. Pekov

University of Silesia in Katowice

Lomonosov Moscow State University

PolandRussiaEditorial Ofﬁce
MDPI
St. Alban-Anlage 664052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal Minerals(ISSN 2075-163X) from 2018 to 2019 (available at: https://www.mdpi.com/journal/minerals/
special issues/new mineral)

For citation purposes, cite each article independently as indicated on the article page online and as
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Contents
About the Special Issue Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Preface to ”New Mineral Species and Their Crystal Structures” . . . . . . . . . . . . . . . . . .

ix

Irina O. Galuskina and Igor V. Pekov
Editorial for Special Issue “New Mineral Species and Their Crystal Structures”
Reprinted from: Minerals 2019, 9, 106, doi:10.3390/min9020106 . . . . . . . . . . . . . . . . . . . .

1

Tonči Balić-Žunić, Anna Garavelli, Daniela Pinto and Donatella Mitolo
Verneite, Na2 Ca3 Al2 F14 , a New Aluminum Fluoride Mineral from Icelandic andVesuvius Fumaroles
Reprinted from: Minerals 2018, 8, 553, doi:10.3390/min8120553 . . . . . . . . . . . . . . . . . . . .

5Igor V. Pekov, Fedor D. Sandalov, Natalia N. Koshlyakova, Marina F. Vigasina,
Yury S. Polekhovsky, Sergey N. Britvin, Evgeny G. Sidorov and Anna G. Turchkova
Copper in Natural Oxide Spinels: The New Mineral Thermaerogenite CuAl2 O4 , Cuprospineland Cu-Enriched Varieties of Other Spinel-Group Members from Fumaroles of the Tolbachik
Volcano, Kamchatka, Russia
Reprinted from: Minerals 2018, 8, 498, doi:10.3390/min8110498 . . . . . . . . . . . . . . . . . . . . 15
Rafał Juroszek, Hannes Krüger, Irina Galuskina, Biljana Krüger, Lidia Jeżak, Bernd Ternes,
Justyna Wojdyla, Tomasz Krzykawski, Leonid Pautov and Evgeny Galuskin
Sharyginite, Ca3 TiFe2 O8 , A New Mineral from the Bellerberg Volcano, Germany
Reprinted from: Minerals 2018, 8, 308, doi:10.3390/min8070308 . . . . . . . . . . . . . . . . . . . . 33
Nikita V. Chukanov, Natalia V. Zubkova, Sergey N. Britvin, Igor V. Pekov, Marina F. Vigasina,
Christof Schäfer, Bernd Ternes, Willi Schüller, Yury S. Polekhovsky, Vera N. Ermolaeva andDmitry Yu. Pushcharovsky
Nöggerathite-(Ce), (Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 , a New Zirconolite-Related Mineral fromthe Eifel Volcanic Region, Germany
Reprinted from: Minerals 2018, 8, 449, doi:10.3390/min8100449 . . . . . . . . . . . . . . . . . . . . 57

Dorota Środek, Rafał Juroszek, Hannes Kr üger, Biljana Kr üger, Irina Galuskina andViktor Gazeev
New Occurrence of Rusinovite, Ca10 (Si2 O7 )3 Cl2 : Composition, Structure and Raman Data of
Rusinovite from Shadil-Khokh Volcano, South Ossetia and Bellerberg Volcano, Germany
Reprinted from: Minerals 2018, 8, 399, doi:10.3390/min8090399 . . . . . . . . . . . . . . . . . . . . 71
Evgeny V. Galuskin, Biljana Krüger, Irina O. Galuskina, Hannes Krüger, Yevgeny Vapnik,
Justyna A. Wojdyla and Mikhail Murashko
New Mineral with Modular Structure Derived from Hatrurite from the Pyrometamorphic Rocksof the Hatrurim Complex: Ariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O, from Negev Desert, Israel
Reprinted from: Minerals 2018, 8, 109, doi:10.3390/min8030109 . . . . . . . . . . . . . . . . . . . . 84

Arkadiusz Krzątała, Taras L. Panikorovskii, Irina O. Galuskina and Evgeny V. Galuskin
Dynamic Disorder of Fe3+ Ions in the Crystal Structure of Natural Barioferrite
Reprinted from: Minerals 2018, 8, 340, doi:10.3390/min8080340 . . . . . . . . . . . . . . . . . . . . 98v

Adam Pieczka, Cristian Biagioni, Bożena Gołębiowska, Piotr Jeleń, Marco Pasero andMaciej Sitarz
Paraﬁniukite, Ca2 Mn3 (PO4 )3 Cl, a New Member of the Apatite Supergroup from the Szklary
Pegmatite, Lower Silesia, Poland: Description and Crystal Structure
Reprinted from: Minerals 2018, 8, 485, doi:10.3390/min8110485 . . . . . . . . . . . . . . . . . . . . 114
Daisuke Nishio-Hamane, Takahiro Tanaka and Tetsuo Minakawa
Aurihydrargyrumite, a Natural Au6 Hg5 Phase from Japan
Reprinted from: Minerals 2018, 8, 415, doi:10.3390/min8090415 . . . . . . . . . . . . . . . . . . . . 125
Luca Bindi, Cristian Biagioni and Frank N. Keutsch
Oyonite, Ag3 Mn2 Pb4 Sb7 As4 S24 , a New Member of the Lillianite Homologous Series from the
Uchucchacua Base-Metal Deposit, Oyon District, Peru
Reprinted from: Minerals 2018, 8, 192, doi:10.3390/min8050192 . . . . . . . . . . . . . . . . . . . . 134
Hans-Jürgen Förster, Luca Bindi, Günter Grundmann and Chris J. Stanley
Cerromojonite, CuPbBiSe3 , from El Dragón (Bolivia): A New Member of the Bournonite Group
Reprinted from: Minerals 2018, 8, 420, doi:10.3390/min8100420 . . . . . . . . . . . . . . . . . . . . 143
Francesco Demartin, Italo Campostrini, Paolo Ferretti and Ivano Rocchetti
Fiemmeite Cu2 (C2 O4 )(OH)2 ·2H2 O, a New Mineral from Val di Fiemme, Trentino, Italy
Reprinted from: Minerals 2018, 8, 248, doi:10.3390/min8060248 . . . . . . . . . . . . . . . . . . . . 157
Cristian Biagioni, Marco Pasero and Federica Zaccarini
Tiberiobardiite, Cu9 Al(SiO3 OH)2 (OH)12 (H2 O)6 (SO4 )1.5 ·10H2 O, a New Mineral Related to
Chalcophyllite from the Cretaio Cu Prospect, Massa Marittima, Grosseto (Tuscany, Italy):
Occurrence and Crystal Structure
Reprinted from: Minerals 2018, 8, 152, doi:10.3390/min8040152 . . . . . . . . . . . . . . . . . . . . 167

Yulia A. Pankova, Sergey V. Krivovichev, Igor V. Pekov, Edward S. Grew and
Vasiliy O. Yapaskurt
Kurchatovite and Clinokurchatovite,
Ideally CaMgB2 O5 :An Example ofModular Polymorphism
Reprinted from: Minerals 2018, 8, 332, doi:10.3390/min8080332 . . . . . . . . . . . . . . . . . . . . 179

viAbout the Special Issue Editors
Irina O. Galuskina is an Associate Professor at the Faculty of Earth Sciences of the University ofSilesia, Poland. She received a PhD (1998) and habilitation (2011) in Earth Sciences in the disciplineof geology at the University of Silesia. She teaches crystallography, mineralogy, genetic mineralogy,
and methods of investigation. Her research interests focus on the crystal chemistry of minerals andmineralogy of rodingites, high-temperature skarns, and pyrometamorphic rocks. She is the author
and co-author of more than 50 new mineral species, and she has published 72 scientiﬁc papers ininternational journals.
Igor V. Pekov is a Professor of Mineralogy at the Lomonosov Moscow State University, Russia.
He graduated in Geology and Geochemistry in 1989 at this University, where hd received a PhD in
Mineralogy and Crystallography in 1997 and a DSc in 2005. He was elected a Corresponding Memberof the Russian Academy of Sciences in 2016 and was elected a member of the Academia Europaea
in 2018. His main research interests are in the ﬁelds of mineralogy and crystal chemistry of rareelements; mineralogy and geochemistry of alkaline rocks, volcanic exhalations, and oxidation zoneof ore deposits; crystal chemistry of oxysalt minerals; crystal chemistry and properties of mineral-likemicroporous materials; and history of mineralogy. To date, he has discovered 145 IMA-approved newmineral species as a senior author and more 98 new mineral species as a co-author. He has authoredmore than 200 articles in international journals, as well as eight books.

vii

Preface to ”New Mineral Species and Their
Crystal Structures”
The discovery of any new mineral is a signiﬁcant event in fundamental science. It enriches
mineralogy and geology as well as crystallography, solid-state physics, and chemistry. Some newminerals are interesting for material science and engineering due to their useful physical properties.
The number of discoveries of new mineral species is the one of the most important quantitative
parameters of success in mineralogical science. The USA and Russia are at present the record-holders
in the number of new mineral found: About eight hundred mineral species have been discoveredon the territories of each of these two countries. Italy and Germany (350 each) are next, followed
by Canada (230), Sweden (183), Australia (162), Japan (143), Chile (133), China (132), the Czech
Republic (127), Great Britain (126), France (119), and Namibia (104), which have also made signiﬁcant
contributions to world mineralogy.
This Special Issue of Minerals is devoted to new mineral species and their crystal structures.
Fourteen articles were published therein. We are very thankful to all authors, who published the dataof their recent investigations in this issue.
All articles were carefully reviewed by experts in mineralogy and crystallography. We are
grateful to all reviewers who found the time to help to improve the submitted manuscripts. Further,of course, we would like to express our deepeset appreciation to the staff of Minerals, as well support
staff, for the big and necessary work on all stages of the manuscript preparation for production.
Irina O. Galuskina, Igor V. Pekov
Special Issue Editorsix

minerals
Editorial

Editorial for Special Issue “New Mineral Species andTheir Crystal Structures”
Irina O. Galuskina 1, * and Igor V. Pekov 2
1
2*

Faculty of Earth Sciences, University of Silesia, B˛edzińska 60, 41-200 Sosnowiec, Poland
Faculty of Geology, Moscow State University, Vorobievy Gory, Moscow 119991, Russia; igorpekov@mail.ru
Correspondence: irina.galuskina@us.edu.pl

Received: 17 January 2019; Accepted: 1 February 2019; Published: 13 February 2019

Mineralogy is the oldest and one of the most important sciences of the geological cycle. Minerals,
the basis of overwhelming mass of solid matter in the universe, are direct subjects of investigation inmineralogy. Minerals, or mineral species, are generally solid crystalline substances. Their deﬁnitionindicates that, they are: (1) naturally occurring; (2) belonging to the distinct structural type; (3) stable,
varying merely in the relatively small limits of chemical composition. If a given mineral differs fromother known species in its structure (2) and/or composition (3) then it can be considered as a newmineral species.
According to the data of the Commission on New Minerals, Nomenclature and Classiﬁcation(CNMNC) of the International Mineralogical Association (IMA) (http://nrmima.nrm.se/), there are
currently about 5600 known mineral species. The number of minerals increases steadily year by
year. Before the 2000s the number of minerals approved by the IMA Commission varied from 50 to
70 species per year. In 2010s this number reached 100–160 species per year, and at present we observe
an increasing tendency again, i.e., in 2018, almost two hundred new mineral species were approvedby the IMA CNMNC. Is it a lot? It depends on what one compares it to. For instance, the numberof biological species known at present is close to two million, and the number of synthetic chemicalcompounds, including organic substances, comes nearer to ten million. In comparison with thesenumbers, the species diversity of the Mineral Kingdom is relatively small, so a discovery of every newmineral species is a signiﬁcant event in science. Investigation of new minerals, many of which possessa unique crystal structure and unusual properties, has a great signiﬁcance for the understanding solids’
structure and processes in the interior of the Earth, on the Earth’s surface and in the Universe.
Articles submitted to the Special Issue of Minerals effectively demonstrate the great chemical,
structural and genetic diversity of new mineral species of the present time, as well as the geographyof discoveries and the variety of analytical methods used in studies of new minerals. Recently an
increasing number of new mineral species discovered in volcanic fumaroles has been observed [1].
In the present issue, there are two articles devoted to the two new volcanic minerals: Verneite,
Na2 Ca3 Al2 F14 , described simultaneously from Eldfell and Hekla volcanoes in Iceland and Vesuvius in
Italy [2], and thermaerogenite, CuAl2 O4 , from the Tolbachik volcano at Kamchatka, Russia [3]. In thesearticles, readers could not only ﬁnd the high level of the analytical investigation, but also the authors’imagination with regards to the choice of names for the new minerals:•“Verneite is named after Jules Verne (1828–1905). In his novel Voyage au centre de la Terre (1864),
Verne describes a group of characters descending through a crater of a quiescent volcano in
Iceland (Snæfell) and, after an adventurous journey through exciting Earth’s underground, ﬁnally
being ejected in South Italy with the eruption of a volcano (Stromboli). Therefore, we consider thename verneite appropriate for a mineral found and described by the same team of researchers on
the best-known Icelandic and Italian volcanoes [2]”.

Minerals 2019, 9, 106; doi:10.3390/min9020106

1

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Minerals 2019, 9, 106•“The name thermaerogenite (spinel group member) is constructed based on the combination of
Greek words, hot,, gas, andthat means “born by”. Thus, in whole it meansborn by hot gas, that reﬂects the fumarolic origin of the mineral” [3].

The next two new minerals, sharyginite, Ca3 TiFe2 O8 (perovskite supegroup member) [4] andnöggerathite-(Ce) (Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 (zirconolite-related mineral) [5], were discoveredin the Eifel region, Rhineland-Palatinate, Germany. Volcanic rocks of Eifel are a unique source of newminerals: more than ﬁfty mineral species were discovered there [6]. In Germany local collectors ofminerals provide substantial assistance in collecting of specimens and so they are rightly the co-authorsof the new mineral descriptions (Christof Schäfer, Bernd Ternes, and Willi Schüller). New data on
the composition and structure of rusinovite Ca10 (Si2 O7 )3 Cl2 , found in altered xenoliths of Eifel andSouthern Ossetia, are presented by Środek et al. [7]. Until now rusinovite was known only from
xenoliths within ignimbrites of the Upper Chegem Caldera at Northern Caucasus, Russia [8].
More than twenty new mineral species were discovered in pyrometamorphic rocks of the
Hatrurim Complex in the Dead Sea rift in the last eight years [9]. In this issue the new mineralariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O (with intercalated antiperovskite structure) is described [10].
It was collected from spurrite rocks in the Negev Desert, Israel and has also been found in several
localities in the Palestinian Autonomous Territory and Jordan [10]. Previously, barioferrite, BaFe12 O19 ,a new mineral of the magnetoplumbite group, was described in rocks of the Hatrurim Complex.
However, due to the small size of crystals, its structure could not be studied [11]. Krzatała
˛with
co-authors reported the structure of barioferrite in a different article [12].
Pieczka and co-authors described the Ca-Mn-ordered new mineral of the apatite supergroup,
paraﬁniukite, Ca2 Mn3 (PO4 )3 Cl, from the Szklary pegmatite in Lower Silesia, Poland. Szklary is thetype locality for previously discovered lepageite, nioboholtite, titanoholtite and szklaryite [13].
Ore minerals in this Special Issue are represented by gold amalgam aurihydrargyrumite,
Au6 Hg5 , found on gold particles in the Iyoki deposit at Shikoku Island, Japan [14]; oyonite,
Ag3 Mn2 Pb4 Sb7 As4 S24 , a new sulphosalt of the lillianite homologous series from the Uchucchacua
deposit in Oyon district, Peru [15]; and cerromojonite, CuPbBiSe3 , a new selenide of the bournonite
group from the El Dragón mine in Potosí, Bolivia [16].
Some new supergene minerals were reported from Italy. Demartin and co-authors present
ﬁemmeite, Cu2 (C2 O4 )(OH)2 ·2H2 O, from Val di Fiemme in Trentino. It occurs in coaliﬁed woods whichwere permeated by mineralizing solutions containing Cu, U, As, Pb and Zn. The oxalate anions haveoriginated from altered plant remnants included in sandstone [17]. Biagioni with co-authors studied
Si-analogue of chalcophyllite from the Cretaio Prospect in Grosseto, which was named tiberiobardiite
in the honour of Tiberio Bardi, a mineral collector who found a specimen which became the holotypeof tiberiobardiite with the simpliﬁed formula Cu9 Al(SiO3 OH)2 (OH)12 (H2 O)6 (SO4 )1.5 ·10H2 O [18].
Repeated studies of early investigated mineral species using modern analytical methods toclarify their formal position in the actual mineralogical classiﬁcation is an important aspect of modernmineralogy. Pankova with co-authors reported the results of a structural investigation of kurchatovite
and clinokurchatovite, two modiﬁcations of CaMgB2 O5 , from their type localities: Solongo in Buryatia,
Russia, and Sayak-IV in Kazakhstan, respectively [19]. As a conclusion of the comparative studyof kurchatovite and clinokurchatovite, the authors stated: “kurchatovite and clinokurchatovite are
not polytypes, but polymorphs, and therefore re-consideration of their status as of separate mineral
species is not warranted. However, the structures of the two minerals are closely related: the crystal
structure of kurchatovite may be considered as a derivative of clinokurchatovite through the modular
approach”.
We hope that the present Special Issue presents an interesting read not only for mineralogists andgeochemists but also for scientists who work in the ﬁelds of crystallography, chemistry, solid-statephysics and materials science, on synthesis and on crystal chemical studies of novel technologicalmaterials related to minerals. We also hope that research articles on new mineral species attract theattention of museum curators and mineral collectors.
2

Minerals 2019, 9, 106

Author Contributions: I.G. and I.P. wrote the paper.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Pekov, I.V.; Koshlyakova, N.N.; Zubkova, N.V.; Lykova, I.S.; Britvin, S.N.; Yapaskurt, V.O.; Agakhanov, A.A.;
Shchipalkina, N.V.; Turchkova, A.G.; Sidorov, E.G. Fumarolic arsenates—A special type of arsenic
mineralization. Eur. J. Mineral. 2018, 30, 305–322. [CrossRef]
Balić-Žunić, T.; Garavelli, A.; Pinto, D.; Mitolo, D. Verneite, Na2 Ca3 Al2 F14 , a new aluminum ﬂuoride mineralfrom Icelandic and Vesuvius fumaroles. Minerals 2018, 8, 553. [CrossRef]
Pekov, I.V.; Sandalov, F.D.; Koshlyakova, N.N.; Vigasina, M.F.; Polekhovsky, Y.S.; Britvin, S.N.; Sidorov, E.G.;
Turchkova, A.G. Copper in natural oxide spinels: The new mineral thermaerogenite CuAl2 O4 , cuprospineland Cu-enriched varieties of other spinel-group members from fumaroles of the Tolbachik Volcano,Kamchatka, Russia. Minerals 2018, 8, 498. [CrossRef]
Juroszek, R.; Krüger, H.; Galuskina, I.; Krüger, B.; Jeżak, L.; Ternes, B.; Wojdyla, J.; Krzykawski, T.; Pautov, L.;
Galuskin, E. Sharyginite, Ca3 TiFe2 O8 , a new mineral from the Bellerberg Volcano, Germany. Minerals 2018,
8, 308. [CrossRef]
Chukanov, N.V.; Zubkova, N.V.; Britvin, S.N.; Pekov, I.V.; Vigasina, M.F.; Schäfer, C.; Ternes, B.; Schüller, W.;
Polekhovsky, Y.S.; Ermolaeva, V.N.; et al. Nöggerathite-(Ce), (Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 , a new
zirconolite-related mineral from the Eifel volcanic region, Germany. Minerals 2018, 8, 449. [CrossRef]
Engelhaupt, B.; Schüller, W. Mineral Reich Eifel; Christian Weise Verlag: München, Germany, 2015; p. 340.
Środek, D.; Juroszek, R.; Krüger, H.; Krüger, B.; Galuskina, I.; Gazeev, V. New occurrence of rusinovite,
Ca10 (Si2 O7 )3 Cl2 : Composition, structure and Raman data of rusinovite from Shadil-Khokh Volcano, South
Ossetia and Bellerberg Volcano, Germany. Minerals 2018, 8, 399. [CrossRef]
Galuskin, E.V.; Galuskina, I.O.; Lazic, B.; Armbruster, T.; Zadov, A.E.; Krzykawski, T.; Banasik, K.;
Gazeev, V.M.; Pertsev, N.N. Rusinovite, Ca10 (Si2 O7 )3 Cl2 : A new skarn mineral from the Upper Chegem
caldera, Kabardino-Balkaria, Northern Caucasus, Russia. Eur. J. Mineral. 2011, 23, 837–844. [CrossRef]
Galuskin, E.V.; Galuskina, I.O.; Gfeller, F.; Krüger, B.; Kusz, J.; Vapnik, Y.; Dulski, M.; Dzierżanowski, P.
Silicocarnotite, Ca5 [(SiO4 )(PO4 )](PO4 ), a new ‘old’ mineral from the Negev Desert, Israel, and theternesite-silicocarnotite solid solution: indicators of high-temperature alteration of pyrometamorphic rocksof the Hatrurim Complex, Southern Levant. Eur. J. Mineral. 2016, 28, 105–123. [CrossRef]
Galuskin, E.V.; Krüger, B.; Galuskina, I.O.; Krüger, H.; Vapnik, Y.; Wojdyla, J.A.; Murashko, M. New mineralwith modular structure derived from Hatrurite from the pyrometamorphic rocks of the Hatrurim Complex:
Ariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O, from Negev Desert, Israel. Minerals 2018, 8, 19. [CrossRef]
Murashko, M.N.; Chukanov, N.V.; Mukhanova, A.A.; Vapnik, E.; Britvin, S.N.; Polekhovsky, Y.S.; Ivakin, Y.D.
Barioferrite BaFe12 O19 : A new mineral species of the magnetoplumbite group from the Haturim Formationin Israel. Geol. Ore Depos. 2011, 53, 558–563. [CrossRef]
Krzatała,˛A.; Panikorovskii, T.L.; Galuskina, I.O.; Galuskin, E.V. Dynamic disorder of Fe3+ ions in the crystal
structure of natural barioferrite. Minerals 2018, 8, 340. [CrossRef]
Pieczka, A.; Biagioni, C.; Goł˛ebiowska, B.; Jeleń, P.; Pasero, M.; Sitarz, M. Paraﬁniukite, Ca2 Mn3 (PO4 )3 Cl, a
new member of the apatite supergroup from the Szklary Pegmatite, Lower Silesia, Poland: Description andcrystal structure. Minerals 2018, 8, 485. [CrossRef]
Nishio-Hamane, D.; Tanaka, T.; Minakawa, T. Aurihydrargyrumite, a natural Au6 Hg5 phase from Japan.
Minerals 2018, 8, 415. [CrossRef]
Bindi, L.; Biagioni, C.; Keutsch, F.N. Oyonite, Ag3 Mn2 Pb4 Sb7 As4 S24 , a new member of the lillianite
homologous series from the Uchucchacua Base-Metal deposit, Oyon District, Peru. Minerals 2018, 8, 192.
[CrossRef]
Förster, H.J.; Bindi, L.; Grundmann, G.; Stanley, C.J. Cerromojonite, CuPbBiSe3 , from El Dragón (Bolivia): A
new member of the bournonite group. Minerals 2018, 8, 420. [CrossRef]
Demartin, F.; Campostrini, I.; Ferretti, P.; Rocchetti, I. Fiemmeite Cu2 (C2 O4 )(OH)2 ·2H2 O, a new mineralfrom Val di Fiemme, Trentino, Italy. Minerals 2018, 8, 248. [CrossRef]

3

Minerals 2019, 9, 106

18.

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Biagioni, C.; Pasero, M.; Zaccarini, F. Tiberiobardiite, Cu9 Al(SiO3 OH)2 (OH)12 (H2 O)6 (SO4 )1.5 ·10H2 O, a newmineral related to chalcophyllite from the Cretaio Cu prospect, Massa Marittima, Grosseto (Tuscany, Italy):
Occurrence and crystal structure. Minerals 2018, 8, 152. [CrossRef]
Pankova, Y.A.; Krivovichev, S.V.; Pekov, I.V.; Grew, E.S.; Yapaskurt, V.O. Kurchatovite and clinokurchatovite,
ideally CaMgB2 O5 : An example of modular polymorphism. Minerals 2018, 8, 332. [CrossRef]© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

4

minerals
ArticleVerneite, Na2Ca3Al2F14, a New Aluminum Fluoride
Mineral from Icelandic and Vesuvius Fumaroles
Tonči Balić-Žunić 1, *, Anna Garavelli 2 , Daniela Pinto 2 and Donatella Mitolo 3
1
2
3

*

Department of Geosciences and Natural Resource Management, University of Copenhagen,Øster Voldgade 10, DK-1350 København K, Denmark
Department of Earth and Geo-environmental Sciences, University of Bari “A. Moro”, via E. Orabona 4,I-70125 Bari, Italy; Anna.garavelli@uniba.it (A.G.); Daniela.pinto@uniba.it (D.P.)
Autorità di Bacino Distrettuale dell’Appennino Meridionale Sede Puglia, Str. Prov. Per Casamassima km 3,
I-70010 Valenzano (BA), Italy; dmitolo@libero.it
Correspondence: toncib@ign.ku.dk; Tel.: +45-35322337

Received: 26 October 2018; Accepted: 21 November 2018; Published: 28 November 2018

Abstract: Verneite, Na2 Ca3 Al2 F14 , is a new mineral ﬁrst discovered in fumarolic samples from both
Hekla, Iceland and Vesuvius, Italy. Additional occurrences are so far from Eldfell and Fimmvörduhals,
both on Iceland. Verneite is cubic, I21 3, a = 10.264(1) Å, V = 1081.4(3) Å3 , Z = 4, and corresponds to
the known synthetic compound. The empirical formula is Na2.01 Ca2.82 Al2.17 F14.02 (scanning electron
microscopy with energy dispersive spectrometer from an unpolished sample). It appears in crystals
up to 20 μm in diameter, with {110}, {100}, and {111} as the main forms. In the crystal structure of its
synthetic analogue, Na is coordinated by 7 F atoms in the form of a capped octahedron, Ca with 8 F
atoms in the form of a bisdisphenoid, and Al with 6 F atoms in the form of an octahedron. The crystal
structure of Na2 Ca3 Al2 F14 contains sinuous chains of Ca coordination polyhedra interlacing withsimilarly sinuous chains of Na coordination polyhedra and forming together with them layers
parallel to {100}. The intersecting layers parallel to three equivalent crystallographic planes forma three-dimensional mesh with Al coordinations imbedded in its holes. The characteristics of Ca
coordinations in ﬂuorides, as well as their relations to other ternary Na–Ca–Al ﬂuorides are discussed.
Verneite is named after Jules Verne.
Keywords: verneite; new mineral; crystal structure; Hekla; Vesuvius; Eldfell; aluminoﬂuoride

1. Introduction
The new mineral verneite, Na2 Ca3 Al2 F14 , was discovered among sublimates collected fromfumaroles on the Eldfell and Hekla volcanoes, where a considerable number of new fumarolic minerals
has been observed [1]; six of them have so far been fully described [2–7]. The same mineral phase
was identiﬁed at approximately the same time in a sublimate sample from Vesuvius, belonging to the
Pelloux collection, which is stored in the mineralogical section of the Museum of Earth Sciences at
Bari University. The original label of this Museum sample, with its 1925 date, indicates “Avogadritefrom Vesuvius”.
Sveinn P. Jakobsson from the Icelandic Institute of Natural History collected the verneite typespecimen on 16 September 1992 on Hekla. He also collected the cotype specimen from the Eldfell
volcano in 1988. The holotype and the cotype are kept in the mineral collection of the Icelandic Instituteof Natural History, Garðabær, Iceland, under sample numbers NI 15509 and NI 12256, respectively.
The mineral was also registered in the samples NI 15518 and NI 17046 from Hekla, and NI 24457 andNI 24565 from Fimmvörduhals, kept in the same museum, and in samples E4-1A, E4-2A and E4-2B,
collected on Eldfell in 2009 and presently kept at the Department of Geosciences and Natural Resource

Minerals 2018, 8, 553; doi:10.3390/min8120553

5

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Minerals 2018, 8, 553

Management of the University of Copenhagen. The Vesuvius sample is kept in the mineralogicalcollection of the Department of Earth and Geo-environmental Sciences, University of Bari.
Verneite is named after Jules Verne (1828–1905), the famous French author of novels, poetry,and plays, best known for his adventure novels and his profound inﬂuence on the literary genre ofscience ﬁction and, through it, the promotion of science, especially among young people. In his novel
Voyage au centre de la Terre (1864), Verne describes a group of characters descending through a crater ofa quiescent volcano in Iceland (Snæfell) and, after an adventurous journey through exciting Earth’s
underground, ﬁnally being ejected in South Italy with the eruption of a volcano (Stromboli). Therefore,
we consider the name verneite appropriate for a mineral found and described by the same team ofresearchers on the best-known Icelandic and Italian volcanoes. Both the mineral and the mineral namehave been approved by the Commission on New Minerals, Nomenclature and Classiﬁcation of the
IMA (no. 2016-112).
In the present work, we give a detailed description of the occurrences of verneite, the morphologicaland chemical analysis by scanning electron microscopy with energy dispersive spectroscopy(SEM–EDS), and the crystallographic analysis by Powder X-ray Diffraction (PXRD) and discuss
its crystal structure details.
2. Materials and Methods
The geological settings and a description of the Hekla and Eldfell fumaroles where the newmineral was found are given in the papers mentioned in the Introduction [1–7]. Recently, the mineral
has also been identiﬁed in samples originating from fumaroles on Fimmvörduhlas, Iceland, active
during and after the eruption in 2010 [8]. Verneite occurs in medium to low temperature (170 ◦ C
at the time of sampling) fumaroles, as white-yellowish to brown crusts and massive aggregates up
to several mm in size, sometimes also in transparent, colorless to pale yellowish crystals. In thesample from Eldfell, crystals up to 20 μm in diameter with a rhombic dodecahedral habit havebeen observed (Figure 1a), whereas in the Vesuvius sample, smaller (up to 10 μm) crystals having
a combination of {100}, {110}, and {111} forms have been noted (Figure 1b). Verneite from Vesuvius wasfound during a reexamination of a sublimate sample belonging to the “Alberto Pelloux mineralogicalcollection”, housed at the “Palace of the Earth Sciences” of Bari University. The original label gives thefollowing indication in Pelloux’s own handwriting: “Avogadrite from Vesuvius collected on 15 July
1925—avogadrite or malladrite?” We conclude, therefore, that the mineral originates from fumaroles
formed after the violent eruption of 1906, which was the last prior to the date reported in the label.
Considering that avogadrite was discovered in 1926 by Professor Ferruccio Zambonini of Naples
University, we conclude that the acquisition of the sample (from the well-known mineral salesman“Roberto Palumbo”, according to the indication on Pelloux’s label) happened after 1926. As indicatedon the label, Pelloux himself pointed to a need for further investigation of the sample. This was initiallyconducted by C.L. Garavelli and coworkers in the 1960s, who reported in it the presence of ralstonite,
matteuccite, avogadrite, malladrite, and of a probably new mineral, MgSiF6 ·6H2 O, suitable for further
studies (C.L. Garavelli, unpublished documents). We could not conﬁrm this last phase during thepresent investigation.
Verneite in Hekla samples forms mixtures with ralstonite and hematite, sometimes also withjakobssonite and “mineral HB” [1] with a still unknown composition, but known PXRD data. The otherminerals, which appear together with verneite in the type specimen and other samples from Hekla,
are leonardsenite, heklaite, malladrite, opal, and ﬂuorite. In the samples from Eldfell, where the cotype
stems from, verneite is associated with jakobssonite, “mineral HB”, anhydrite, leonardsenite, ralstonite,jarosite, and meniaylovite. In the present investigation of the sample from Vesuvius, we found verneite
associated with ralstonite and, to a lesser degree, to hieratite and knasibﬁte.
For the determination of the chemical composition, samples of verneite were analyzed by
SEM-EDS. The Eldfell sample was analyzed by a S 360 Cambridge SEM, coupled with an Oxford-Link
Ge ISIS EDS equipped with a Super Atmosphere thin window, whereas a 50XVP LEO SEM and6

Minerals 2018, 8, 553

Oxford AZtec system with an Oxford SDD XMax (80 mm2 ) detector were used for the Vesuvius
sample. The samples were sputtered with a 30 nm thick carbon ﬁlm before analysis. As we had tomeasure inclined surfaces, a “noncritical” working distance was utilized [9,10]. X-ray intensities wereconverted to wt % values by the ZAF4/FLS quantitative analysis software support of Oxford-Link
Analytical. For standards, we used synthetic LiF (F), albite (Na), wollastonite (Ca), corundum (Al),and orthoclase (K).
The crystallographic data were obtained by PXRD on diffractometers with Bragg–Brentano
geometry, ﬁrst the Panalytical (formerly Philips) PW3710 diffractometer with a long ﬁne focus Cu
sealed tube, secondary-beam graphite monochromator, and a variable-slit for the beam divergence.
Subsequently, a Bruker-AXS D8 diffractometer with a ceramic Cu tube, primary-beam Ge111
monochromator, ﬁxed divergence slit, and Lynx-Eye silicon strip detector was used. Bruker–AXS
program Topas was used for the Rietveld reﬁnement.
The crystallographic data for verneite and other compared crystal structures were calculated byprogram IVTON [11].

(a)

(b)

Figure 1. (a) SEM image of the crystals of verneite in association with ﬁne-grained jakobssonite in the
cotype sample from Eldfell. (b) SEM image of verneite crystals from the Vesuvius sample.

3. Results
3.1. Chemical Formula and Physical Properties
The empirical formulae (based on 7 cations pfu) are: Na2.01 Ca2.82 Al2.17 F14.02 for the Eldfell sampleand (Na1.47 K0.09 )Σ1.56 Ca3.25 Al2.19 F14.33 for the Vesuvius sample. The ideal formula is Na2 Ca3 Al2 F14 ,which requires: F = 54.71, Na = 9.46, Ca = 24.73, and Al = 11.10 wt %.
The calculated density of verneite, from the empirical formula and the unit–cell data, is 2.974 g/cm3 .
The calculated refractive index using the Gladstone–Dale constants of Mandarino [12] is 1.357.
The cleavage, hardness, streak, and lustre of verneite could not be accurately determined due tothe minute size of the crystals and the admixture with other minerals. No ﬂuorescence was observedon the investigated samples, either under short-wavelength or long-wavelength ultraviolet radiation.
It could be expected that verneite is piezoelectric due to its space group symmetry.
3.2. Crystal Structure Data
Verneite is analogous to synthetic Na2 Ca3 Al2 F14 investigated by Courbion and Ferrey [13]. It is
cubic, I21 3, a = 10.264(1) Å, V = 1081.4(3) Å3 , Z = 4. The atomic parameters and a list of bond lengths
and angles are given in Reference [13].
Verneite was identiﬁed by PXRD in samples from all localities mentioned above. Rietveld
reﬁnements of verneite using the atomic parameters of Courbion and Ferrey [13] match the observed
data very well in intensities. Due to this, and the fact that it was impossible to obtain a pure diagram
7

Minerals 2018, 8, 553

of the mineral, which was always mixed with at least three other components in all the investigatedsamples, a full crystal structure reﬁnement from powder diffraction data, with the inclusion of atomic
parameters, was not attempted. Table 1 presents the PXRD data for the sample from the type locality(Hekla), containing verneite, ralstonite, hematite, and jakobssonite, as well as a minor undeterminedamount of the still not fully investigated “mineral HB” [1].
Table 1. X-ray powder diffraction data of verneite from Hekla (recorded with automatic variable
divergence slit) compared with the experimental diagram of synthetic Na2 Ca3 Al2 F14 (PDF 36-1496).
HB = mineral HB [1]; R = ralstonite; J = jakobssonite; H = hematite.
hkl/Minerald (Å) 1

I/I0 % 1

d (Å) 2

I/I0 % 2

011
R
002
211
?
H
022
HB
?
031
HB, J
R
222
R
321
H
H
411
?
402
H
332
HB
422
R, J
3 4 1, 4 3 1
R
251
H
044
433+R
006
H
6 1 1, 5 3 2,
352
062
H
451
622
361

7.24
5.72
5.11
4.18
3.84
3.67
3.62
3.54
3.30
3.23
3.17
2.99
2.95
2.87
2.73
2.70
2.512
2.414
2.349
2.288
2.201
2.184
2.127
2.088
2.042
2.009
1.915
1.871
1.840
1.811
1.755
1.697

17.4
39.0
17.6
76.2
15.3
25.6
54.7
22.4
25.6
68.1
28.7
56.9
100.0
41.6
38.2
74.6
69.1
40.5
20.0
40.5
35.7
78.3
20.5
20.2
22.1
98.2
37.1
75.1
37.9
84.1
40.5
51.3

7.24
5.11
4.18
3.62
3.24
2.96
2.74
2.413
2.289
2.183
2.090
2.008
1.877
1.810
1.756
1.708
-

20
14
91
55
60
85
24
33
21
72
5
100
72
72
9
7
-

1.663

66.2

1.661

55

1.607
1.582
1.545
1.512

23.0
28.4
45.9
30.6

1.620
1.581
1.544
1.510

1
8
38
10

1 verneite Hekla. 2 PDF 36-1496.

Table 2 gives the results of the Rietveld reﬁnement. The reﬁnement shows that the sample is madeup by 53(1) wt % of verneite, 30(1) wt % of ralstonite, 15.9(6) wt % of hematite, and 1.6(4) wt % ofjakobssonite, as well as a minor, not determined quantity of “mineral HB”.

8

Minerals 2018, 8, 553

Table 2. Rietveld reﬁnement results. “Mineral HB” [1], also present in the sample, was not included in
reﬁnement because structural details are unknown. Average crystallite size modelled by Lorentzianfunction. Global parameters: Rexp = 6.58%, Rwp = 11.6%, GoF = 1.76, proﬁle function: Fundamental
parameters, background: Chebyshev polynomials.

average crystallite size (nm)
a (Å)
b (Å)
c (Å)
β (◦ )
atomic parameters
R-Bragg

Verneite

Ralstonite

Hematite

Jakobssonite

121(13)
10.264(1)ﬁxed [13]
4.7%

35(2)
9.963(2)ﬁxed [14]
6.5%

56(5)
5.035(1)13.824(4)ﬁxed [15]5.8%ﬁxed to 200
8.63(3)6.36(2)
7.25(2)114.4(5)ﬁxed [4]7.2%

4. Discussion
4.1. Description of the Crystal Structure
The original description of the crystal structure [13] presents it in an unconventional form as
a combination of the cation-centered [AlF6 ] groups (octahedra) and anion-centered [FNaCa3/2 ]2framework. This helps in relating the structure to some complex oxide structures presenting it as
their “negative” (with the roles of cations and anions in frameworks exchanged), but ignores thecoordinations of Na and Ca and makes the comparison with other ﬂuorides difﬁcult. Here, we give
another view on the crystal structure, based solely on cation coordinations.
As expected from the Al:F ratio, verneite is an aluminoﬂuoride with isolated [AlF6 ] octahedralgroups and additional F atoms not bonded to Al. The arrangement of the six F atoms around theAl site is almost perfectly octahedral with a perfect sphericity [16] and a volume distortion (υ) [17]of only 0.0018. The Al atom sits on the three-fold axis 0.036 Å from the centroid of coordination,with eccentricity [16] of 0.0232. The average Al–F bond distance is 1.804 Å [13].
Ca atoms have an eight-fold coordination in the form of a bisdisphenoid (Figure 2). This type of
coordination can achieve a conﬁguration with the minimum ratio of the volume of a circumscribed
sphere and the volume of the polyhedron for the coordination number 8; in other words, it isa maximum-volume polyhedron for this coordination number [18]. The Ca coordination polyhedronin verneite does not completely fulﬁll the conditions of a maximum-volume polyhedron because thefour F atoms that form the shortened-disphenoid part of the coordination approach a square-planararrangement (equatorial atoms on Figure 2). Consequently, its υ parameter (or volume distortion
compared to the maximum-volume bisdisphenoid) is larger than zero (Table 3). This type of
coordination is unique among the Ca–F coordinations in mineral ﬂuorides and related synthetic
compounds represented in Table 3.

Figure 2. Bisdisphenoidal coordination of Ca in verneite. Projection based on crystal structure datafrom Reference [13].

9Minerals 2018, 8, 553

Table 3. The coordination parameters of [CaFn ] coordination polyhedra in ﬂuoride minerals andrelated synthetic compounds; υ = volume distortion; O. = Octahedron; P.b. = Pentagonal bipyramid;
S.o. = Split octahedron; Bis. = Bisdisphenoid; S.a. = Square antiprism; C. = Cube; T.t.p. = Tricapped
trigonal prism; T.c. = Tricapped cube.
CompoundSite

CN<Ca–F> (Å)υ

Asphericity

Eccentricity

Polyhedron

Ref.

CaLiAlF6

Ca1

6

2.281

0.0038

0

0

O.

[19]

Na4 Ca4 Al7 F33

Ca1

6

2.246

0.0232

0

0

O.

[20]

β-NaCaAlF6

Ca1

6

2.308

0.04390.0049

0.023

O.

[20]

KCaAl2 F9

Ca1
Ca2

6
6

2.302
2.292

0.07030.05590.03480.04260.0165
0.017

O.

[21]

CaAlF5

Ca1

7

2.318

0.02650.03350.0521P.b.

[22]

Ca2 AlF7

Ca1
Ca2

7
7

2.316
2.346

0.03440.1391

0.02170.01120.02480.0404

P.b.
S.o.

[23]

α-NaCaAlF6

Ca1
Ca2

7
7

2.351
2.343

0.09430.08690.03310.04050.054
0.0467transitional
P.b./S.o.

[24]

Na2 Ca3 Al2 F14

Ca1

8

2.374

0.02340.02540.0286

Bis.

[13]

BaCaAlF7

Ca1
Ca2

8
8

2.372
2.372

0.0467
0.05690.03090.02710.03610.0362S.a.

[25]

Ba2 CaMgAl2 F14

Ca1

8

2.371

0.05650.04530.0349S.a.

[26]

Ca2 PbAlF9

Ca1
Ca2

8
8

2.360
2.361

0.088
0.08990.01810.02340.02370.0241transitional
C./S.a.

[27]

CaNa3 Mg3 AlF14

Ca1

8

2.421

0.1136

0.05830

distorted C.

[28]

CaF2

Ca1

8

2.365

0.1522

0

0

C.

[29]

Ca13 Y6 F43

Ca3
Ca1*
Ca2

8
9
10

2.341
2.617
2.491

0.14440.02960.04780.021
0.06270.04070.01420.06280.0576

distorted C.
T.t.p.
T.c.

[30]

The crystal structure data for the table are compiled from Reference [31]. As can be seen,
the coordination number (CN) of Ca (coordinated with F atoms only) varies from 6 to 8 or even,rarely, 9 or 10. In the crystal structures with CN6, the coordination is in the form of a nearly
perfect octahedron (in colquirite [19]) or subsequently more distorted octahedra in Na4 Ca4 Al7 F33 [20],β-NaCaAlF6 [20], and KCaAl2 F9 [21]. Ca with CN7 appears in moderately distorted pentagonal
bipyramids in jakobssonite [4,22] and Ca2 AlF7 (Ca1) [23]. A regular pentagonal bipyramid is the
maximum-volume polyhedron for CN7 and the υ values are calculated in comparison with it. Ca2 in
Ca2 AlF7 and the Ca coordinations in α-NaCaAlF6 [24] have this parameter signiﬁcantly different fromzero because they represent a different type of coordination geometry, close to a “split octahedron”,
the ideal form of which would have υ = 0.1333 [17]. From values in Table 3, we can see that Ca2
in Ca2 AlF7 closely approaches this form, whereas the coordinations in α-NaCaAlF6 are transitional
between a pentagonal bipyramid and a “split octahedron”. For CN8, the ideal types of coordination
polyhedra are, in addition to bisdisphenoid, the cube (0.1522), the hexagonal bipyramid (0.0462),
the square antiprism (0.0351), and the bicapped trigonal prism (0.0733). The numbers in parentheses
are the υ values in comparison with the maximum-volume bisdisphenoid. The forms observed in
Ca ﬂuorides from Table 3 are a regular cube in ﬂuorite and distorted cubes in tveitite (Ca3) [30]and coulsellite [28]. The coordinations in calcioaravaipaite [27] are transitional between the cubeand square antiprism, whereas the coordinations in usovite [26] and BaCaAlF7 [25] are moderately
distorted square antiprisms. In tveitite, the CN9 and CN10 Ca sites are also present. Ca1 witha tricapped trigonal prism coordination actually occupies a larger CN12 icosahedral void and is a split
site, whereas the coordination of Ca2 can be described as a cube where one corner has been substitutedby an F3 triangle [30].
Na atoms have a seven-fold coordination with a capped octahedron as the coordination
polyhedron (Figure 3). The capped octahedral face is broader than the opposite one (Figure 3).
10Minerals 2018, 8, 553

The average Na–F bond distance is 2.431 Å [13], whereas the polyhedron distortion parameters are
eccentricity = 0.0043, asphericity = 0.0615, and υ = 0.0849 (compared to regular pentagonal bipyramid,
which is the maximum-volume polyhedron for CN7). There is a considerably larger number of
Na containing ﬂuorides than those containing Ca, with a larger variability of coordination types;
a comparative study of Na coordinations in ﬂuorides will not be presented here, but in a special
following article. We only mention that the Na coordination polyhedron type in verneite seems to be
unique compared to other mineral ﬂuorides, similar to the case of the Ca coordination polyhedron.

Figure 3. Na coordination in verneite. The three-fold axis is vertical. Projection based on crystal
structure data from Reference [13].

The Ca coordination polyhedra build sinuous chains by sharing edges. They are interlacedby the sinuous chains of equally edge-sharing Na coordination polyhedra, and together these two
types of chains build layers parallel to {100} (Figure 4). The layers, parallel to the three equivalent
crystallographic planes, form a three-dimensional mesh that houses Al coordination octahedra in its
interstices (Figure 5). The Al coordination octahedra share one face with a Na coordination polyhedronand three edges with Ca coordination polyhedra. Three F atoms at the corners that belong to the sharedface are characterized by longer F–Al bonds (1.824 Å) and are each bonded to one Al, one Ca, and two
Na. The three F atoms in the corners of the opposite face with shorter F–Al bonds (1.784 Å) makebonds to one Al and two Ca each. The directions of the sinuous chains of Ca and Na coordination
polyhedra on the adjacent {100} faces, e.g., on the (100) and (010) faces, are mutually perpendicular(in this case [010] and [001], respectively), as required by the space group symmetry (Figure 5).

Figure 4. A {100} layer from the crystal structure of verneite, formed by sinuous chains of Ca coordinations
and Na coordinations, and overlaid/underlain by the [AlF6 ] octahedra. Projection based on crystal
structure data from Reference [13].

11Minerals 2018, 8, 553

Figure 5. A general direction view of coordination polyhedra in one unit cell of verneite. Red arrows
indicate the directions of sinuous chains of coordination polyhedra on the (100) and (010) surfaces of
the unit cell. Projection based on crystal structure data from Reference [13].

4.2. NaF–CaF2 –AlF3 System and the Natural Occurrences of Phases
Only three ternary phases have been originally reported in the phase system NaF–CaF2 –AlF3 [32],the low-temperature form of NaCaAlF6 , its high-temperature polymorph, and NaCaAl2 F9 .
Later investigations showed that the material investigated by Craig and Brown [32] and reported as
the low-temperature form of NaCaAlF6 actually has the composition Na2 Ca3 Al2 F14 [13]. In addition,
the composition of NaCaAl2 F9 was corrected to the structural formula Na4 Ca4 Al7 F33 [20]. NaCaAlF6
was conﬁrmed to have a high-temperature (stable above 620 ◦ C) and a low-temperature form, based ondeterminations of their structures [20,24]. The crystal structure data for the four conﬁrmed phasesin the phase system are represented in Table 4. To the best of our knowledge, none of the phasesfrom this phase system have been observed before in nature, and Na2 Ca3 Al2 F14 reported here is theﬁrst example. The synthetic work showed that this compound is stable up to 719 ◦ C at atmosphericpressure and decomposes at higher temperatures to a mixture of the high-temperature NaCaAlF6
and ﬂuorite [13]. It forms readily at temperatures lower than 600 ◦ C in the part of the phase systemcomprising approximately equal molar amounts of Na, Ca, and Al, and it crystallizes rather thanthe low-temperature form of NaCaAlF6 , which is metastable and could be synthesized only throughhydrothermal synthesis [20]. The fourth ternary phase in this phase system, Na4 Ca4 Al7 F33 , is reported
to form through a sluggish reaction between Na2 Ca3 Al2 F14 and AlF3 [32]. The metastability of
NaCaAlF6 below 600 ◦ C and the slow-forming reaction of Na4 Ca4 Al7 F33 most probably explain
the appearance of verneite as the only ternary Na–Ca–Al ﬂuoride in fumaroles.

12Minerals 2018, 8, 553

Table 4. Crystal structure data for the ternary phases from the NaF–CaF2 –AlF3 system.
FormulaSpace Group

Crystal latticeParameters

Structure
Type

Aluminoﬂuoride Part

Ref.

β-NaCaAlF6

P321

isolated [AlF6 ][20]

P21 /c8.9295(9), 5.0642(2) Å
8.7423(3), 5.1927(2),20.3514(9) Å, 91.499(2)◦Na2 SiF6

α-NaCaAlF6

uniqueisolated [AlF6 ][24]

Na2 Ca3 Al2 F14I21 3

10.257(1) ÅuniqueNa4 Ca4 Al7 F33

Im3m

10.781(3) Åuniqueisolated [AlF6 ] plus
additional F[Al7 F33 ] 3D framework

[13]
[20]

Author Contributions: Investigation, T.B.-Ž., A.G., D.P. and D.M.; Methodology, T.B.-Ž., A.G., D.P. and D.M.;
Software, T.B.-Ž.; Writing—original draft, T.B.-Ž.; Writing—review & editing, A.G. and D.P.
Funding: This research received no external funding.
Acknowledgments: Authors are indebted to the late S.P. Jakobsson for the collection of the Hekla and Eldfell
samples and the initiation of this work, and to E. Leonardsen for the ﬁrst XRD measurements of verneite. We thankJ. Bailey for the correction of the English text and the three anonymous referees for suggestions that improved
the manuscript.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Balić-Žunić, T.; Garavelli, A.; Acquafredda, P.; Leonardsen, E.; Jakobsson, S.P. Eldfellite, NaFe(SO4 )2 , a new
fumarolic mineral from Eldfell volcano, Iceland. Mineral. Mag. 2009, 73, 51–57. [CrossRef]
Garavelli, A.; Balić-Žunić, T.; Mitolo, D.; Acquafredda, P.; Leonadsen, E.; Jakobsson, S.P. Heklaite, KNaSiF6 ,a new fumarolic mineral from Hekla volcano, Iceland. Mineral. Mag. 2010, 74, 147–157. [CrossRef]
Balić-Žunić, T.; Garavelli, A.; Mitolo, D.; Acquafredda, P.; Leonardsen, E. Jakobssonite, CaAlF5 , a newmineral from fumaroles at the Eldfell and Hekla volcanoes, Iceland. Mineral. Mag. 2012, 76, 751–760.
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Mitolo, D.; Garavelli, A.; Balić-Žunić, T.; Acquafredda, P.; Jakobsson, S.P. Leonardsenite, MgAlF5 (H2 O)2 ,a new mineral species from Eldfell volcano, Heimaey Island, Iceland. Can. Mineral. 2013, 51, 377–386.
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minerals
ArticleCopper in Natural Oxide Spinels: The New Mineral
Thermaerogenite CuAl2O4, Cuprospinel and
Cu-Enriched Varieties of Other Spinel-Group
Members from Fumaroles of the Tolbachik Volcano,Kamchatka, Russia
Igor V. Pekov 1, *, Fedor D. Sandalov 1 , Natalia N. Koshlyakova 1 , Marina F. Vigasina 1 ,Yury S. Polekhovsky 2,† , Sergey N. Britvin 2,3 , Evgeny G. Sidorov 4 and Anna G. Turchkova 1
1

2
3
4

*
†

Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia;
fyodor.sandalov@yandex.ru (F.D.S.); nkoshlyakova@gmail.com (N.N.K.); vigasina@geol.msu.ru (M.F.V.);
annaturchkova@rambler.ru (A.G.T.)
Institute of Earth Sciences, St. Petersburg State University, Universitetskaya Nab. 7/9,
199034 St Petersburg, Russia; sbritvin@gmail.com (S.N.B.)
Nanomaterials Research Center, Kola Science Centre, Russian Academy of Sciences, Fersman str. 14,
184209 Apatity, Murmansk Region, Russia
Institute of Volcanology and Seismology, Far Eastern Branch of Russian Academy of Sciences,
Piip Boulevard 9, 683006 Petropavlovsk-Kamchatsky, Russia; mineral@kscnet.ru
Correspondence: igorpekov@mail.ru; Tel.: +7-495-939-4676
Deceased 28 September 2018.

Received: 11 October 2018; Accepted: 29 October 2018; Published: 1 November 2018

Abstract: This paper is the ﬁrst description of natural copper-rich oxide spinels. They were
found in deposits of oxidizing-type fumaroles related to the Tolbachik volcano, Kamchatka, Russia.
This mineralization is represented by nine species with the following maximum contents of CuO(wt.%, given in parentheses): a new mineral thermaerogenite, ideally CuAl2 O4 (26.9), cuprospinel,
ideally CuFe3+ 2 O4 (28.6), gahnite (21.4), magnesioferrite (14.7), spinel (10.9), magnesiochromite (9.0),
franklinite (7.9), chromite (5.9), and zincochromite (4.8). Cuprospinel, formerly known only as aphase of anthropogenic origin, turned out to be the Cu-richest natural spinel-type oxide [sample withthe composition (Cu0.831 Zn0.100 Mg0.043 Ni0.022 )Σ0.996 (Fe3+ 1.725 Al0.219 Mn3+ 0.048 Ti0.008 )Σ2.000 O4 from
Tolbachik]. Aluminum and Fe3+ -dominant spinels (thermaerogenite, gahnite, spinel, cuprospinel,
franklinite, and magnesioferrite) were deposited directly from hot gas as volcanic sublimates.
The most probable temperature interval of their crystallization is 600–800 ◦ C. They are associated witheach other and with tenorite, hematite, orthoclase, ﬂuorophlogopite, langbeinite, calciolangbeinite,aphthitalite, anhydrite, ﬂuoborite, sylvite, halite, pseudobrookite, urusovite, johillerite, ericlaxmanite,tilasite, etc. Cu-bearing spinels are among the latest minerals of this assemblage: they occur in cavities
and overgrow even alkaline sulfates. Cu-enriched varieties of chrome-spinels (magnesiochromite,
chromite, and zincochromite) were likely formed in the course of the metasomatic replacementof a magmatic chrome-spinel in micro-xenoliths of ultrabasic rock under the inﬂuence of volcanic
gases. The new mineral thermaerogenite, ideally CuAl2 O4 , was found in the Arsenatnaya fumaroleat the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption.
It forms octahedral crystals up to 0.02 mm typically combined in open-work clusters up to 1 mm
across. Thermaerogenite is semitransparent to transparent, with a strong vitreous lustre. Its colour
is brown, yellow-brown, red-brown, brown-yellow or brown-red. The mineral is brittle, with theconchoidal fracture, cleavage is none observed. D(calc.) is 4.87 g/cm3 . The chemical compositionof the holotype (wt.%, electron microprobe) is: CuO 25.01, ZnO 17.45, Al2 O3 39.43, Cr2 O3 0.27,
Fe2 O3 17.96, total 100.12 wt.%. The empirical formula calculated on the basis of 4 O apfu is:(Cu0.619 Zn0.422 )Σ1.041 (Al1.523 Fe3+ 0.443 Cr0.007 )Σ1.973 O4 . The mineral is cubic, Fd-3m, a = 8.093(9) Å,Minerals 2018, 8, 498; doi:10.3390/min8110498

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V = 530.1(10) Å3 . Thermaerogenite forms a continuous isomorphous series with gahnite. The strongest
lines of the powder X-ray diffraction pattern of thermaerogenite [d, Å (I, %) (hkl)] are: 2.873 (65) (220),2.451 (100) (311), 2.033 (10) (400), 1.660 (16) (422), 1.565 (28) (511) and 1.438 (30) (440).
Keywords: thermaerogenite; cuprospinel; gahnite; magnesioferrite; CuAl2 O4 ; CuFe2 O4 ; copperoxide; new mineral; spinel supergroup; fumarole sublimate; Tolbachik volcano; Kamchatka

1. Introduction
Oxide spinels compose one of the most studied mineral families and have numerous implicationsin the geosciences, chemistry and materials science [1]. In spite of the apparent simplicity,
the spinel-type crystal structure exhibits a remarkable ﬂexibility towards cation and anion substitutions,which results in the appearance of minerals accommodating more than two dozens of chemicalelements [2]. Copper–bearing oxide spinels, being well studied as synthetic phases in materials science,
are however virtually unknown in nature. The only reported Cu-rich spinel-type oxide mineral,cuprospinel, ideally CuFe2 3+ O4 , is in fact an anthropogenic phase as it has never been found in thenatural environments unaffected by anthropogenic inﬂuence factors. Cuprospinel was described as a
new mineral species from burnt dumps on the property of Consolidated Rambler Mines Limited near
Baie Verte, Newfoundland, Canada, where, together with hematite and a Cu-rich (13.9 wt.% CuO)
variety of magnesioferrite, it was formed in the result of a spontaneous ﬁre of the mined copper-zincore [3]. Thus, these oxides have deﬁnite anthropogenic origin, as well as cuprospinel formed as anincidental product of ore processing in ancient and modern smelters [4,5]. Cuprospinel was mentioned
in a volcanic material from Mahanadi, Orissa, India [6], and in ores of the Chahnaly gold deposit in SE
Iran [7], however, no analytical evidences were given and the identiﬁcation of this mineral from both
localities seems doubtful.
In the course of ongoing research of oxidizing-type fumaroles related to the Tolbachik volcano
at Kamchatka, Russia, we have encountered more than twenty oxide minerals. Among those,ten mineral species belonging to the spinel supergroup [2] were identiﬁed, including the new mineraldeltalumite, (Al0.67 0.33 )Al2 O4 , the delta-alumina dimorphous with corundum [8]. A remarkablechemical feature of Tolbachik spinels is the common presence of a signiﬁcant amount of copper.
CuO contents ranging from 1 to 18 wt.% are typical for spinel, magnesioferrite, gahnite, franklinite,magnesiochromite, chromite and zincochromite. Besides these Cu-rich mineral varieties, two minerals
with species-deﬁning copper were therein discovered: the genuine natural cuprospinel and a newmineral ﬁrst described in the present paper, thermaerogenite, ideally CuAl2 O4 . The latter wasrecently approved by the IMA Commission on New Minerals, Nomenclature and Classiﬁcation(IMA no. 2018-021). The name thermaerogenite (Cyrillic: термaэрoгенит) is constructed based on the
combination of Greek words θερμóς, hot, αέριoν, gas, and γενής that means “born by”. Thus, in whole
it means born by hot gas, that reﬂects the fumarolic origin of the mineral. This name also contains
the allusion that Cu-rich spinel-type oxides form in volcanic fumaroles and their anthropogenic
counterparts, unlike Cu-free and Cu-poor oxide spinels, are numerous and widespread in othergeological formations. The type specimen of thermaerogenite is deposited in the collections of theFersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with theregistration number 5192/1.
The Mg-Al-Fe oxide spinels, namely magnesioferrite, magnetite and spinel were previously
reported from fumaroles of several active volcanoes [9–13]. However, no information on Cu contents
in these minerals was published. Moreover, we did not ﬁnd any references on Cu-enriched (with
CuO content higher than 1 wt.%) natural oxide spinels. Thus, this work is the ﬁrst report on natural
copper-rich oxide members of the spinel supergroup.

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2. Occurrence and Mineral Associations
The studied material was collected by us during ﬁeldwork in the period of 2012–2018. The majorityof studied samples originate from the Arsenatnaya fumarole, one of the brightest in the mineralogicalaspect examples in the world of fumaroles belonging to the oxidizing type (in fumaroles ofthis type, the increase of oxygen fugacity is a result of the mixing of volcanic gases with theatmospheric air [14,15]). Arsenatnaya is located at the apical part of the Second scoria cone of theNorthern Breakthrough of the Great Tolbachik Fissure Eruption (below–NB GTFE), Tolbachik volcano,Kamchatka Peninsula, Far-Eastern Region, Russia (55◦ 41 N 160◦ 14 E, 1200 m asl). This scoria cone,
formed in 1975, is a monogenetic volcano about 300 m high and approximately 0.1 km3 in volume [16].
Now, more than 40 years after the eruption, its fumarole ﬁelds remain active: numerous gas ventswith temperatures up to 490 ◦ C were observed by us in 2012–2018. Now the fumarolic gases at theSecond scoria cone are compositionally close to atmospheric air, with the contents of <1 vol.% water
vapour and <0.1 vol.% acid species, mainly CO2 , HF and HCl [17], while in 1976–1977 these gases
were signiﬁcantly more enriched in H2 O, CO2 , SO2 , HCl and, in some fumaroles, HF [14].
The Arsenatnaya fumarole was uncovered and ﬁrst studied by us during ﬁeldworks in July 2012.
This active fumarole described in papers [18,19] is a near-meridional linear system of mineralized
pockets (up to 10–15 cm wide) and cracks situated between blocks of basalt scoria and volcanic bombsin the near-surface part of the scoria cone. The length of the hot area belonging to Arsenatnaya is
about 15 m and its width varies from 1–1.5 m in the southern end to 3–4 m in the northern part.
Numerous strongly mineralized pockets occur at depths from 0.3 to 4 m. The sublimate minerals form
incrustations in the open space of the pockets, ﬁll cracks and pores or replace basalt. Arsenatnaya is
one of the hottest fumaroles at the Second scoria cone of the NB GTFE: the temperature measured
by us using chromel-alumel thermocouple in 2012–2018 in different pockets immediately after their
partial uncovering varies from 360 to 490 ◦ C and, in general, increases with depth. About 160 valid
minerals (including 46 new species ﬁrst discovered here) and >30 insufﬁciently studied mineral phases
have been identiﬁed in this unique mineralogical site.
The Cu-bearing oxide spinels in the Arsenatnaya fumarole were found in two
mineral assemblages.
The minerals with Al or Fe3+ as species-deﬁning components (spinel, gahnite, thermaerogenite,magnesioferrite, franklinite, and cuprospinel) occur in the intermediate in depth, in the polymineraliczone of the fumarole [19] and are associated with tenorite, hematite, orthoclase (As-bearing
variety), ﬂuorophlogopite, langbeinite, calciolangbeinite, aphthitalite-type sulfates, anhydrite,krasheninnikovite, vanthofﬁte, ﬂuoborite, sylvite, halite, pseudobrookite, rutile, corundum andvarious arsenates: urusovite, johillerite, ericlaxmanite, kozyrevskite, popovite, lammerite, lammerite-β,tilasite, svabite, nickenichite, bradaczekite, dmisokolovite, shchurovskyite, etc. Cu-bearing spinels
are among the latest minerals of this assemblage: they occur in cavities and overgrow not only
earlier oxides (hematite, tenorite) and silicates but also arsenates and even “saline” sulfates, suchas langbeinite-calciolangbeinite series and aphthitalite group minerals (Figures 1a–c and 2a,b).
Overgrowing of hematite by Cu-rich oxide spinels is very common; these minerals form epitacticintergrowths (the crystal face {111} of a spinel-group member is coplanar to the face {0001} of hematite
crystal) or clusters of randomly oriented crystals (Figure 3a–c). Sometimes, spinels cover hematite as
massive crusts (Figure 4a,b). Some associations contain two or more spinel-group minerals. In suchcases, the Cu-richest species are typically the latest (Figure 4b).

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Minerals 2018, 8, 498

(a)

(b)

(c)
Figure 1. (a) Numerous small brown crystals of thermaerogenite on colourless to white langbeinite
with light blue urusovite, iron-black tenorite and minor green ericlaxmanite. Field width: 2.2 mm.
(b) Abundant small brown crystals of thermaerogenite on white aphthitalite and iron-black hematite
and tenorite. Field width: 3.5 mm. (c) Small brown crystals of thermaerogenite on colourless
calciolangbeinite iron-black tenorite, light blue urusovite and green kozyrevskite. Field width: 0.8 mm.

(a)

(b)

Figure 2. (a) Octahedral crystals of Cu-bearing spinel on crystal crust of tilasite. (b) Flattened spinel-law
twins on {111}, additionally combined in complex twinned lattice-like aggregates, of Cu-bearing gahnite
on crystal crust of anhydrite. Scanning electron microscopic (SEM)-secondary electron (SE) images.

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Minerals 2018, 8, 498

(a)

(b)

(c)
Figure 3. (a) Distorted (ﬂattened on {111}) octahedral crystals of cuprospinel epitactically overgrowing
the {0001} face of hematite crystal. (b,c) Octahedral crystals of Cu-bearing gahnite overgrowing
columnar hematite crystals. Both epitaxy and random orientation of gahnite crystals on hematite areobserved. SEM (SE) images.

(a)

(b)

Figure 4. (a) Solid crust of thermaerogenite (Thag) overgrowing hematite (Hem) crystal. (b) Crustconsisting of Cu-bearing gahnite (Gh) and later thermaerogenite (Thag) overgrowing skeletal crystal ofhematite (Hem). Polished section, SEM-back-scattered electron (BSE) images.

Cu-enriched chrome-spinels were found in Arsenatnaya in several micro-xenoliths (up to 1 mm
across) of ultrabasic rock mainly consisting of olivine (Fo83-85 ). These micro-xenoliths embedded inbasalt scoria were strongly altered by fumarolic gases. As a result of gas inﬂuence, the peripheral part ofolivine grains was signiﬁcantly replaced by hematite (Figure 5a) and primary, magmatic chrome-spinel(crystals up to 0.06 mm) was altered to Zn- and Cu-bearing species. Figure 5b demonstrates such

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Minerals 2018, 8, 498

pseudomorphosed crystal with a core represented by twinned lattice-like aggregates of the newly
formed Cu-enriched chromite and a rim consisting of Cu-enriched zincochromite.

(a)

(b)

Figure 5. (a) Altered by fumarolic gases, micro-xenolith of ultrabasic rock mainly consisting ofolivine (Ol) and containing Cu-bearing chrome-spinels (Chr); Hem—hematite. (b) Crystal of primary,magmatic chrome-spinel replaced by Cu-bearing varieties of chromite (Chr, core) and zincochromite(Zchr, rim). Polished section, SEM (BSE) images.

Besides Arsenatnaya, Cu-bearing varieties of gahnite, magnesioferrite and spinel were found indeposits of extinct fumaroles belonging to the Western paleo-fumarole ﬁeld at Mountain 1004, a scoria
cone located 2 km south of the Second scoria cone of the NB GTFE. Mountain 1004 was formed as aresult of an ancient eruption of Tolbachik; the age of this monogenetic volcano and its fumarole ﬁelds
is evaluated as ca. 2000 years [16]. Spinels occur here in cavities of basalt scoria altered by fumarolicgas. The associated sublimate minerals are diopside, ﬂuorophlogopite, potassic feldspar, indialite,
hematite, tenorite, ﬂuorite, sellaite, anglesite, and baryte.
3. Methods
Reflectance spectra for thermaerogenite were obtained in air using a MSF-21 micro-spectrophotometer(LOMO company, St. Petersburg, Russia) with the monochromator slit width of 0.4 mm and beam diameterof 0.1 mm; SiC (Reflectionstandard 474251, No. 545, Germany) was used as a standard.
The Raman spectrum of thermaerogenite was obtained using an EnSpectr R532 Raman microscope(Department of Mineralogy, Moscow State University, Moscow, Russia) with a solid-state laser diode withgreen radiation (532 nm) at room temperature. The spectrum was processed in the range from 100 to
4000 cm−1 with the use of a holographic diffraction grating with 1800 mm−1 and a resolution equal
to 6 cm−1 . The microscope was focused onto the sample using a PLNC 40X objective (NA = 0.65).
Backscattered Raman signals were collected at 1 s exposure time with 400 spectra accumulations.
The spectrum was obtained for a randomly oriented crystal with the diameter of the focal spot on thesample about 1 μm. The output laser power was about 12 mW. The power of laser radiation on the surfaceof the mineral was significantly less. No thermal damage of the sample was observed. Before conducting
the experiments, the instrument was calibrated with Raman line of crystalline silicon (520 cm−1 ).
Scanning electron microscopic (SEM) studies in secondary electron (SE) and back-scatteredelectron (BSE) modes were carried out and chemical composition was determined for all studied
samples using a Jeol JSM-6480LV scanning electron microscope equipped with an INCA-Wave
500 wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial
Resolution, Department of Petrology, Moscow State University, Moscow, Russia), with an acceleration
voltage of 20 kV, a beam current of 20 nA, and a 3 μm beam diameter. The standards used are: MgAl2 O4(Mg,Al), Ni (Ni), Cu (Cu), ZnS (Zn), V (V), FeCr2 O4 (Cr), FeS2 (Fe), and MnTiO3 (Mn,Ti).
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Minerals 2018, 8, 498

Powder X-ray diffraction data were collected using a Rigaku RAXIS Rapid II diffractometer with acurved image plate detector, a rotating anode with VariMAX microfocus optics, using CoKα radiation,
in Debye-Scherrer geometry, at an accelerating voltage of 40 kV, a current of 15 mA and an exposure
time 15 min. The distance between the sample and the detector was 127.4 mm. Data processing was
carried out using osc2xrd software [20].
Single-crystal X-ray diffraction studies were carried out using an Xcalibur S diffractometerequipped with a CCD detector (MoKα radiation).
4. Results
4.1. General Appearance and Physical Properties of Thermaerogenite and Other Cu-Rich Oxide Spinelsfrom Tolbachik
Thermaerogenite forms octahedral crystals up to 0.02 mm across, sometimes skeletal, typicallycombined in open-work clusters (Figure 6a) up to 1 mm across. Areas “sprinkled” by crystals of the
new mineral (Figure 1b) are up to 0.5 cm × 0.5 cm. Besides the major form {111}, narrow {110} faces
were observed on some crystals of thermaerogenite. Being visually perfect (Figure 6a), crystals ofthermaerogenite have blocky inner structure and these micro-blocks are slightly misoriented, whichhampers the single-crystal X-ray diffraction study of the mineral.

(a)

(b)

(c)

(d)

Figure 6. (a) Cluster of octahedral, with minor faces {110}, crystals of thermaerogenite. (b) Crustof octahedral crystals of cuprospinel with minor faces {100}. (c) Aggregate of octahedral crystalsof Cu-bearing magnesioferrite partially covered by thin “ﬂuffy” crust of supergene opal from
paleo-fumarole (Mountain 1004). (d) Crystal of Cu-bearing magnesioferrite of rhombic dodecahedral
shape. SEM (SE) images.

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Minerals 2018, 8, 498

Other Cu-rich Al- and Fe3+ -dominant spinels in Tolbachik fumaroles occur as octahedral crystals(Figures 2a, 3 and 6a–c), sometimes with minor {110} or/and {100} faces, up to 0.1 mm across,only magnesioferrite was observed as larger (up to 1 mm) crystals, typically rhombic dodecahedra(Figure 6d). Gahnite was also found as ﬂattened spinel-law twins on {111} additionally combined in acomplex lattice-like aggregates (Figure 2b). A similar lattice-like shape was observed for Cu-bearing
chromite (Figure 5b). Spinel, magnesioferrite and gahnite sometimes form crusts up to 1 cm acrossand up to 0.5 mm thick. Solid crusts of cuprospinel (up to 2 mm across) were observed on hematite.
Thermaerogenite is semitransparent to transparent, with a yellowish streak and strong vitreous
lustre. Its colour is brown, yellow-brown, red-brown, brown-yellow or brown-red (Figure 1).
Colour, transparency and lustre of other Cu-rich Al- and Fe3+ -dominant spinels from Tolbachik
are in general correlated with the Al:Fe ratio. The Fe-poor varieties are very light (some samplesof spinel and gahnite from Mountain 1004 are colourless or greyish-white) with vitreous lustre.
Spinel, gahnite and Al-enriched cuprospinel in typical samples from the Arsenatnaya fumarole arevisually indistinguishable from the above-described thermaerogenite. Fe-rich cuprospinel is dark
reddish-brown with strong vitreous to adamantine lustre. Magnesioferrite has dark brown to iron-black
colour and submetallic lustre.
Thermaerogenite is brittle, with conchoidal fracture (observed under the scanning electron
microscope); cleavage or parting is not observed. Its Mohs hardness is ca. 7. Density calculatedusing the empirical formula of the holotype specimen is 4.870 g/cm3 .
4.2. Optical Data for Thermaerogenite
In reﬂected light, thermaerogenite is optically isotropic, grey, with yellowish internal reﬂections.
The reﬂectance values are given in Table 1.
Table 1. Reﬂectance data for thermaerogenite. Reﬂectance values for four wavelengths recommended
by the IMA Commission on Ore Microscopy are given in bold type.
λ (nm)Rλ (nm)

R

400
420
440
460
470
480
500
520
540
546

16.4
16.0
15.7
15.4
15.2
15.1
14.8
14.5
14.2
14.2

560
580
589
600
620
640
650
660
680
700

14.0
13.7
13.6
13.4
13.2
13.0
12.9
12.8
12.5
12.3

4.3. Raman Spectroscopy of Thermaerogenite
The Raman spectrum of thermaerogenite (Figure 7) is typical for spinel-type oxides [21]. It contains
four distinct bands with following wavenumbers of maxima (cm−1 , s—strong band): 762s, 590, 284,
and 125s. They could be assigned based on the data reported for different oxide spinels in [21].
The strongest band at 762 cm−1 could correspond to A1g mode, stretching vibrations of O atoms
surrounding Al in tetrahedral coordination. The presence of part of Al in the tetrahedral position (8a)is typical for synthetic Cu-rich members of the series Zn1−x Cux Al2 O4 [22]. Broad band with maximum
at 590 cm−1 probably corresponds to F2g (2) or F2g (3) mode involving divalent cations, (Cu,Zn)–O,
whereas weak band at 284 cm−1 corresponds to F2g (1) mode. Strong band at 125 cm−1 can be assigned
as corresponding to lattice modes.

22

Minerals 2018, 8, 498

Figure 7. The Raman spectrum of thermaerogenite.

4.4. Chemical Composition of Cu-Rich Oxide Spinels from Tolbachik
Representative electron-microprobe analyses for the studied minerals are given in Table 2(Cu-bearing Al- and Fe3+ -dominant spinels) and Table 3 (Cu-bearing chrome-spinels). There are 32 spot
analyses of their different chemical varieties which demonstrate chemical variability of Cu-enrichedoxide spinels from Tolbachik fumaroles.
Valent states of cations were not determined by direct methods. For properly sublimate minerals(thermaerogenite, cuprospinel, gahnite, spinel, magnesioferrite and franklinite) Fe and Mn are considered
as Fe3+ and Mn3+ , respectively, taking into account extremely oxidizing conditions of mineral deposition
in Arsenatnaya and similar fumaroles [18]. This is clearly confirmed by the calculation of the empiricalformulae using the scheme common for oxide spinels, on the basis of four O atoms per formula unit (apfu):
Sums of divalent and trivalent cations are close to 2.00 and 3.00 apfu, respectively (Tables 2 and 3) that is
typical for “2-3 spinels” with the general formula A2+ B3+ 2 O4 [2,23]. For the Cu-bearing chrome-spinels
formed in the result of the alteration of a primary, magmatic chrome-spinel, we assume the presenceof part of iron in a divalent state (while manganese content is below detection limit in these minerals).
Their formulae were calculated on the basis of four O apfu with fixing of the cation sum A2+ + B3+,4+ = 3.00
apfu and the Fe2+ :Fe3+ ratio was calculated by charge balance.
Maximum contents of copper detected in different spinel-group minerals from Tolbachik
fumaroles are presented in Table 4 and atomic ratios of the major bivalent (A) and trivalent (B)cations are shown in Figure 8a,b (based on 124 spot electron-microprobe analyses).
Table 2. Chemical composition of Cu-bearing Al- and Fe3+ -dominant spinel-group oxides from
Tolbachik fumaroles: 1–4: thermaerogenite, 5–11: gahnite, 12–16: spinel, 17–20: magnesioferrite, 21:
franklinite, and 22–28: cuprospinel.
No.

1*

2

3

4

5

6

7

MgO

-

-

-

0.44

-

5.50

-

CuO

25.01 (23.64–26.86)/1.46

25.47

25.67

20.25

20.07

15.00

17.55

wt.%

ZnO

17.45 (14.46–18.71)/2.00

18.33

17.43

18.99

23.79

18.29

21.71

Al2 O339.43 (34.59–45.43)/4.60

45.43

28.30

22.79

49.05

53.90

56.08
-

Cr2 O30.27 (0.17–0.33)/0.07

0.17

0.86

-

0.15

0.03

Mn2 O3

-

-

-

0.36

-

0.10

-

Fe2 O317.96 (11.47–22.21)/4.76

11.47

24.48

34.29

7.72

5.99

4.63

TiO2

-

-

2.59

2.61

-

0.22

0.24

Total

100.12100.87

99.33

99.73

100.7899.03

100.2123

Minerals 2018, 8, 498

Table 2. Cont.
No.

1*

2

3

4

5

6

7

formula calculated on the basis of 4 O apfu
Mg

-

-

-

0.023

-

0.242

-

Cu

0.619

0.609

0.673

0.540

0.472

0.334

0.396

Zn

0.422

0.428

0.447

0.496

0.547

0.398

0.479
1.973

Al

1.523

1.695

1.156

0.949

1.801

1.874

Cr

0.007

0.004

0.017

-

0.003

0.001

-

Mn

-

-

-

0.011

-

0.002

-

Fe3+

0.443

0.273

0.639

0.911

0.181

0.133

0.104

Ti

-

-

0.068

0.069

-

0.005

0.006

ΣA2+

1.041

1.037

1.119

1.059

1.019

0.975

0.874

ΣB3+,4+

1.973

1.972

1.881

1.940

1.984

2.015

2.082

No.

8

9

10

11

12

13

14

0.59

2.66

13.27

17.54

19.61

wt.%
MgO

0.42

-

NiO

-

-

-

-

0.87

-

-

CuO

13.72

13.06

13.02

2.94

10.89

6.86

4.83

ZnO

30.66

31.91

29.25

35.66

5.98

8.42

9.37

Al2 O3

44.70

54.11

44.30

55.81

32.93

61.12

48.44

Cr2 O3

0.19

-

0.18

-

-

-

-

Mn2 O3

0.16

-

0.10

-

1.88

0.37

0.93

Fe2 O3

8.82

1.75

11.40

2.11

34.29

4.77

13.88

TiO2

1.06

0.11

0.80

-

-

0.41

2.02

Total

99.73

100.94

99.64

99.18

100.11

99.49

99.08
0.807

formula calculated on the basis of 4 O apfu
Mg

0.020

-

0.028

0.119

0.601

0.688

Ni

-

-

-

-

0.021

-

-

Cu

0.333

0.301

0.314

0.066

0.250

0.136

0.100

Zn

0.726

0.718

0.690

0.789

0.134

0.164

0.191

Al

1.690

1.944

1.669

1.970

1.180

1.897

1.576

Cr

0.004

-

0.004

-

-

-

-

Mn

0.004

-

0.003

-

0.048

0.008

0.022

Fe3+

0.213

0.040

0.274

0.048

0.784

0.094

0.288

Ti

0.025

0.003

0.019

-

-

0.008

0.042

ΣA2+

1.079

1.019

1.033

0.974

1.006

0.989

1.098

ΣB3+,4+

1.935

1.987

1.969

2.018

2.012

2.008

1.928

24

Minerals 2018, 8, 498

Table 2. Cont.
No.

15

16

17

18

19

20

21
8.54

wt.%
MgO

21.94

16.04

17.43

17.65

10.56

20.81

NiO

-

-

-

-

0.35

-

-

CuO

2.18

4.34

5.90

3.22

14.73

1.26

7.91

ZnO

5.24

12.28

3.09

0.60

0.94

0.60

17.99

Al2 O3

59.10

58.36

5.30

0.66

5.51

5.72

20.24

Cr2 O3

-

0.09

0.33

-

-

-

0.17

Mn2 O3

1.56

0.26

0.24

1.32

1.01

1.24

2.29

Fe2 O3

8.76

8.10

66.24

77.29

66.64

69.46

36.51

TiO2

0.48

0.24

0.92

-

-

-

5.50

Total

99.26

99.70

99.45

100.74

99.74

99.09

99.15

Mg

0.843

0.640

0.873

0.884

0.552

1.014

0.424

Ni

-

-

-

-

0.010

-

-

Cu

0.042

0.088

0.150

0.082

0.391

0.031

0.199

Zn

0.100

0.243

0.077

0.015

0.024

0.015

0.443
0.795

formula calculated on the basis of 4 O apfu

Al

1.796

1.842

0.210

0.026

0.228

0.220

Cr

-

0.002

0.006

-

-

-

0.005

Mn

0.031

0.005

0.007

0.034

0.027

0.031

0.058

Fe3+

0.170

0.163

1.675

1.953

1.760

1.709

0.915

Ti

0.009

0.005

0.023

-

-

-

0.138

ΣA2+

0.986

0.971

1.099

0.980

0.977

1.060

1.066

ΣB3+,4+

2.006

2.018

1.922

2.013

2.015

1.960

1.910

No.

22

23

24

25

26

27

28

MgO

0.75

0.63

4.31

3.68

2.32

0.48

5.05

NiO

0.70

-

-

-

-

-

-

CuO

28.55

27.13

24.48

24.36

25.92

20.81

23.44

ZnO

3.51

7.76

3.39

2.48

4.53

18.82

1.94

Al2 O3

4.82

7.52

3.44

1.69

4.12

18.69

3.46

Cr2 O3

-

0.22

-

0.23

-

0.24

0.27

wt.%

Mn2 O3

1.65

0.94

2.15

0.85

1.38

0.33

1.20

Fe2 O3

59.49

53.49

59.77

65.82

61.51

37.06

63.40

TiO2

0.28

1.75

1.54

-

0.30

2.63

0.41

Total

99.75

99.44

99.08

99.11

100.08

99.06

99.17

25

Minerals 2018, 8, 498

Table 2. Cont.
No.

22

23

24

25

26

27

28
0.283

formula calculated on the basis of 4 O apfu
Mg

0.043

0.036

0.244

0.211

0.133

0.026

Ni

0.022

-

-

-

-

-

-

Cu

0.831

0.781

0.704

0.707

0.748

0.573

0.665

Zn

0.100

0.219

0.096

0.071

0.128

0.506

0.054

Al

0.219

0.338

0.154

0.076

0.185

0.802

0.153

Cr

-

0.005

-

0.006

-

0.005

0.006

Mn

0.048

0.030

0.069

0.027

0.045

0.010

0.038

Fe3+

1.725

1.535

1.711

1.902

1.768

1.016

1.793

Ti

0.008

0.050

0.044

-

0.008

0.072

0.011

ΣA2+

0.996

1.036

1.044

0.988

1.008

1.105

1.002

ΣB3+,4+

2.000

1.958

1.979

2.012

2.006

1.904

2.002

* The holotype specimen of thermaerogenite: averaged from four spot analyses value (range)/standard deviation;dash means the content below detection limit.

Table 3. Chemical composition of Cu-bearing chrome-spinels from the Arsenatnaya fumarole,
Tolbachik: 1–2: magnesiochromite, 3: chromite, 4: zincochromite.
No.

1

2

3

4

wt.%
MgO

10.48

7.28

7.52

2.60

FeO

2.30

6.19

16.38

1.07

CuO

8.97

6.25

5.86

3.96

ZnO

8.54

12.83

2.16

27.88

Al2 O3

11.32

11.52

11.06

10.44

V2 O3

0.19

0.14

0.16

0.19

Cr2 O3

47.45

46.06

48.20

43.42

Fe2 O3

10.73

9.70

7.56

10.61

TiO2

0.83

0.89

0.88

0.81

Total

100.82100.8699.78

100.98

formula calculated on the basis of 4 O apfu with A + B = 3.00 apfu *
Mg

0.524

0.374

0.388

0.140

Fe2+

0.058

0.160

0.427

0.029

Cu

0.227

0.163

0.153

0.108

Zn

0.212

0.326

0.056

0.745

Al

0.448

0.467

0.451

0.445

V

0.005

0.004

0.004

0.006

Cr

1.259

1.254

1.319

1.243

Fe3+

0.246

0.228

0.179

0.263

Ti

0.021

0.023

0.023

0.022

ΣA2+

1.021

1.023

1.023

1.022

1.979

1.977

1.977

1.978

ΣB3+,4+* Fe2+ :Fe3+ ratio is calculated by charge balance.

26

Minerals 2018, 8, 498

Table 4. Maximum contents of copper detected in different spinel-group members from Tolbachik.
Mineral Species

Ideal Formula

CuO, wt.%

Cu, apfu

Cuprospinel
Thermaerogenite
GahniteMagnesioferrite
SpinelMagnesiochromite
Franklinite
Chromite
Zincochromite

CuFe3+ 2 O4
CuAl2 O4
ZnAl2 O4
MgFe3+ 2 O4
MgAl2 O4
MgCr2 O4
ZnFe3+ 2 O4
Fe2+ Cr2 O4
ZnCr2 O4

28.6
26.9
21.4
14.7
10.9
9.0
7.9
5.9
4.8

0.83
0.69
0.51
0.39
0.25
0.23
0.20
0.15
0.13

Figure 8. Atomic ratios of the major bivalent (a) and trivalent (b) cations in Cu-bearing
spinel-group oxides from Tolbachik fumaroles. Legend: 1—thermaerogenite, 2—cuprospinel, 3—spinel,
4—magnesioferrite, 5—gahnite, 6—franklinite, 7—zincochromite, 8—chromite, 9—magnesiochromite.

4.5. X-ray Diffraction Data of Cu-Rich Oxide Spinels from Tolbachik
Powder X-ray diffraction (XRD) data of thermaerogenite in comparison with the calculated data
for synthetic CuAl2 O4 are given in Table 5. All diffraction reﬂections are well-indexed in the cubic
unit cell, by analogy with common spinel-group minerals [2,23]. Unit-cell dimensions reﬁned from thepowder XRD data are presented in Table 6.

27

Minerals 2018, 8, 498

Table 5. Powder X-ray diffraction data (d in Å) of thermaerogenite and its synthetic end-member analogue.
Thermaerogenite

Synthetic CuAl2 O4 *

Imeas

dmeas

dcalc

Icalc

dcalc

3
65
100
1
10
6
16
28
30
4
6
1
1

4.659
2.873
2.451
2.329
2.033
1.865
1.660
1.565
1.438
1.286
1.240
1.226
1.174

4.694
2.875
2.452
2.347
2.033
1.865
1.660
1.565
1.437
1.286
1.240
1.226
1.174

2
51
100
0.2
17
1
12
31
38
3
6
1
1

4.664
2.856
2.436
2.332
2.020
1.853
1.649
1.555
1.428
1.277
1.232
1.218
1.166

hkl
111
220
311
222
400
331
422
511
440
620
533
622
444

* Calculated from the structure data from [24].

Both single-crystal and powder XRD data for all other studied spinel-group oxides from Tolbachik
fumaroles demonstrate their cubic symmetry and systematic absences typical for space group Fd-3m
characteristic for “2-3 spinels” [2,23]. The obtained unit-cell parameters a and V are in agreement withthe chemical composition of the minerals (Table 6). The major factor which inﬂuences these parametersof the studied spinels is the Al:Fe3+ ratio. In particular, unit-cell dimensions and the majority ofd-spacings of thermaerogenite are slightly higher in comparison with its synthetic end-member
analogue CuAl2 O4 (Tables 5 and 6) that is mainly caused by admixture of Fe3+ which partiallysubstitutes Al in the mineral (Table 2).
Single-crystal XRD data of thermaerogenite allowed to obtain the a parameter of the cubic unit
cell (Table 6), however, the crystal structure of the new mineral was not studied due to the low qualityof single-crystal diffraction patterns caused by the imperfectness of all tested crystals which consist ofslightly misoriented micro-blocks and in fact can be considered as crystal intergrowths.
Table 6. Unit-cell dimension a (Å) and volume (V, Å3 ) of Cu-rich spinel-group oxides from Tolbachik,cuprospinel from its type locality and synthetic CuAl2 O4 .
Mineral/CompoundaV

Method *SourceThermaerogenite
Thermaerogenite
Synthetic CuAl2 O4
Cuprospinel
Cuprospinel **GahniteGahniteSpinelMagnesioferrite

8.093(9)8.131(1)8.079(3)8.402(11)8.369
8.124(4)8.1327(6)8.149(1)8.344(13)530.1(10)537.6(2)527(3)
593(1)
586
536.2(5)537.9(1)541.2(2)581(1)

SCXRD
PXRD
SCXRD
SCXRD
PXRD
SCXRD
PXRD
PXRD
SCXRD

this work
this work[24]
this work[3]
this work
this work
this work
this work* Abbreviations SCXRD and PXRD mean single-crystal and powder X-ray diffraction methods, respectively; ** typespecimen from Consolidated Rambler Mines Limited, Baie Verte, Newfoundland, Canada.

Chemical composition of thermaerogenite varies from crystal to crystal in both Cu:Zn and Al:Fe
ratios (Figure 8). The latter is a cause of slight discrepancy between unit-cell parameters of the newmineral obtained from single-crystal and powder X-ray diffraction data (Table 6).

28Minerals 2018, 8, 498

5. Discussion
All spinel-group oxides found in the middle zones of the Arsenatnaya fumarole are copper-bearing
and the majority of the studied samples contain >1 wt.% CuO (Tables 2 and 3). The maximum contentsof copper in these minerals are reported in Table 4. For Tolbachik cuprospinel, the range 20.8–28.6 wt.%
CuO (=0.57–0.83 apfu Cu) is found and for thermaerogenite 16.7–26.9 wt.% CuO (=0.41–0.69 apfu Cu).
Zinc is the most typical admixture in thermaerogenite. Gahnite and this mineral form here thecontinuous solid-solution series with the main substitution scheme Cu2+ →Zn2+ (Figure 8a). Even the
Zn-poorest specimen of thermaerogenite contains 14.5 wt.% ZnO (=0.36 apfu Zn), and in the Cu-poorest
sample of gahnite from Arsenatnaya, 2.9 wt.% CuO (=0.07 apfu Cu) was detected. The continuous
isomorphous series with the general formula Zn1–x Cux Al2 O4 was reported for synthetic spinels [22].
Some crystals of thermaerogenite contain signiﬁcant Mg admixture (up to 5.4 wt.% MgO),
however, a continuous solid solution between spinel and the new mineral is not observed (Figure 8a),unlike the synthetic system Mg1–x Cux Al2 O4 with full isomorphism [25]. The solid-solution series
between cuprospinel and thermaerogenite, with the main substitution scheme Al3+ →Fe3+ , also
demonstrates a signiﬁcant gap (Figure 8b). Cuprospinel shows the most stable chemical composition
among all studied “2-3 spinels” from Tolbachik fumaroles. It is typically characterized by not very
high contents of admixtures, including Mg, Zn and Al, and does not form continuous solid-solution
series with other oxide spinels (Table 2, Figure 8).
Cuprospinel was described as a new mineral species from burning dumps [3] and its other ﬁnds
were related to ore smelters [4,5]. These anthropogenic spinel-forming systems are close in conditions
to volcanic fumaroles of the oxidizing type in which copper-rich spinel-type oxides crystallize in acompletely natural environment at Tolbachik. The similarity of products formed in both systems,
as well as data on the synthesis and thermodynamic stability of these phases [26–28] and the absenceof data on such minerals in other geological formations, demonstrate that the physical and chemical
conditions in fumaroles are optimal for the formation of Cu-enriched oxide spinels: there is the
combination of high temperature, atmospheric pressure and high oxygen fugacity.
It is doubtless that Cu-rich Al- and Fe3+ -dominant oxide spinels in fumarole systems of Tolbachik
where deposited directly from hot gas as volcanic sublimates. The most convincing evidence of their
exhalation origin is the location over crusts of very typical sublimate minerals, especially over alkaline
sulfates (Figure 1) in the open space of pockets within a recently formed and still active fumarole at thesummit part of a volcanic scoria cone. No sign of occurrence of any other process that could result in theformation of spinels (crystallization from melt or solution, solid-state transformation, etc.) is observed
there. Hot volcanic gas was a carrier of “ore” constituents, foremost Cu, Zn and Fe. Basalt scoria which
composes the walls of fumarole chambers can be a source of elements having low volatilities in such
post-volcanic systems, namely Mg, Al and Ti [29,30]. Our temperature measurements in fumarolesof the Second scoria cone of the NB GTFE and fumaroles born by the Ploskiy Tolbachik eruption of
2012–2013 [31] together with data based on the halite–sylvite solid-solution thermometry [19] showthat the mineral assemblages with Cu-bearing oxide spinels were formed at temperatures deﬁnitely not
lower than 500 ◦ C and probably not higher than 800 ◦ C. This assumption is in agreement with data on
synthetic CuAl2 O4 prepared under the atmospheric pressure in the temperature range 600–1100 ◦ C [28].
Thus, the most probable temperature interval of crystallization of Cu-rich oxide spinels in Tolbachik
fumaroles seems to be 600–800 ◦ C.
For the Cu- and Zn-enriched chrome-spinels found in altered micro-xenoliths of ultrabasic rock
(Figure 5), we assume the same physical conditions but rather another mechanism of formation.
They could be formed due to gas metasomatism, in terminology by Naboko and Glavatskikh [32],i.e., in the result of the replacement of a primary, magmatic chrome-spinel by chemically different
spinel-group species under the inﬂuence of hot volcanic gas enriched by “ore” components.
Chromium, Mg, Al, V and probably part of Fe (which remained as Fe2+ ) were inherited from theprimary chrome-spinel phase whereas Cu and Zn were taken from gas and Fe2+ was partly oxidized
to Fe3+ in this process.
29Minerals 2018, 8, 498

The distribution of cations between tetrahedral and octahedral sites in Cu-rich oxide spinels from
Tolbachik fumaroles was not examined by us. We only can consider, based on the Raman spectrum (seeabove), that thermaerogenite contains part of Al in tetrahedral coordination. The cation distribution iswell-studied for synthetic Cu-bearing spinel-type oxides including the end-member CuAl2 O4 and it
was shown that they are “largely normal” spinels (in terminology by [33], i.e., with bivalent cations
occupying the tetrahedral site for 2/3 or more) but part of Cu2+ typically occurs in the octahedral site
and the Cu-B3+ disorder in general increases with the temperature increase [22,25,27,28,34]. It should
be noted that the distribution of bivalent and trivalent cations between tetrahedral and octahedral
sites is not a species-deﬁning sign in the light of the IMA-accepted nomenclature of spinel-supergroupminerals: The classiﬁcation is based only on chemical formulae as resulting from chemical data only [2].
Copper-rich oxide spinels are extremely rare minerals reliably known in Nature only in Tolbachik
fumaroles. Unlike them, the chalcogenide members of the spinel supergroup are not so rare. Nine valid
minerals with species-deﬁning Cu belong to the thiospinel and the selenospinel groups [2] and one ofthem, carrollite CuCo2 S4 , is a common sulﬁde in many ore deposits. Such strong difference in diversityand distribution in nature between oxide and chalcogenide Cu-rich spinels is probably caused by thestrong chalcophile character (it is worthy to note that the term chalcophile originates from Greek wordχαλκóς, copper) of copper.
6. Conclusions
Natural copper-rich oxide spinels are characterized for the ﬁrst time. They were found in
the deposits of active and extinct fumaroles of the oxidizing type related to the Tolbachik volcano,Kamchatka, Russia. This mineralization is represented by nine species belonging to the “2-3 spinels”
with the general chemical formula A2+ B3+ 2 O4 , i.e., to the spinel subgroup of the oxyspinel groupwithin the spinel supergroup [2]. There are (content of CuO in wt.% is given in parentheses for eachmineral) two minerals with species-deﬁning Cu2+ , namely cuprospinel (20.8–28.6) and the new species
thermaerogenite (16.7–26.9), and Cu-enriched varieties of gahnite (up to 21.4), magnesioferrite (upto 14.7), spinel (up to 10.9), magnesiochromite (up to 9.0), franklinite (up to 7.9), chromite (up to 5.9),
and zincochromite (up to 4.8).
The new mineral species thermaerogenite [ideally CuAl2 O4 , cubic, space group Fd-3m,
a = 8.093(9) Å, V = 530.1(10) Å3 ] forms a continuous isomorphous series with gahnite.
Cuprospinel, ideally CuFe3+ 2 O4 , earlier reliably known only as a phase of the anthropogenic origin or
as a synthetic compound, is a typical oxide mineral in the Arsenatnaya fumarole at Tolbachik. Its sample
with composition (Cu0.831 Zn0.100 Mg0.043 Ni0.022 )Σ0.996 (Fe3+ 1.725 Al0.219 Mn3+ 0.048 Ti0.008 )Σ2.000 O4 represents
the Cu-richest natural spinel-type oxide so far described.
Cu-bearing oxide spinels at Tolbachik have a properly fumarolic origin. Aluminum- andFe3+ -dominant species (thermaerogenite, gahnite, spinel, cuprospinel, franklinite, and magnesioferrite)
were deposited directly from hot gas as volcanic sublimates. The most probable temperatureinterval of their crystallization seems to be 600–800 ◦ C. Copper-enriched varieties of chrome-spinels(magnesiochromite, chromite and zincochromite) were formed probably under the same physicalconditions but in the result of the metasomatic replacement of a magmatic chrome-spinel inmicro-xenoliths of ultrabasic rock under the inﬂuence of volcanic gas.
The rarity of Cu-rich oxide spinels in nature is probably caused by the very speciﬁc characterof conditions optimal for their formation: there is the combination of high temperature, atmosphericpressure, high oxygen fugacity, rich source of copper and hot gas as effective carrier of this element.
Such combination is mostly realized in volcanic fumaroles of the oxidizing type enriched by “ore”
components: there are really rare, exotic geological objects. The strong afﬁnity of Cu-rich oxide
spinels to such environments is conﬁrmed by earlier reported [3–5] ﬁndings of their anthropogenic
counterparts in burning copper mine dumps and ore smelters.

30

Minerals 2018, 8, 498

Author Contributions: I.V.P. and S.N.B. wrote the paper. N.N.K. and F.D.S. obtained, processed and designed
chemical data. I.V.P. and S.N.B. obtained and processed X-ray diffraction data. I.V.P., E.G.S., A.G.T. and F.D.S.
collected the material and F.D.S. prepared it for laboratory studies. M.F.V. obtained and processed Raman
spectrum. Y.S.P. measured reﬂectance values.
Funding: This work was supported by the Russian Foundation for Basic Research, grant no. 18-05-00051 (in partof mineralogical studies), and the Russian Science Foundation, grant 14-17-00071 (in part of powder XRD studies).
Acknowledgments: We are grateful to Nadezhda V. Shchipalkina for her assistance in the obtaining of powder
XRD data for spinel and to Vasiliy O. Yapaskurt for his help with the SEM studies. We thank X-Ray Diffraction
Resource Center and Centre for Physical Methods of Surface Investigation of St. Petersburg State University forproviding instrumental and computational resources.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Sidorov, E.G.; Pushcharovsky, D.Y. Wulfﬁte, K3 NaCu4 O2 (SO4 )4 , and parawulfﬁte, K5 Na3 Cu8 O4 (SO4 )8 ,two new minerals from fumarole sublimates of the Tolbachik volcano, Kamchatka, Russia. Can. Mineral.
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minerals
ArticleSharyginite, Ca3TiFe2O8, A New Mineral from theBellerberg Volcano, Germany
Rafał Juroszek 1, *, Hannes Krüger 2 , Irina Galuskina 1 , Biljana Krüger 2 , Lidia Jeżak 3 ,Bernd Ternes 4 , Justyna Wojdyla 5 , Tomasz Krzykawski 1 , Leonid Pautov 6 and Evgeny Galuskin 1
1

2
3
4
5
6

*

Department of Geochemistry, Mineralogy and Petrography, Faculty of Earth Sciences,
University of Silesia, B˛edzińska 60, 41-200 Sosnowiec, Poland; irina.galuskina@us.edu.pl (I.G.);
tomasz.krzykawski@us.edu.pl (T.K.); evgeny.galuskin@us.edu.pl (E.G.)
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria;
Hannes.Krueger@uibk.ac.at (H.K.); Biljana.Krueger@uibk.ac.at (B.K.)
Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw, Al. Żwirki and Wigury 93,
02-089 Warszawa, Poland; l.jezak@uw.edu.pl
Dienstleistungszentrum Ländlicher Raum (DLR) Westerwald-Osteifel-Aussenstelle Mayen,
Bahnhofstrasse 45, DE-56727 Mayen, Germany; Bernd.Ternes@dlr.rlp.de
Swiss Light Source, Paul Scherrer Institute, 5232 Villigen, Switzerland; justyna.wojdyla@psi.ch
Fersman Mineralogical Museum RAS, Leninskiy pr, 18/2, 115162 Moscow, Russia; pla58@mail.ru
Correspondence: rjuroszek@us.edu.pl; Tel.: +48-516-491-438

Received: 27 June 2018; Accepted: 17 July 2018; Published: 21 July 2018

Abstract: The new mineral sharyginite, Ca3 TiFe2 O8 (P21 ma, Z = 2, a = 5.423(2) Å, b = 11.150(8) Å,c = 5.528(2) Å, V = 334.3(3) Å3 ), a member of the anion deﬁcient perovskite group, was discovered inmetacarbonate xenoliths in alkali basalt from the Caspar quarry, Bellerberg volcano, Eifel, Germany.
In the holotype specimen, sharyginite is widespread in the contact zone of xenolith with alkali basalt.
Sharyginite is associated with ﬂuorellestadite, cuspidine, brownmillerite, rondorﬁte, larnite andminerals of the chlormayenite-wadalite series. The mineral usually forms ﬂat crystals up to
100 μm in length, which are formed by pinacoids {100}, {010} and {001}. Crystals are ﬂattenedon (010). Sharyginite is dark brown, opaque with a brown streak and has a sub-metallic lustre.
In reﬂected light, it is light grey and exhibits rare yellowish-brown internal reﬂections. The calculated
density of sharyginite is 3.943 g·cm-3 . The empirical formula calculated on the basis of 8 O
apfu is Ca3.00 (Fe3+ 1.00 Ti4+ 0.86 Mn4+ 0.11 Zr0.01 Cr3+ 0.01 Mg0.01 )Σ2 (Fe3+ 0.76 Al0.20 Si0.04 )Σ1.00 O8 . The crystal
structure of sharyginite, closely related to shulamitite Ca3 TiFeAlO8 structure, consists of double layersof corner-sharing (Ti, Fe3+ ) O6 octahedra, which are separated by single layers of (Fe3+ O4 ) tetrahedra.
We suggest that sharyginite formed after perovskite at high-temperature conditions >1000◦ C.
Keywords: sharyginite; new mineral; crystal structure; Raman spectroscopy; Bellerberg volcano; Germany

1. Introduction
Sharyginite, Ca3 TiFe2 O8 , is a new mineral which was found in thermally-metamorphosed
limestone xenoliths in alkali basalts of the Bellerberg volcano lava ﬁeld, Caspar quarry, Eastern
Eifel region, Rhineland-Palatinate, Germany (50◦ 21 6” N, 7◦ 14 2” E). Sharyginite (P21 ma, Z = 2,
a = 5.423(2) Å, b = 11.150(8) Å, c = 5.528(2) Å, V = 334.3(3) Å3 ) is a Fe3+ -analogue of shulamitite,
Ca3 TiFeAlO8 [1], a member of the anion deﬁcient perovskite group [2] and also an intermediate
member of the pseudobinary perovskite CaTiO3 —brownmillerite Ca2 (Fe,Al)2 O5 series [3].
“Grenier phase” is the name of the Ca3 TiFe3+ 2 O8 compound synthesized in 1976 [4]. This phase
was intensively studied due to its ionic and electronic conductivity [4,5]. The crystal structure of
Ca3 TiFe3+ 2 O8 was ﬁrst determined by Rodriguez-Carvajal and co-authors [6]. Some information on

Minerals 2018, 8, 308; doi:10.3390/min8070308

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Minerals 2018, 8, 308

magnetic properties of this phase were reported by Causa et al. as result of an electron paramagnetic
resonance (EPR) study [7].
Previously, sharyginite was documented as a mineral in ye’elimite-larnite pyrometamorphic rocksof the Hatrurim Complex [3] and in high-temperature skarns occurring in sedimentary carbonate
xenoliths within ignimbrite conﬁned to the Upper Chegem volcanic structure of the North Caucasus,
Kabardino-Balkaria, Russia [8]. Some sharyginite crystals were recognized in mineral association of
Ca-rich xenoliths in Klöch Basalt quarry, Bad Radkersburg, Styria, Austria [9]. This phase was also
described in metacarbonate rocks from some burned dumps of the Donetsk [10] and Chelyabinsk [11]coal basins. Moreover, it was identiﬁed in xenoliths from the Eifel region [12], but because of the small
size of the crystals, this mineral was not studied in detail, until now.
Sharyginite (IMA 2017-14) was approved by the Commission of New Minerals, Nomenclature
and Classiﬁcation of IMA. Type material was deposited in the mineralogical collection of Fersman
Mineralogical Museum RAS, Leninskiy pr., 18/2, 115162 Moscow, Russia, catalogue numbers: 4958/1.
The name sharyginite was given in honour of Victor Victorovich Sharygin (b.1964) from the
Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russia. Victor Sharygin is a mineralogist,
author and co-author of many publications concerning the mineralogy and petrology of different
alkaline rocks (alkali basalts, lamproites, kimberlites, carbonatites, etc.) and pyrometamorphic rocks.
He is the author of new mineral descriptions; for example, shulamitite Ca3 TiFeAlO8 —Al-analogue ofsharyginite [1]. He has also worked on xenolith specimens with sharyginite from Eifel and publishedsome preliminary data on this mineral [12].
In the present paper, we report the detailed description of the new mineral sharyginite fromthe type locality—Caspar quarry, Bellerberg volcano, Eifel, Germany. In addition, we discuss
the composition and Raman spectroscopic data of sharyginite from the other two localities:
Upper Chegem volcanic Caldera, Great Caucasus, Russia and Jabel Harmun (Hatrurim Complex),Palestinian Autonomy.
2. Occurrence, Geological Settings, Physical and Optical Properties of Sharyginite
The type locality of sharyginite, is a part of a quaternary volcano region in the Eastern Eifelarea, Rhineland-Palatinate, Germany [13]. The Bellerberg volcano is a well-preserved cinder cone:
the western part of the crater wall is the actual Ettringer Bellerberg and the eastern one is theKottenheimer Büden [14]. More than 20 new mineral species were discovered in the area of
the Bellerberg volcano [13,15–17]. The active quarry is characterized by the presence of various
thermally-metamorphosed silicate and carbonate-silicate xenoliths within a leucite tephrite lava [13,14].
The signiﬁcant variety of xenoliths is based on different protolith composition and metamorphic
transformations. The calcium-rich xenoliths in the crater area of the Bellerberg are mainly composed ofgehlenite, pyroxene (diopside), garnet (grossular-andradite), anorthite, and wollastonite. As accessory
minerals, chalcopyrite, hematite, magnetite, or spinel are noted. Some of these xenoliths contain larnite,jasmundite, brownmillerite, mayenite, spurrite, and monticellite. The appearance of such mineral
assemblages is a result of metamorphism at very high-temperature conditions (pyrometamorphism).
The altered limestone xenoliths are known for their diversity of secondary, low-temperature minerals.
Ettringite is the most abundant. Portlandite, thaumasite, hydrocalumite, afwillite, tobermorite,
CaCO3 polymorphs, and zeolites belong to the low-temperature mineral association [14].
The new mineral sharyginite occurs in altered Ca-rich xenolith, which consists of a few
multi-coloured zones characterized by variable mineral associations (Figure 1).
The ﬁrst
zone (light brown and pink) is composed of cuspidine, andradite, and minerals of thebrownmillerite-srebrodolskite series. Lakargiite, sphalerite, and some Ni-phosphides occur as
accessory minerals. Secondary phases are represented by hydrocalumite, ettringite, calcite, brucite,hydrogarnets, and unidentiﬁed Ca-hydrosilicates. The second zone (white and creamy) consistsof cuspidine and minerals of the brownmillerite-srebrodolskite and chlormayenite-wadalite series.

34

Minerals 2018, 8, 308

Rankinite, ﬂuorellestadite, vorlanite, lakargiite, and bismoclite appear in this zone as accessory
minerals. The low-temperature mineral association is the same as in the ﬁrst zone.

Figure 1. Xenolith specimen from Bellerberg with characteristic multi-coloured zones: I-III—xenolith,
IV—altered alkali basalt (description in text).

Sharyginite is common in the next zone (Figure 1, zone III). It is associated with ﬂuorellestadite,cuspidine, brownmillerite, rondorﬁte, larnite, and minerals of the chlormayenite-wadalite series(Figure 2a). Rankinite, magnesioferrite, perovskite, and ﬂuorite are less common in this zone.
Low-temperature mineral assemblages are represented by calcite, ettringite-thaumasite, brucite,
gypsum and Ca-hydrosillicates. Sharyginite forms euhedral crystals and their intergrowths, which canbe up to 200 μm in size (Figure 2b). Abundant inclusions of cuspidine, ﬂuorellestadite, and also
chlormayenite are characteristic for sharyginite crystals (Figure 2c). The next zone is represented
by completely altered basalt (Figure 1, zone IV). This zone, about 2 cm thick, consists of cuspidine,
perovskite, minerals of the chlormayenite-wadalite series, and magnesioferrite. Sharyginite is also
noted. In some cavities, clinoenstatite and spinel relicts occur. The low-temperature association isrepresented by calcite, gypsum, gibbsite, and minerals of ettringite-thaumasite series.
Sharyginite crystals are formed by pinacoids {100}, {010}, {001}, and the rhombic pyramid isobserved rarely. Crystals are ﬂattened on (010) (Figure 3). Sharyginite exhibits a dark brown colour
and a brown streak. The mineral is opaque in transmitted light and exhibits a sub-metallic lustre.
Sharyginite shows good cleavage on (010) and imperfect along (001) and (100). Parting was notobserved, tenacity is brittle and fracture is uneven.

35Minerals 2018, 8, 308

Figure 2. (a) General view of a metacarbonate xenolith with sharyginite from the Caspar
quarry, Bellerberg volcano; fragment in the frame is magniﬁed in Figure 2b; (b) Euhedral
crystals and intergrowths of sharyginite; (c) Poikilitic crystal of sharyginite with inclusionsof ﬂuorellestadite and chlormayenite.
BSE (backscattered electron) images.
Cal—calcite;
Ca-Si-OH—Ca-hydrosilicates; Cus—cuspidine; Ell—ﬂuorellestadite; Fl—ﬂuorite; May—chlormayenite;
Prv—perovskite; Shg—sharyginite; Wad—wadalite.

Figure 3. Sharyginite crystals formed by pinacoids {100}, {010}, and {001}, and rhombic pyramid andﬂattened on (010) at the part of the strongly altered rock. BSE image.

36

Minerals 2018, 8, 308

The density could not be measured because of abundant ﬂuorellestadite, chlormayenite,and cuspidine inclusions. The density calculated using the average composition and unit-cellparameters as obtained from single crystal XRD (X-ray Powder Diffraction) analysis is 3.943 g·cm−3 .
Micro-hardness measurements were carried out using a VHN (Vickers Hardness Number) load of 25 gwhich gave a mean value of 635 kg/mm3 (range from 621 to 649 kg/mm3 ) based on 35 measurements.
This value corresponds to a hardness of ~5.5–6 in the Mohs scale.
In reﬂected light, sharyginite was characterized by light grey colour and weak pleochroism fromgrey to very light grey. Under crossed polarizers, weak anisotropy was noted. Rare yellowish browninternal reﬂections were observed in sharyginite crystals in polarized light (Figure 4).

Figure 4. Sharyginite crystals with characteristic yellowish brown internal reﬂections from theBellerberg volcano. Reﬂected light, crossed polarizers. Cal—calcite; Cus—cuspidine; Fl—ﬂuorite;
Prv—perovskite; Shg—sharyginite; Wad—wadalite.

Quantitative reﬂectance measurements for sharyginite from the Bellerberg volcano were
measured in air relative to a SiC standard using a UMSP 50D Opton microscope-spectrophotometer(Table 1). Reﬂectance percentages for the four Rmax and Rmin COM (Commission on Ore Mineralogy)
wavelengths were: 16.1, 15.5 (470 nm); 14.9, 14.2 (546 nm); 14.6, 14.1 (589 nm), and 14.5, 13.9 (650 nm).
Table 1. Reﬂectance values for sharyginite.
Rmax /Rmin (%)λ (nm)

Rmax /Rmin (%)λ (nm)

18.7/17.6
18.3/17.4
17.0/16.0
16.4/15.6
16.1/15.5
15.9/15.4
15.5/14.9
15.2/14.5
15.0/14.3
14.9/14.2

400
420
440
460
470 (COM)
480
500
520
540
546 (COM)

14.8/14.2
14.7/14.1
14.6/14.1
14.6/14.1
14.6/14.0
14.6/14.0
14.5/13.9
14.4/13.7
14.3/13.5
14.1/13.4

560
580
589 (COM)
600
620
640
650 (COM)
660
680
700

37

Minerals 2018, 8, 308

Sharyginite was also detected in other localities. It was found in the xenolith No.1—the largest of
the 11 described altered carbonate xenoliths within ignimbrites of the Upper Chegem volcanic Caldera,
North Caucasus, Kabardino-Balkaria, Russia. This unique xenolith was discovered by Victor Gazeev
and Alexandr Zadov [18] and speciﬁed as a pyrometamorphic rock, formed at high temperatureand low-pressure conditions [19]. The Upper Chegem Caldera situated in the eastern part of the
Elbrus-Kyugen volcanic region is a type locality for more than 20 new mineral species e.g.: lakargiite,
CaZrO3 [8], chegemite, Ca7 (SiO4 )3 (OH)2 [20], elbrusite, Ca3 (Zr1.5 U6+ 0.5 )Fe3+ 3 O12 [21], rusinovite,
Ca10 (Si2 O7 )3 Cl2 [22], pavlovskyite, Ca8 (SiO4 )2 (Si3 O10 ) [23], edgrewite, Ca9 (SiO4 )4 F2 [24], dzhuluite,
Ca3 SbSnFe3+ 3 O12 [25], etc. More geological information concerning this locality can be found inGazeev et al. [18], Galuskin et al. [8,20] and Galuskina et al. [26].
In the analysed specimen, sharyginite occurs as an accessory mineral. Cuspidine, ﬂuorellestadite,wadalite, and larnite are widespread rock-forming minerals. Less commonly sharyginite is
associated with perovskite, lakargiite, periclase, ﬂuorite, and baryte. Secondary minerals are
represented by low-temperature phases belonging to the ettringite-thaumasite series and unidentiﬁed
Ca-hydrosilicates. Sharyginite from this locality occurs as euhedral and subhedral crystals up to 200 μm
in size (Figure 5a). Most of them are partially or totally altered and replaced by phases exhibiting
compositions similar to iron oxides. Unaltered sharyginite up to 50 μm in size was observed rarely as
single crystals in a matrix of ellestadite-wadalite. Mostly, they are characterized by the presence of
wadalite inclusions (Figure 5b).

Figure 5. (a) Sharyginite crystal in larnite-wadalite rock from the Upper Chegem volcanic
Caldera; (b) Poikilitic euhedral crystal of sharyginite with wadalite inclusions. BSE images.
Ca-Si-OH—Ca-hydrosilicates, Ell—ﬂuorellestadite, Lrn—larnite, Shg—sharyginite, Wad—wadalite.

Sharyginite was also found in larnite-ye’elimite nodules from pseudoconglomerates of thepyrometamorphic rocks of the Hatrurim Complex, Jabel Harmun locality, Judean Mountains,
Palestinian Autonomy. Jabel Harmun, as well as other Hatrurim Complex localities, consists ofhigh-temperature rocks represented, mainly, by larnite-, gehlenite- and spurrite-bearing rocks [27].
It is also the type locality for a few new minerals such as: vapnikite, Ca3 UO6 [27], harmunite,
CaFe2 O4 [28], ﬂuormayenite, Ca12 Al14 O32 F2 [29], nabimusaite, KCa12 (SiO4 )4 (SO4 )2 O2 F [30], gazeevite,
BaCa6 (SiO4 )2 (SO4 )2 O [31] and dzierżanowskite, CaCu2 S2 [32]. Larnite-bearing rocks, which wereformed during spontaneous combustion metamorphism of bituminous chalks and marls from theGhareb and Taqiye Formations [33–36], were collected during ﬁeldworks in 2015. Detailed geologicaldescription of the Jabel Harmun locality and the main hypotheses of the Hatrurim Complex formation
were described by Galuskina et al. [28].
Sharyginite from Jabel Harmun was recognized in dark-brown homogenous larnite-ye’elimiterock samples, wherein magnesioferrite and minerals of the ﬂuorellestadite-ﬂuorapatite series occurred
38

Minerals 2018, 8, 308

also as main phases. Accessory and secondary minerals are represented by vorlanite, cuprite, tenorite,
baryte, and hydrocalumite. Most frequently, sharyginite forms xenomorphic crystals from20 to140 μm in size. Poikilitic crystals of sharyginite contain numerous inclusions of larnite, ye’elimite,and ﬂuorellestadite. Homogenous grains occur rarely. Aggregates of sharyginite-magnesioferriteintergrowths up to 70 μm in size were more often noted (Figure 6).

Figure 6. Xenomorphic sharyginite crystals in larnite-ye’elimite rock from Jabel Harmun. BSE image.
Ell—ﬂuorellestadite, Lrn—larnite, Mfr—magnesioferrite; Shg—sharyginite, Ye—ye’elimite.

3. Materials and Methods
Mineral association and crystal morphology of sharyginite were examined using a scanningelectron microscope Phenom XL, PhenomWorld, ThermoFisher Scientiﬁc, Eindhoven, The Netherlands
equipped with an EDS (energy-dispersive X-ray spectroscopy) detector (Faculty of Earth Sciences,
University of Silesia, Sosnowiec, Poland).
Quantitative chemical analyses of sharyginite were carried out using a CAMECA SX100
electron-microprobe operating in WDS (wavelength dispersive X-ray spectroscopy) mode (Institute ofGeochemistry, Mineralogy and Petrology, University of Warsaw, Warsaw, Poland) at 15 kV and 10 nA,
beam size ~1 μm. The following lines and standards were used: CaKα, SiKα, MgKα—diopside;
AlKα—orthoclase; TiKα—rutile; CrKβ—Cr2 O3 ; FeKα—Fe2 O3 ; SnLα—cassiterite, SrLα—celestine;
MnKα—rhodonite; NbLα—Nb; ZrLα—zircon.
Powder X-ray diffraction data were obtained using CuKα radiation (λ = 1.541874 Å) with 40 kV
and 40 mA generator settings, on a Panalytical, X’Pert PRO PW3040/60 diffractometer equippedwith an X’Celerator strip detector with an active angle of 2.122 ◦ 2θ, Malvern PANalytical, Almelo,
Netherlands (Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland). Incident beam optics
with divergence slit (1/8◦ ) and anti-scatter slit (1/4◦ ) was used. A Ni ﬁlter was used for reduction of
the Kβ line intensity. Conditions for recording the XRD pattern were as follow: scan range 5–90 ◦ 2θ,step size 0.01 ◦ 2θ, the counting time was set to 800 s. The total measurement time was 18 h.
Synchrotron radiation diffraction experiments were carried out at the Swiss Light Source(Paul Scherrer Institute, Villigen, Switzerland) beamline X06DA. The measurement was performed atambient conditions and the data was collected with the Pilatus 2M-F detector placed at a distance of
120 mm from the crystal. The detector was vertically translated by 67 mm to increase the maximum
resolution to 0.67 Å. The X-ray diffraction data were collected from a single crystal mounted onto themulti-axis PRIGo goniometer [37] using the DA+ acquisition software [38]. Data were collected at the39

Minerals 2018, 8, 308

0.7085 Å wavelength with 0.1 s exposure time and 0.1◦ oscillation range using a focused 80 × 45 μm(h × v) beam. Data reduction, including empirical absorption correction, was performed with the XDS
software package [39]. Further details of the intensity data collection and crystal-structure reﬁnementof sharyginite are reported in Table 2.
Table 2. Parameters for X-ray data collection and crystal-structure reﬁnement for sharyginite.
Crystal Data
Chemical formula
Crystal system
Space group
Unit-cell dimensions

Ca3 TiFe1.68 Al0.32 O8
orthorhombic
P21 maa = 5.423(2) Å
b = 11.150(8) Åc = 5.528(2) Å
334.3(3) Å3
398.6
3.943 g/cm3
2
60 × 40 × 40 μm

Unit cell volume
Formula weight
Density (calculated)
Z
Crystal size
Data Collection

Reﬂection measured
Rintbeamline X06DA, Swiss Light Source
multi-axis goniometer PRIGo, PILATUS 2M-F
detector
0.7085 Å
120 mm
0.1◦
1800
0.1 s
−7 ≤ h ≤ 7;
−16 ≤ k ≤ 10;−7 ≤ l ≤ 7
1810
0.026

Reﬁnement of Structure

Full Matrix Least-squares on f

No. of unique reﬂections
No. of observed unique reﬂ. [I > 3σ(I)]Final R values [I > 3σ(I)]Final R values (all data)
S (all data)
Reﬁned parameters
Weighting schemeΔρmin [e Å−3 ]Δρmax [e Å−3 ]951
943
R = 0.024; wR = 0.034
R = 0.024; wR = 0.034
1.6274
w = 1/(σ2 (F) + 0.0001F2 )
−0.62
0.49

Diffractometer
Radiation wavelength
Detector to sample distanceOscillation range
No. of frames measured
Time of exposure
Reﬂection ranges

The Raman spectra of sharyginite from Bellerberg, Jabel Harmun and Upper Chegem Caldera
were recorded on a WITec alpha 300R Confocal Raman Microscope, WITec, Ulm, Germany (Faculty ofEarth Science, University of Silesia, Poland) equipped with an air-cooled solid laser 532 nm and a CCD(closed circuit display) camera operating at −61◦ C. The laser radiation was coupled to a microscopethrough a single-mode optical ﬁbre with a diameter of 3.5 μm. An air Zeiss (LD EC Epiplan-Neoﬂuan
DIC–100/0.75NA) objective was used. Raman scattered light was focused by effective Pinhole size
about 30 μm and monochromator with a 600 mm−1 grating. The power of the laser at the sample
position was 20–30 mW. Integration times of 5 s with an accumulation of 15 scans and a resolution of
3 cm−1 were chosen. The monochromator was calibrated using the Raman scattering line of a siliconplate (520.7 cm−1 ). Spectra processing such as baseline correction and smoothing was performed using
the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Bands ﬁtting40

Minerals 2018, 8, 308

were done using a Gauss-Lorentz cross-product function, with the minimum number of component
bands used for the ﬁtting process.
4. Results
4.1. Chemical Composition
The analytical data for sharyginite from the Bellerberg volcano lava ﬁeld (holotype locality),
Jabel Harmun (Hatrurim Complex, Palestinian Autonomy) and Upper Chegem Volcanic Caldera(Kabardino-Balkaria, Russia) are presented in Table 3. Chemical data of other authors [1,3,12,40] areincluded in Table 4.

41

Nb2 O5
MnO2
SiO2
SnO2
TiO2
ZrO2
Al2 O3Cr2 O3Fe2 O3
CaO
FeO
MgO
MnO
SrO
Total

wt %

n.d.
2.27
0.58
n.d.
17.04
0.27
2.49
0.20
34.87
41.59
n.d.
0.13
n.d.
n.d.
99.44

n=9

Mean

1.04–3.22
0.40–0.80
16.29–19.34
0.07–0.57
2.22–3.89
0.07–0.42
32.81–35.85
40.99–42.09
0.08–0.24

0.89
0.14
0.53
0.11
0.85
0.37
0.05

Range

0.71
0.13

S.D.

Bellerberg (Holotype Locality)

n.d.
n.d.
1.19
n.d.
17.97
0.43
3.83
0.25
32.80
42.19
n.d.
0.08
n.d.
0.18
98.91

n = 19

Mean

0.05

0.02

0.61
0.11
0.14
0.14
0.79
0.27

0.09

S.D.

0.08–0.32

0.06–0.11

16.65–18.76
0.25–0.62
3.62–4.13
0.00–0.47
31.70–34.54
41.64–42.60

1.07–1.43

Range

Jabel Harmun (Hatrurim Complex)

n.d.
n.d.
0.17
0.37
17.38
0.39
1.86
n.d.
37.43
40.71
n.d.
0.05
0.09
n.d.
98.45

n=4

Mean

0.04–0.06
0.01–0.13

36.40–38.72
40.53–40.85

1.03
0.14
0.01
0.06

0.07–0.32
0.15–0.61
16.97–17.76
0.16–0.67
1.36–2.29

Range

0.11
0.26
0.38
0.23
0.45

S.D.

Upper Chegem Caldera

Table 3. The representative chemical composition of sharyginite from different localities.

Minerals 2018, 8, 308

42

Ca2+
Sr2+
Sum A
Nb5+
Mn4+
Sn4+
Ti4+
Zr4+
Cr3+
Fe3+
Fe2+
Mg2+
Mn2+
Sum B
Si4+
Al3+
Fe3+
Sum T

43

0.01
2.00
0.08
0.30
0.59
0.97

0.01

2.00
0.04
0.20
0.76
1.00

0.01
0.01
2.00
0.01
0.15
0.87
1.03

1.06

0.01
0.90
0.01

3.00
0.00
3.00

Upper Chegem Caldera

Footnotes: S.D. = 1σ standard deviation; n—number of analyses; n.d.—not detected.

0.91
0.01
0.01
1.06

3.03
0.01
3.04

Calculated on 8 O

Jabel Harmun (Hatrurim Complex)

0.86
0.01
0.01
1.00

0.11

3.00

3.00

Bellerberg (Holotype Locality)

Table 3. Cont.

Minerals 2018, 8, 308

Nb2 O5
SiO2
TiO2
ZrO2
Al2 O3Cr2 O3Fe2 O3
V2 O3
La2 O3Ce2 O3
CaO
FeO
MgO
MnO
SrO
ZnO
Total

Sample wt %

n=4
0.18
0.71
20.98
0.03
3.21
0.01
30.74
n.d.
n.d.
n.d.
41.64
0.51
0.30
0.38
0.55
n.d.
99.24

Mean
n = 19

Mean

0.17
0.53
19.55
0.19
4.27
0.12
31.74
n.d.
n.d.
n.d.
41.12
0.06
n.d.
0.09
1.78
n.d.
99.62

[B]
M7-184[A]

E-2011

0.17
0.71
18.81
0.12
3.65
0.01
34.34
n.d.
n.d.
n.d.
41.68
0.03
n.d.
0.22
0.41
n.d.
100.15

n=1

Mean

E-2-1

[C]

n.d.
0.60
18.59
0.35
4.85
0.45
32.27
n.d.
n.d.
n.d.
42.48
n.d.
0.12
0.05
0.20
0.07
100.03

n=2

Mean

H-201

[D]

n.d.
2.67
17.85
n.d.
4.65
n.d.
30.94
n.d.
n.d.
n.d.
42.76
n.d.
0.45
0.54
n.d.
n.d.
99.86

n=1

Mean

YV-411

[E]

n.d.
0.94
17.20
n.d.
5.20
n.d.
31.13
n.d.
n.d.
n.d.
42.79
n.d.
0.06
0.95
0.15
n.d.
98.42

n=2

Mean

YV-568

[F]

0.06
0.62
19.66
0.45
5.51
0.03
30.35
n.d.
0.13
0.17
42.73
0.01
0.07
n.d.
0.16
n.d.
99.95

n=2

Mean

42-17g[G]

Table 4. The chemical composition of sharyginite from different localities-literature data.
[H]

0.04
0.67
18.67
0.31
4.30
0.02
33.52
0.18
n.d.
n.d.
41.21
n.d.
n.d.
n.d.
1.41
n.d.
100.33

n=1

Mean

M-4

[I]

0.06
0.37
20.23
0.41
3.55
0.06
31.95
0.10
n.d.
n.d.
41.10
0.48
n.d.
n.d.
1.29
n.d.
99.60

n=4

Mean

M-4

Minerals 2018, 8, 308

44

45

2.00
0.04
0.34
0.62
1.00

0.01

3.02
0.01
0.98
0.01
0.01
0.98

2.95
0.07

[C]

2.00
0.05
0.25
0.69
0.99

1.05

0.85
0.01
0.03
0.03
0.02
2.01
0.05
0.29
0.67
1.01

0.01

<0.01

2.99
0.01
0.94
<0.01

2.97
0.02

E-2-1

3.00
0.01
1.05
<0.01

2.98
0.02

[B]
M7-184[A]

E-2011

[D]

[E]

0.01
<0.01
<0.01
1.98
0.04
0.38
0.58
1.00

0.88

0.92
0.01
0.02
1.02

2.00
0.17
0.36
0.47
1.00

0.04
0.03

1.05

3.00

3.00

Calculated on 8 O

YV-411

3.01

3.00
0.01

H-201

[F]

2.00
0.06
0.41
0.46
0.93

0.01

1.12

0.87

3.09

3.08
0.01

YV-568

[G]

1.96
0.04
0.43
0.53
1.00

<0.01
0.01

3.02
0.01
0.01
3.04
<0.01
0.97
0.01
<0.01
0.97

42-17g

[H]

2.01
0.04
0.34
0.62
1.00

2.99
<0.01
0.93
0.01
<0.01
1.06
0.01

2.94
0.05

M-4

[I]

1.99
0.03
0.28
0.69
1.00

3.01
<0.01
1.02
0.01
<0.01
0.92
0.01
0.03

2.96
0.05

M-4

Footnotes: S.D. = 1σ standard deviation; n—number of analyses; n.d.—not detected; [A–C]—metacarbonate xenoliths in alkali basalt, the Bellerberg volcano, Eastern Eifel, Germany[1,12]; [D]—larnite-ye’elimite rocks from the Hatrurim Basin, Israel [1,3]; [E–F]—larnite bearing rocks from the Hatrurim Basin, Israel [40]; [G]—metacarbonate rocks in the contact withparabasalt, burned dump of mine 42, Kopeisk, Chelyabinsk coal basin, Russia [1,11]; [H–I]—metacarbonate rocks, burned dump of the Kalinin mine, Donetsk coal basin, Ukraine [1,10].

Sum A
Nb5+
Ti4+
Zr4+
Cr3+
Fe3+
V3+
Fe2+
Mg2+
Mn2+
Zn2+
Sum B
Si4+
Al3+
Fe3+
Sum T

La2+ +Ce2+

Sr2+

Ca2+

SampleTable 4. Cont.

Minerals 2018, 8, 308

Minerals 2018, 8, 308

The empirical formula of the holotype sharyginite calculated on the basis of 8 O apfu
was Ca3,00 (Fe3+ 1.00 Ti4+ 0.86 Mn4+ 0.11 Zr0.01 Cr3+ 0.01 Mg0.01 )Σ2.00 (Fe3+ 0.76 Al0.20 Si0.04 )Σ1.00 O8 , which may
be simpliﬁed to Ca3 (Fe3+ Ti) Fe3+ O8 , as a reﬂecting end-member formula: Ca3 (TiFe)FeO8 or
Ca3 TiFe2 O8 . The holotype sharyginite is represented by a complex solid-solution, which contains
the following end members: 64% of sharyginite, 20% of shulamitite and 11% of Mn-analogue ofsharyginite. Other components like Ca3 (Zr4+ Fe3+ )Fe3+ O8 , Ca3 (Fe3+ Fe3+ )Si4+ O8 , Ca3 (Cr3+ Fe3+ )Si4+ O8
and Ca3 (Mg2+ Ti4+ )Si4+ O8 , were minor and their contents were less than 1–2%. The chemicalcomposition of sharyginite from Bellerberg metacarbonate xenoliths was also measured by Sharyginand Wirth [1,12]. The measurements were made for three different samples: E-2011, M7-184, and E-2-1,
which are presented in Table 4 as A, B and C, respectively. The differences between these data
and the holotype sharyginite were observed in Ti content, which varied between 0.9–1.1 apfu in
Sharygin’s analyses. In the holotype specimen, lower Ti content was related to Mn4+ substitution.
Moreover, the iron content in the tetrahedral site indicated that the holotype sharyginite was moreenriched in Fe3+ . Sharygin also reported small concentrations of Nb5+ and Sr2+ in his samples [12].
Sharyginite from Jabel Harmun was presented by a solid-solution with prevalent sharyginite
Ca3 TiFeFeO8 (62%) and subordinate Ca3 TiFeAlO8 (30%) and Ca3 Fe(Fe,Cr)SiO8 (8%) end members.
The mean formula (average of 19 analysis) was (Ca3.03 Sr0.01 )Σ3.04 (Fe3+ 1.06 Ti4+ 0.91 Zr0.01 Cr3+ 0.01 Mg0.01 )3+Σ2.00 (Fe 0.59 Al0.30 Si0.08 )Σ0.97 O8 . Vapnik et al. [40] described opaque minerals occurring within
larnite-bearing rocks from the Hatrurm Basin and published some chemical analyses of sharyginite,which were also included in Table 4 (E–F). Sharyginite from these rocks had a similar Ti content as
in the holotype sample from the Bellerberg, but relatively lower in comparison with sharyginitefrom Jabel Harmun. The increase in silicon content (~0.2 apfu) indicated that the iron content
in the tetrahedral position does not exceed 0.5 apfu. Despite this, sharyginite end-member
prevailed over shulamitite end-member. For example sample YV-411 (E in Table 4) consistedof following end-members: 47% sharyginite, 36% shulamitite, 12% Ca3 (Cr3+ Fe3+ )Si4+ O8 and 5%
Ca3 (Mg2+ Ti4+ )Si4+ O8 . Furthermore, chemical analysis for sharyginite from Hatrurim Basin were
performed and calculated by Sharygin et al. (D in Table 4) [1,3]. They are very similar incomparison to analysis from Jabel Harmun, mostly in Fe3+ and Ti contents, but contained less SiO2 inchemical composition.
The chemical composition of sharyginite from the Upper Chegem Volcanic Caldera corresponded
to the ideal composition. It contained ≈ 85% of sharyginite, 14% of shulamitite, and 1% of Sn
or Zr-bearing end members. The empirical formula yielded on four analysis was as follows:
Ca3.00 (Fe3+ 1.06 Ti4+ 0.90 Sn4+ 0.01 Zr0.01 Mn2+ 0.01 Mg0.01 )Σ2.00 (Fe3+ 0.87 Al0.15 Si0.01 )Σ1.03 O8 .
In literature, there are also chemical data of sharyginite from other metacarbonate rocks, collected
on a burned dump of mines in Ukraine [10] and Russia [11]. Sharyginite from Chelyabinsk (G inTable 4) and Donetsk (H-I in Table 4) exhibited similar content of main elements: Fe2 O3 , CaO, TiO2 andAl2 O3 , like in other localities. Differences were observed in impurities. Sharyginite from Chelyabinsk
burned dump contained rare earth elements (REE) such as La and Ce, which were below the detection
limit for other mentioned localities. We could observe that samples from Donetsk showed similarities
to the Bellerberg samples and in turn, samples from Chelyabinsk were more similar to these from
Israel, mainly in the context of tetrahedral position occupation by Al3+ and Fe3+ .
4.2. X-ray Crystallography
The measured powder data, as well as a calculated pattern, are reported in Table 5. The calculated
pattern is based on the structure model obtained from single-crystal data and is generated using
XPow [41]. Powder data were reﬁned with the Rietveld method using the semi-automatic modulein HighScore+ software [42]. Unit-cell parameters reﬁned from the powder data are as follows:a = 5.4262(4) Å, b = 11.1468(7) Å, c = 5.5308(3) Å, V = 334.5(3) Å3 , Z = 2.

46

Minerals 2018, 8, 308

Table 5. Measured and calculated X-ray powder diffraction data (d in Å) for sharyginite. The strongest
diffraction lines are given in bold.
Calculated from the Results ofSingle-crystal Structure Reﬁnement

Powder Data
h

k

l

I

dmeas.

dcalc.

I

d

0
0
0
0
1
0
1
0
0
2
1
0
1
1
0
2
2
2
1
2
0
1
2
0
1
0
3
1
3
0
2
3
2
0
4
2
3
4
1

1
2
1
2
0
3
2
3
0
0
3
4
0
4
3
3
3
0
5
1
6
4
2
2
6
3
2
3
3
6
6
4
3
0
0
6
3
0
9

0
0
1
1
1
0
1
1
2
0
1
1
2
1
2
0
1
2
1
2
0
2
2
3
1
3
1
3
1
2
0
1
3
4
0
2
3
2
1

9
5
3
3
8
5
4
6
32
27
100
7
4
7
3
3
2
36
4
3
19
2
3
3
2
4
2
18
12
8
8
2
3
3
3
11
4
2
3

11.09095.59914.93853.91603.8701
3.7127
3.18363.07832.76332.71192.67912.48742.45782.26122.18682.03861.93591.90801.8566
1.85111.82861.75541.68591.65071.63931.58001.5592
1.54501.40641.35591.34101.21931.2176
1.180611.16415.58204.95443.92813.87163.72143.18133.08712.76432.71182.68302.49152.46282.26412.2191
2.19162.0374
1.93581.9342
1.90741.86071.84671.82901.75001.67711.65151.6423
1.57981.56001.54361.5342
1.46321.41051.38211.35591.3415
1.21931.21731.1813

7
4
3
3
11
5
4
5
28
36
100
6
4
6
2
3
2
41
4
3
18
2
3
2
2
4
3
17
16
8
9
2
3
3
4
13
5
2
4

11.1500
5.57504.95273.92543.8712
3.71673.17983.08442.76402.71152.68112.48902.46262.26212.21792.19052.03651.93561.93231.90711.8583
1.84551.82851.74961.67531.65091.64191.5793
1.5596
1.5422
1.53291.46261.41011.38201.35571.3405
1.21901.2172
1.17994.3. Crystal Structure
The crystal structure was reﬁned with Jana2006 [43], starting from the structural model of thesynthetic compound Ca3 TiFe2 O8 reported by Rodriguez-Carvajal et al. [6]. As EPMA (electron probe
microanalysis) data indicated the presence of up to 3% of Al2 O3 , aluminium was introduced as anadditional species on the octahedral as well as on the tetrahedral position. Half of the octahedral site
had to be occupied by titanium. Neutral scattering factors and anisotropic displacement parameters
were utilized. Furthermore, inversion twinning and isotropic extinction were taken into account.
Results showed that an equal amount of twins were present (0.51(3):0.49(3)). Some other crystals
showed another type of twinning; a pseudo four-fold axis along b. This was evident in the violationof the h = 2n extinction condition of the a-glide plane. The reciprocal space layer hk0 showed weak
reﬂections at positions h = 1, 3, 5. However, these reﬂections are slightly displaced with respect to47

Minerals 2018, 8, 308

a*. Furthermore, reﬂections with h = 4 and h = 6, showed splitting along a*. This was caused by the90◦ -rotation twinning along b, as this superimposed the 0kl layer onto the layer hk0. Figure 7 showsa part of the reconstructed hk0 layer of a crystal affected by this type of twinning. The reﬂections
belonging to the 90◦ rotation twin are marked with arrows. With increasing index h, their distancefrom integer h positions increased.

Figure 7. Reconstruction of the reciprocal space layer hk0 of a crystal affected by 90◦ rotation twinning.
Arrows point to reﬂections belonging to the smaller twin individual. Synchrotron diffraction data.
The curved white bands are caused by gaps between the elements of the Pilatus 2M-F detector andcontain no measured data.

The ﬁnal stage of the reﬁnement included 74 parameters. According to the reﬁnement,
the octahedral site is occupied by 3.5% Al, the tetrahedral site exhibits 25% of Al. This corresponds to
a composition of x = 0.32 (in Ca3 TiFe2-x Alx O8 ). In four additional reﬁnements (using data from further
crystals), x was found to vary between 0.26 and 0.38.
Finally, atoms were renumbered, and a cell transformation and origin shift were applied to closely
match the model reported for shulamitite [1].
Atom coordinates (x,y,z), occupancies, and equivalent isotropic displacement parameters(Uiso , Å2 ) for sharyginite are given in Table 6. Anisotropic displacement parameters (Å2 ) weresummarized in Table 7. The main interatomic distances (Å) are reported in Table 8.

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Minerals 2018, 8, 308

Table 6. Atom coordinates (x,y,z), occupancies and equivalent isotropic displacement parameters(Uiso , Å2 ) for sharyginite.
Site

Atom

x

y

z

Uiso

Occupancy

Ti1
Fe1
Al1
Fe2
Al2
Ca1
Ca2
O1
O2
O3
O4
O5

Ti
Fe
Al
Fe
Al
Ca
Ca
O
O
O
O
O

0.25010(10)0.25010(10)0.25010(10)0.19825(16)0.19825(16)−0.26824(16)0.2676(3)0.2758(5)0.3553(6)0.0161(4)0.2444(6)−0.0146(5)0.16862(4)0.16862(4)0.16862(4)0.5
0.5
0.31319(6)00.35744(18)0.50.15042(14)00.20160(16)0.23966(9)0.23966(9)0.23966(9)0.18318(13)0.18318(13)0.27602(11)−0.26624(16)0.3210(3)−0.1222(5)0.5135(5)0.1700(5)0.0157(5)0.00672(15)0.00672(15)0.00672(15)0.00750(19)0.00750(19)0.01048(18)0.0141(3)0.0136(6)0.0127(8)0.0105(5)0.0132(8)0.0127(6)0.5
0.465(5)0.035(5)0.751(7)0.249(7)
1
1
1
1
1
1
1

Table 7. Anisotropic displacement parameters (Å2 ) for sharyginite.
Site

Atom

U11

U22

U33

U23

U13

U12

Ti1
Fe1
Al1
Fe2
Al2
Ca1
Ca2
O1
O2
O3
O4
O5

Ti
Fe
Al
Fe
Al
Ca
Ca
O
O
O
O
O

0.0055(3)0.0055(3)0.0055(3)0.0071(4)0.0071(4)0.0131(4)0.0166(6)0.0176(12)0.0128(15)0.0105(9)0.0159(17)0.0134(11)0.0109(2)0.0109(2)0.0109(2)0.0094(3)0.0094(3)0.0115(3)0.0142(4)0.0131(9)0.0130(13)0.0129(7)0.0122(12)0.0177(8)0.0038(3)0.0038(3)0.0038(3)0.0060(4)0.0060(4)0.0069(3)0.0116(4)0.0099(9)0.0124(14)0.0080(9)0.0115(13)0.0071(9)−0.0001(3)−0.0001(3)−0.0001(3)00−0.0012(4)00.0016(8)00.0001(8)00.0007(8)−0.00028(18)−0.00028(18)−0.00028(18)0.0003(3)0.0003(3)0.0015(3)−0.0004(4)−0.0004(9)0.0032(13)0.0014(8)−0.0009(13)−0.0022(8)0.00037(14)0.00037(14)0.00037(14)00−0.00057(18)00.0011(7)0−0.0008(8)0−0.0005(9)The crystal structure of synthetic sharyginite, Ca3 TiFe2 O8 , has been described before [6]. It
is closely related to the structure of shulamitite [1] and consists of double layers of corner-sharing(Ti, Fe3+ )O6 octahedra which are separated by single layers of (Fe3+ O4 ) tetrahedra, which form zweier
single-chains (Figure 8). These chains are characteristic for sharyginite, shulamitite and the structurally
related brownmillerite.
The sharyginite structure exhibited one octahedrally coordinated site, which hosted titanium andiron, as well as minor amounts of aluminium. The occupation factor of titanium was 0.5, whichwas
imposed by the stoichiometry. The tetrahedral site was occupied by iron ( 34 ) and aluminium ( 14 ).
The independent calcium cations were located between the two octahedral layers (Ca2) andbetween octahedral and tetrahedral layers (Ca1).

49

Minerals 2018, 8, 308

Table 8. Selected interatomic distances (Å) for sharyginite.
Atom

Atom

distance (Å)

Ti1

O1
O3
O3
O4
O5
O5

2.157(4)1.985(4)1.996(3)1.919(3)1.931(3)1.938(3)1.988
1.812(3)1.891(3)1.890(3)1.851
2.481(3)2.549(3)2.887(4)2.640(4)2.415(3)2.892(3)2.643
2.534(3)2.294(3)2.347(3)2.719(3)2.453(3)2.348(3)2.436(3)
2.448

Mean
Fe2

Mean
Ca2

Mean
Ca1

O1 × 2O2O2
O3 × 2O3 × 2O4O4O4O5 × 2
O1
O1
O2
O3
O3
O5
O5

Mean

Figure 8. Projections along (001) of the crystal structures of sharyginite (a) and shulamitite (b).
Calcium atoms are shown as orange spheres, 0.5Fe3+ /0.5TiO6 octahedra are in grey, green, and blue
spheres, and bonds corresponding to the tetrahedra centred mainly by Fe3+ and Al3+ ions, respectively.
In shulamitite (b), the tetrahedral chains are disordered and both possible conﬁgurations areshow simultaneously.

50

Minerals 2018, 8, 308

4.4. Raman Spectroscopy
In the Raman spectrum of the holotype sharyginite the following main bands were noted (cm−1 )(Figure 9a): 114, 145, 190, 248, 307, 389, 486, 560, 710, 752, 785 and 1415, 1475 (overtone). Bands in
the OH region were not observed. The spectrum of sharyginite was dominated by an intense Raman
band at 710 cm−1 which was attributed to ν1 (Fe3+ O4 ) tetrahedra symmetric stretching vibrations. Twoshoulders observed on this band with relative lower intensity at 752 and 785 cm−1 were associated with
ν1 (AlO4 ) and ν3 (Fe3+ O4 ) modes, respectively. Bands at 486 and 560 cm−1 were related to tetrahedral
bending vibrations of (Fe3+ O4 ) group. The low wavenumber region below 400 cm−1 was ascribed tothe polyhedral CaO8 and octahedral (Fe3+ , Ti)O6 vibrations [1]. Raman spectra of sharyginite fromthe other two localities (Figure 9b,c) were similar to the sharyginite spectrum from the Bellerberg.
The Raman spectrum was also similar to that of shulamitite [1], with the main distinction in theposition of the main band.

Figure 9. Raman spectra of sharyginite from the Bellerberg volcano (a), Jabel Harmun (b) and Upper
Chegem Volcanic Caldera (c) localities.

51

Minerals 2018, 8, 308

5. Discussion
The difference between sharyginite, Ca3 TiFe2 O8 and shulamitite, Ca3 TiFeAlO8 , is related
to the occupancy of the tetrahedral site by Fe3+ and Al, respectively and the space groupsymmetry. The holotype sharyginite from the Bellerberg volcano is close to Ca3 TiFe(Fe0.8 Al0.2 )O8 ,whereas sharyginite compositions from the Upper Chegem Caldera and the Hatrurim Complex
show greater diversity (Tables 3 and 4). The iron content in the holotype specimen, obtained fromthe reﬁnement calculation, may be slightly overestimated, due to the Mn4+ substitution (up to
2.3%, Table 3). Comparing results of chemical composition from the different localities and otherauthors data we also note the presence of insigniﬁcant impurities such as Zr, Sr, Nb, Sn, Mn and REE.
The wide variation of trivalent iron and aluminium in the chemical composition allows us to claimthat a continuous solid solution exists between sharyginite and shulamitite. This solid solution may becompared with the brownmillerite, Ca2 (Fe3+ ,Al)O5 —srebrodolskite, Ca2 Fe3+ 2 O5 series. All mentioned
minerals belong to anion-deﬁcient, perovskites and show differences in TO4 tetrahedra orientation
between end-members which are related to the adopted space group [2].
At a ﬁrst glance, the main structural differences between shulamitite and sharyginite appear to be
in tetrahedral layers: shulamitite exhibits disorder of two possible conﬁgurations of the tetrahedralchain, whereas in sharyginite, only one type of chain exists, which also results in the structure
being acentric. Looking more closely, another striking difference can be noted in the octahedral layers:
Figure 10 shows a view perpendicular to the octahedral layers of both structures. In shulamitite (Pmma),
the oxygen atoms O3 and O5 reside on the two-fold axis 2[010]. As a consequence, the octahedra
cannot be rotated around the b-axis, as their O3-O5 edges are constrained to be parallel to a and c(Figure 10a). The lower symmetry in sharyginite (P21 ma) does not restrict O3 and O5 on a special
position, thus allowing the octahedra to rotate around b. The rotation, which can be seen in Figure 10b,is 3.15◦ . The reason for this rotation is most likely an adaption to the conﬁguration of the tetrahedralchains. As discussed earlier [44], the tetrahedral chains exhibit a dipole moment. In shulamitite,
all chains are disordered, resulting in a net dipole moment of zero. Only one type of chain occurs insharyginite, therefore dipole moments have to be compensated by distortions of the octahedral layers
and displacement of Ca cations.
The occurrence of a morphotropic phase boundary (depending on the Al-content) between
sharyginite and shulamitite is comparable to the one between srebrodolskite and brownmillerite [45]and will be the subject of another study.

Figure 10. The arrangement of the octahedral layers in shulamitite (a) and sharyginite (b). Rotation of
the octahedra is restricted by symmetry in shulamitite, as O3 and O5 reside on special positions.
The lower symmetry of sharyginite allows the octahedra to rotate.

52

Minerals 2018, 8, 308

In metacarbonate xenoliths from the Bellerberg locality, the occurrence of sharyginite in association
with perovskite is common. The phase relationship indicates that sharyginite is a later phase andovergrows the perovskite or even replaces it, which is very well presented in Figure 4. We suggest thatnew mineral sharyginite is formed at high-temperature conditions. According to the CaTiO3 -Ca2 Fe2 O5diagram [46], the stoichiometric Ca3 TiFe2 O8 phase and disordered cubic Fe-perovskite are stable attemperatures above 1160◦ C. Under this temperature in the composition ﬁeld 0.5 < x < 0.67, Ca3 TiFe2 O8
coexists with Ca4 Ti2 Fe2 O11 phase. In turn in 0.67 < x < 1.00, two ordered phases: Ca3 TiFe2 O8 and
Ca2 Fe2 O5 coexist under 1400◦ C [3,46]. Moreover, for some larnite rocks of the Hatrurim Complex,
where sharyginite was also identiﬁed, the minimum temperature of formation was estimated from
Fe-perovskite-Ca3 TiFe2 O8 mineral paragenesis. This temperature is in the range of 1170–1200◦ C [3].
Synthetic data also indicate the formation of Ca3 TiFe2 O8 phase at high temperature (>1000◦ C) [4,6].
Furthermore, the coexistence of Fe-perovskite and Ca3 TiFe2 O8 phases (sharyginite) may be used for thetemperature evaluation of mineral associations in pyrometamorphic systems. However, it is necessary
to take into account the speciﬁc composition of Fe-perovskite and the modal phase relationship in theassociation and to be sure that the phases were crystallized under conditions close to equilibrium [10].
Supplementary Materials: The sharyginite_cif.ﬁle is available online at http://www.mdpi.com/2075-163X/8/7/
308/s1.
Author Contributions: R.J., H.K., B.K., I.G. and E.G. wrote the paper. R.J. performed the petrological investigation,measurements of chemical composition from different localities, and Raman studies. H.K. and B.K. performed
SC-XRD investigation and reﬁned the sharyginite structure. J.W. helped in the single-crystal investigation usingsynchrotron radiation. L.P. carried out optical studies. T.K. performed X-ray powder diffraction data of sharyginite.
L.J. helped with chemical measurements as an operator of microprobe analyzer. B.T. collected the samples.
Funding: The investigations were partially supported by the National Science Centre (NCN) of Poland, Grant
No. 2016/23/N/ST10/00142.
Acknowledgments: The authors would like to thank Jakub Kaminski for technical assistance at the X06DA
beamline, Klaus Zöll and Daniela Schmidmair for assistance during the synchrotron experiments, and ChristianPott for helping with German literature translation. The authors also thank the anonymous reviewers for theiruseful and constructive comments which helped to improve a previous version of the manuscript.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Zadov, A.E.; Dzierżanowski, P.; Pertsev, N.N.; et al. Pavlovskyite Ca8 (SiO4 )2 (Si3 O10 ): A new mineralof altered silicate-carbonate xenoliths from the two Russian type localities, Birkhin massif, Baikal Lake area
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Dzierżanowski, P.; Zadov, A.E.; et al. Edgrewite Ca9 (SiO4 )4 F2 -hydroxyledgrewite Ca9 (SiO4 )4 (OH)2 , a newseries of calcium humite-group minerals from altered xenoliths in the ignimbrite of Upper Chegem caldera,
Northern Caucasus, Kabardino-Balkaria, Russia. Am. Miner. 2012, 97, 1998–2006. [CrossRef]
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Russia: Occurrence, crystal structure, and new data on the mineral assemblages. Can. Miner. 2015,
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Ca12 Al14 O32 [4 F2 ], and ﬂuorkyuygenite, Ca12 Al14 O32 [(H2 O)4 F2 ], two new minerals from pyrometamorphic
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Dzierżanowski, P. New minerals with a modular structure derived from hatrurite from the pyrometamorphic
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minerals
ArticleNöggerathite-(Ce), (Ce,Ca)2Zr2(Nb,Ti)(Ti,Nb)2Fe2+O14,
a New Zirconolite-Related Mineral from the Eifel
Volcanic Region, Germany
Nikita V. Chukanov 1, *, Natalia V. Zubkova 2 , Sergey N. Britvin 3,4 , Igor V. Pekov 2,5 ,Marina F. Vigasina 2 , Christof Schäfer 6 , Bernd Ternes 7 , Willi Schüller 8 , Yury S. Polekhovsky 3 ,Vera N. Ermolaeva 9 and Dmitry Yu. Pushcharovsky 2
1
2

3
4
5
6
7
8
9

*

Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka,
142432 Moscow, Russia
Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia;
n.v.zubkova@gmail.com (N.V.Z.); igorpekov@mail.ru (I.V.P.); vigasina55@mail.ru (M.F.V.);
dmitp@geol.msu.ru (D.Y.P.)
Institute of Earth Sciences, St Petersburg State University, Universitetskaya Nab. 7/9,
199034 St Petersburg, Russia; sbritvin@gmail.com (S.N.B.); yury1947@mail.ru (Y.S.P.)
Nanomaterials Research Center, Kola Science Centre, Russian Academy of Sciences, Fersman str. 14,
184209 Apatity, Murmansk, Russia
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
Kosygin str. 19, 119991 Moscow, Russia
Gustav Stresemann-Strasse 34, 74257 Untereisesheim, Germany; mspech612@gmail.com
Bahnhofstrasse 45, 56727 Mayen, Germany; ternes-mayen@web.de
Im Straußenpesch 22, 53518 Adenau, Germany; maschwisch@web.de
Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka,
142432 Moscow, Russia; cvera@mail.ru
Correspondence: chukanov@icp.ac.ru; Tel.: +7-4965221556

Received: 7 September 2018; Accepted: 3 October 2018; Published: 12 October 2018

Abstract: The new mineral nöggerathite-(Ce) was discovered in a sanidinite volcanic ejectum from theLaach Lake (Laacher See) paleovolcano in the Eifel region, Rhineland-Palatinate, Germany. Associatedminerals are sanidine, dark mica, magnetite, baddeleyite, nosean, and a chevkinite-group mineral.
Nöggerathite-(Ce) has a color that ranges from brown to deep brownish red, with adamantine luster;thel streak is brownish red. It occurs in cavities of sanidinite and forms long prismatic crystals
measuring up to 0.02 × 0.03 × 1.0 mm, with twins and random intergrowths. Its density calculatedusing the empirical formula is 5.332 g/cm3 . The Vickers hardness number (VHN) is 615 kgf/mm2 ,which corresponds to a Mohs’ hardness of 51⁄2. The mean refractive index calculated using the
Gladstone–Dale equation is 2.267. The Raman spectrum shows the absence of hydrogen-bearing groups.
The chemical composition (electron microprobe holotype/cotype in wt %) is as follows: CaO 5.45/5.29,
MnO 4.19/4.16, FeO 7.63/6.62, Al2 O3 0.27/0.59, Y2 O3 0.00/0.90, La2 O3 3.17/3.64, Ce2 O3 11.48/11.22,
Pr2 O3 1.04/0.92, Nd2 O3 2.18/2.46, ThO2 2.32/1.98, TiO2 17.78/18.69, ZrO2 27.01/27.69, Nb2 O5
17.04/15.77, total 99.59/99.82, respectively. The empirical formulae based on 14 O atoms per formula
unit (apfu) are: (Ce0.59 La0.165 Nd0.11 Pr0.05 )Σ0.915 Ca0.82 Th0.07 Mn0.50 Fe0.90 Al0.045 Zr1.86 Ti1.88 Nb1.07 O14(holotype), and (Ce0.57 La0.19 Nd0.12 Pr0.05 Y0.06 )Σ0.99 Ca0.79 Th0.06 Mn0.49 Fe0.77 Al0.10 Zr1.89 Ti1.96 Nb1.00 O14(cotype). The simplified formula is (Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 . Nöggerathite-(Ce) is
orthorhombic, of the space group Cmca. The unit cell parameters are: a = 7.2985(3), b = 14.1454(4),c = 10.1607(4) Å, and V = 1048.99(7) Å3 . The crystal structure was solved using single-crystal X-ray
diffraction data. Nöggerathite-(Ce) is an analogue of zirconolite-3O, ideally CaZrTi2 O7 , with Nb
dominant over Ti in one of two octahedral sites and REE dominant over Ca in the eight-fold coordinatedsite. The strongest lines of the powder X-ray diffraction pattern (d, Å (I, %) (hkl)) are: 2.963 (91) (202),
2.903 (100) (042), 2.540 (39) (004), 1.823 (15) (400), 1.796 (51) (244), 1.543 (20) (442), and 1.519 (16) (282),

Minerals 2018, 8, 449; doi:10.3390/min8100449

57

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Minerals 2018, 8, 449

respectively. The type material is deposited in the collections of the Fersman Mineralogical Museum of
the Russian Academy of Sciences, Moscow, Russia (registration number 5123/1).
Keywords: nöggerathite-(Ce); new mineral; zirconolite; laachite; sanidinite; crystal structure; alkaline
volcanic rock; Laacher See; Eifel

1. Introduction
Zirconolite-related Ca-REE-Zr-Ti-Nb oxides have been described in numerous publications as advanced
materials suitable for the immobilization of actinides, which are components of high-level radioactive
waste [1–5]. Natural zirconolites and related minerals (zirconolite-3O (Ca,REE)2 Zr2 (Ti,Nb)3 FeO14 ,zirconolite-3T (Ca,REE)2 Zr2 (Ti,Nb)3 FeO14 , zirconolite-2M (Ca,REE)2 Zr2 (Ti,Nb)3 FeO14 , laachite
Ca2 Zr2 Nb2 TiFeO14 , stefanweissite (Ca,REE)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 (IMA 2018-020) and thehere-described nöggerathite-(Ce)) are characterized by a wide compositional diversity and can beconsidered as prototypes of such materials. These minerals usually contain uranium and thorium,
the total content of which can reach 15–20 wt % [6]. As a result of exposure to alpha radiation, most such
samples are X-ray amorphous, metamict. Some exceptions are crystalline samples of zirconolite-typeminerals from young volcanic rocks [7–10].
This paper describes nöggerathite-(Ce), a new zirconolite-related mineral species from theLaach Lake area situated in the Eifel paleovolcanic region, Germany. This mineral is non-metamict
because of the very young geological age of the mother rock [11,12].
The root name of the new mineral is given in honor of Johann Jacob Nöggerath (1788–1877),
a prominent German mineralogist and geologist. From 1818 Nöggerath was a professor of mineralogy
and geology at the University of Bonn. Among his publications is a geological description of the
Laach Lake (Laacher See) paleovolcanic region. The Levinson’s modiﬁer -(Ce) in the mineral namereﬂects the predominance of Ce among rare earth elements, which together are dominant in one
structure position. The mineral and its name have been approved by the Commission on New
Minerals, Nomenclature and Classiﬁcation of the International Mineralogical Associatiob (IMA number
2017-107). The type material is deposited in the collections of the Fersman Mineralogical Museum of
the Russian Academy of Sciences, Moscow, Russia (registration number 5123/1).
2. Materials and Methods
The new mineral was found in the In den Dellen (Zieglowski) pumice quarry, 1.5 km
to the notrheast of Mendig, in the Laach Lake (Laacher See) paleovolcano of the Eifel region,
Rhineland-Palatinate, Germany. Two specimens have been investigated and considered as the holotype
and the cotype, which are fragments of the same sanidinite volcanic ejectum. Associated minerals are
sanidine, dark mica, magnetite, baddeleyite, nosean, and a chevkinite-group mineral.
The origin of sanidinites of the Laacher See area has been discussed earlier [9,12–14]. These rocks
are derivatives of different foyaite magmas which are comagmatic with haüyne-bearing rocks (haüyne
foyaite, haüyne syenite, haüyne monzonite, etc.), or with noseane–cancrinite–nepheline syenites.
Sanidinites of the latter type are enriched in rare element (Nb, Zr, REE, U, Th) accessory minerals.
These rocks are cogenetic with the phonolitic host magma, and the crystallization took place in an
intrusive syenite–carbonatite complex at temperatures below 700 ◦ C in the host rock surrounding
the top of the magma chamber 5000 to 20,000 years prior to the eruption of the magma chamber [12].
Most probably, nöggerathite-(Ce) and associated minerals forming crystals on the walls of cavities insanidinite crystallized from above-critical ﬂuid.
Chemical analyses (ﬁve for the holotype and three for the cotype) were carried out using an
Oxford INCA Wave 700 electron microprobe (WDS mode, 20 kV, 600 pA, 300-nm beam diameter,

58

Minerals 2018, 8, 449

Oxford Instruments plc, London, UK) housed at the Institute of Experimental Mineralogy RAS.
The counting time per peak was 100 s.
The Raman spectrum of a randomly oriented nöggerathite-(Ce) crystal was obtained on the cotype
sample using an EnSpectr R532 spectrometer based on an OLYMPUS CX 41 microscope coupled with adiode laser (λ = 532 nm) at room temperature (Enhanced Spectrometry, San Jose, USA). The spectrum
was recorded in the range from 100 to 4000 cm−1 with a diffraction grating (1800 mm−1 ) and spectral
resolution of about 6 cm−1 . The output power of the laser beam was about 9 mW. The diameter of the
focal spot on the sample was less than 10 μm. The backscattered Raman signal was collected with a
40× objective; thel signal acquisition time for a single scan of the spectral range was 2 s, and the signal
was averaged over 100 scans.
The Raman spectrum of an oriented laachite crystal used for comparison was obtained with a
HORIBA instrument based on the OLYMPUS BX 41/51 microscope (HORIBA Jobin Yvon, Bensheim
Germany) with a diode laser (λ = 532 nm) at room temperature. The power of the laser beam at the
sample was about 1.5 mW. The spectrum was recorded in a range from 100 to 3800 cm−1 , with adiffraction grating (1800 mm−1 ) and spectral resolution of about 1 cm−1 . The diameter of the focal spot
on the sample was less than 7 μm. The backscattered Raman signal was collected with 50× objective;
signal acquisition time for a single scan of the spectral range was 10 s and the signal was averaged
over ﬁve scans.
Both Raman spectrometers were housed at Moscow State University.
Maximal and minimal reﬂectance values (Rmax /Rmin ) were measured in air using a MSF-21
micro-spectrophotometer (LOMO company, St. Petersburg, Russia) with a monochromator slit widthof 0.4 mm and beam diameter of 0.1 mm. SiC (reﬂection standard 474251, number 545, Jena, Germany)
was used as a standard. The spectrophotometer was housed at St. Petersburg State University.
Powder X-ray diffraction data were collected using a Rigaku RAXIS Rapid II diffractometer(Rigaku Corporation, Tokyo, Japan) with a curved image plate detector and rotating anode in
Debye–Scherrer geometry, with an accelerating voltage of 40 kV, current of 15 mA, and exposure
time 15 min. The distance between sample and detector was 127.4 mm. Data processing was carriedout using osc2xrd software [15]. The diffractometer was housed at St. Petersburg State University.
The single-crystal X-ray diffraction experiment was carried out using a Bruker Kappa APEX DUO
CCD diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The diffractometer was housed at
Moscow State University. Experimental details are given in Table 1.
Table 1. Crystal data, data collection information and structure reﬁnement details for the holotype
specimen of nöggerathite-(Ce).
Characteristics

Data and Methods

Crystal sizes, mm
Temperature, K
Radiation and wavelength, ÅF000θ range for data collection, ◦h, k, l ranges
Reﬂections collected
Independent reﬂections
Independent reﬂections with I > 2σ(I)Data reduction
Structure solution
Reﬁnement method
Weighting coefﬁcients a, b
Number of reﬁned parameters
Final R indices (with I > 2σ(I))
R indices (with all data)
GoF
Largest diffraction peak and hole, e/Å30.01 × 0.01 × 0.10
293(2)
MoKα; 0.71073
1530
2.88–26.98–9 → 7, –18 → 18, –12 → 124962
617 (Rint = 0.0215)
574
Bruker SAINT
Direct methods
Full-matrix least-squares on F2
0.0251, 7.1886
71
R1 = 0.0198, wR2 = 0.0518
R1 = 0.0224, wR2 = 0.0550
1.161
1.53 and –0.70

59Minerals 2018, 8, 449

3. Results
3.1. General Appearance and Mechanical Properties
Nöggerathite-(Ce) forms prismatic crystals measuring up to 0.1 × 0.1 × 1.0 mm, elongatedalong (001), and simple twins, isolated or combined in random aggregates (Figure 1a,b) in cavities
in sanidinite. The main observed crystal forms are pinacoids {100} and {010}, as well as prisms {110}and {120}. The other forms are {111} and minor {001}. In most cases, the twinning plane is (130);
thel angle between the c axes of the twin components is 65◦ . The exception is a growth (possibly atwin) with the angle between the c axes of the twin components of 90◦ (Figure 1c). Some crystals ofnöggerathite-(Ce) are embedded in sanidine.
The new mineral is translucent to transparent, with a color ranging from brown to very dark
reddish brown, almost black, with adamantine luster. The streak is brownish red.
Nöggerathite-(Ce) is brittle with uneven fractures; no cleavage was observed. Hardness,
as determined by the micro-indentation method (Vickers hardness number (VHN) load of 20 g),
is equal to 615 kgf/mm2 which corresponds to a Mohs’ hardness of 5 12 . The density calculated using
the empirical formula is 5.332 g/cm3 .

Figure 1. (a) Aggregates of brown nöggerathite-(Ce) crystals on sanidine. Photographer: Stefan Wolfsried.
Field width: 4 mm; (b) Nöggerathite-(Ce) crystal on sanidine. Photographer: Marko Burkhardt.
Field width: 0.5 mm; (c) Twin of nöggerathite-(Ce) on sanidine. Photographer: Marko Burkhardt.
Field width: 1 mm.

60Minerals 2018, 8, 449

3.2. Raman Spectroscopy
The Raman spectrum of nöggerathite-(Ce) (Figure 2) shows the absence of absorption bands of
H2 O molecules, OH groups, and CO3 2– anions. The bands in the range of 400–800 cm−1 correspondto (Ti,Nb,Zr)–O-stretching vibrations, and the bands in the range of 100–400 cm−1 are due to(REE,Ca)–O-stretching and O–(Ti,Nb,Zr)–O bending vibrations. Broad features above 900 cm−1in the Raman spectrum of nöggerathite-(Ce) correspond to luminescence due to high amounts of REE.
In Raman spectra of laachite (Figure 3), which is a mineral related to nöggerathite-(Ce)but is characterized by much lower REE:Ca and Ti:Nb ratios, the bands of (REE,Ca)–O- and(Ti,Nb,Zr)–O-stretching vibrations are shifted towards higher and lower values, respectively.
This regularity is in accordance with mean masses of corresponding groups of atoms.

Figure 2. Raman spectrum of nöggerathite-(Ce) (a.u. = arbitrary units).

Figure 3. Raman spectrum of laachite Ca2 Zr2 Nb2 TiFeO14 obtained with the polarization of the laser
beam parallel to the a axis of the crystal [9].

61Minerals 2018, 8, 449

3.3. Optical Properties
In reﬂected light, nöggerathite-(Ce) is optically anisotropic, with ΔR589 = 1.27%. The color is light
grey, with reddish brown internal reﬂections. The reﬂectance values in the visible range are given in
Table 2. Mean refractive index calculated from the Gladstone–Dale equation is 2.267.
Table 2. Reﬂectance values (Rmax /Rmin ) for nöggerathite-(Ce). Reﬂectance values for four wavelengths
recommended by the Commission on Ore Microscopy of the International Mineralogical Association
are given in bold type.
Wavelength, nm

R1

R2

400
420
440
460
470
480
500
520
540
546
560
580
589
600
620
640
650
660
680
700

17.3
16.8
16.4
16.0
15.8
15.6
15.3
15.3
15.0
15.0
15.0
14.9
14.9
14.8
14.8
14.8
14.8
14.8
14.7
14.7

16.8
16.4
16.0
15.5
15.3
15.2
15.0
14.8
14.7
14.7
14.6
14.6
14.5
14.5
14.5
14.4
14.4
14.4
14.4
14.3

3.4. Chemical Composition
Chemical data for nöggerathite-(Ce) are given in Table 3. Contents of other elements with atomic
numbers >8 are below detection limits. Based on structural data (see below) and by analogy withlaachite [9], iron and manganese are considered as Fe2+ and Mn2+ , respectively.
Table 3. Chemical data for nöggerathite-(Ce). Upper and lower values for each constituent correspondto the holotype and the cotype, respectively.
Constituent wt %
CaO
MnO
FeO
Al2 O3
Y2 O3
La2 O3Ce2 O3Pr2 O3

5.45
5.29
4.19
4.16
7.63
6.62
0.27
0.59
0.00
0.90
3.17
3.64
11.48
11.22
1.04
0.92

Range

Standard
Deviation

5.27–5.55
5.12–5.39
4.07–4.32
4.06–4.34
7.46–7.79
6.23–6.83
0.18–0.38
0.48–0.78–
0.61–0.99
3.05–3.28
3.47–3.84
11.27–11.73
10.95–11.69
0.89–1.24
0.90–0.97

0.10
0.34
0.09
0.13
0.14
0.28
0.07
0.14
–
0.16
0.10
0.16
0.19
0.33
0.12
0.03

62

Standard
Used

X-ray Line
Measured

WollastoniteK

MnTiO3K

Fe

K

Albite

K

YPO4

L

LaPO4

L

CePO4

L

PrPO4

L

Minerals 2018, 8, 449

Table 3. Cont.
Range

Standard
Deviation

2.10–2.34
2.28–2.81
2.11–2.50
1.79–2.17
17.45–18.12
18.49–18.90
26.82–27.26
27.51–27.86
16.72–17.37
15.53–15.99

0.08
0.25
0.15
0.16
0.27
0.16
0.19
0.11
0.28
0.19

-

-

Constituent wt %Nd2 O3
ThO2
TiO2
ZrO2
Nb2 O5
Total

2.18
2.46
2.32
1.98
17.78
18.69
27.01
27.69
17.04
15.77
99.59
99.82

Standard
Used

X-ray Line
Measured

NdPO4

L

ThO2

M

TiO2

M

ZrO2

L

LiNbO3L--

The empirical formulae (based on 14 O apfu) are: (Ce0.59 La0.165 Nd0.11 Pr0.05 )Σ0.915 Ca0.82 Th0.07 Mn0.50
Fe0.90 Al0.045 Zr1.86 Ti1.88 Nb1.07 O14 (holotype) and (Ce0.57 La0.19 Nd0.12 Pr0.05 Y0.06 )Σ0.99 Ca0.79 Th0.06 Mn0.49
Fe0.77 Al0.10 Zr1.89 Ti1.96 Nb1.00 O14 (cotype).
The simpliﬁed formula is (Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 .
3.5. X-ray Diffraction Data and Crystal Structure
Powder X-ray diffraction data are presented in Table 4. Diffraction peaks are readily indexed in
the orthorhombic unit cell, space group Cmca. The unit cell parameters calculated from the powder
data are: a = 7.296(1), b = 14.147(2), c = 10.161(1) Å, and V = 1048.9(2) Å3 .
Table 4. Powder X-ray diffraction data for the holotype specimen of nöggerathite-(Ce).
Iobs , %dobs , Å

Icalc , %dcalc , Åhkl

Iobs , %dobs , Å

Icalc , %dcalc , Å

h

k

l

4
2
3
5
1
2
10
2
3
2
91
100
2
2
39
1
1
5
3
1
3
1
1
2
1
1
1
3
-

7.068
5.806
5.463
5.085
4.131
4.001
3.689
3.536
3.343
3.238
2.963
2.903
2.731
2.575
2.540
2.393
2.367
2.343
2.298
2.270
2.166
2.139
2.113
2.082
2.062
2.033
2.001
1.956
-

2
1
1
1
1
2
8
1
2
1
100
93
1
1
14
27
1
1
5
1
3
1
2
1
1
1
1
1
1
2
1

7.074
5.805
5.466
5.081
4.126
3.999
3.690
3.537
3.340
3.242
2.963
2.903
2.733
2.574
2.540
2.539
2.391
2.365
2.342
2.341
2.297
2.271
2.168
2.138
2.114
2.081
2.063
2.032
2.000
1.957
1.953

0
0
1
0
0
1
1
0
0
2
2
0
2
1
0
2
0
1
2
1
0
2
3
1
3
1
0
2
2
3
0

2
2
1
0
2
1
3
4
4
2
0
4
2
3
0
4
2
1
2
5
6
4
1
3
3
5
4
4
2
1
2

0
1
1
2
2
2
1
0
1
0
2
2
2
3
4
0
4
4
3
2
1
2
2
4
1
3
4
3
4
3
5

2
1
1
1
4
1
2
1
2
1
20
10
16
1
1
6
6
1
1
3
2
1
1

1.743
1.735
1.721
1.710
1.690
1.667
1.646
1.600
1.571
1.561
1.543
1.532
1.519
1.500
1.494
1.482
1.451
1.441
1.427
1.413
1.407
1.400
1.381

1
1
1
1
1
3
1
1
1
1
1
1
1
1
20
2
10
11
17
1
1
6
1
6
1
1
1
2
1
1
1

1.743
1.742
1.734
1.722
1.709
1.688
1.668
1.647
1.646
1.639
1.601
1.572
1.568
1.562
1.544
1.536
1.536
1.527
1.519
1.501
1.492
1.482
1.481
1.451
1.440
1.428
1.425
1.413
1.406
1.399
1.381

3
0
3
2
2
1
4
0
3
1
4
2
0
2
4
3
2
0
2
2
3
4
3
0
2
4
1
3
1
1
5

1
8
5
2
6
7
2
2
3
1
4
8
8
6
4
7
0
4
8
2
5
0
3
8
8
6
5
7
7
9
3

4
1
2
5
3
3
2
6
4
6
1
1
3
4
2
1
6
6
2
6
4
4
5
4
3
1
6
3
5
3
1

63

Minerals 2018, 8, 449

Table 4. Cont.
Iobs , %dobs , Å

Icalc , %dcalc , Åhkl

Iobs , %dobs , Å

Icalc , %dcalc , Å

h

k

l

3
6
1
15
51
10

1.940
1.914
1.847
1.823
1.796
1.769

1
1
3
1
1
15
1
56
12

1.944
1.935
1.913
1.845
1.830
1.824
1.822
1.796
1.768

2
0
1
2
1
4
3
2
0

6
6
7
6
5
0
3
4
8

1
3
1
2
4
0
3
4
0

1
1
1
1

1.365
1.334
1.307
1.288

1
1
1
1
1
1
1
1
1

1.367
1.363
1.334
1.334
1.333
1.333
1.309
1.305
1.289

4
1
5
0
4
3
3
0
5

4
3
1
8
2
3
9
10
3

4
7
3
5
5
6
1
3
3

Note: The dcalc values are calculated for unit cell parameters obtained from single-crystal data.

The crystal structure of the holotype sample (see Tables 5–7) was solved by direct methods basedon single-crystal X-ray diffraction data and reﬁned to R = 0.0198 for 574 unique reﬂections with
I > 2σ(I). Nöggerathite-(Ce) is orthorhombic, with space group Cmca. The reﬁned unit cell parameters
are: a = 7.2985(3), b = 14.1454(4), c = 10.1607(4) Å, V = 1048.99(7) Å3 ; and Z = 4.
Solving the crystal structure of nöggerathite-(Ce) reveals the alternation of two types of bent
polyhedral layers, namely an octahedral layer (Figure 4A) and a layer of cations with sevenand eight-fold coordination (Figure 4B). The octahedral layer is built by vertex-sharing M(3)O6
and M(4)O6 octahedra forming three- and six-membered rings, whereas M(5) and M(6) sites arelocated in the centers of six-membered rings. The adjacent sites M(5) and M(6), with coordination
numbers 4 and 5, respectively, are statistically occupied and contain Fe2+ as the major cation (Figure 4C).
The M(1)-centered polyhedron is a distorted cube which shares edges with neighboring M(1)-centeredcubes to form rows along the a axis. Similar rows are formed by seven-fold M(2)-centered polyhedra(mono-capped octahedra). Adjacent rows of eight- and seven-fold polyhedra are linked with each othervia common edges forming a dense layer. A general view of the crystal structure of nöggerathite-(Ce)is shown in Figure 5.
The refined crystal/chemical formula of nöggerathite-(Ce) is as follows (Z = 4, REE are modeled byCe, coordination numbers of cations are indicated with Roman numerals): VIII (LREE0.88 Ca0.80 Mn0.24 Th0.08 )VII (Zr
VI
VI
IV
V
1.88 Mn0.12 ) (Nb1.22 Ti0.78 ) (Ti1.48 Nb0.48 Al0.04 ) (Fe0.48 Mn0.08 ) (Fe0.40 Mn0.04 )2 O14 .
Table 5. Atom coordinates, equivalent thermal displacement parameters (Ueq , Å2 ), site populations
and site multiplicities (Q) in the structure of nöggerathite-(Ce).
Site

x

y

z

U eq

Site Population

Q

VIII M(1)

0.25
0.5
0.0
0.25
0.4211(8)0.5
0.1948(4)0.2131(4)
0.5
0.0
0.5

0.11753(3)0.23411(4)0.0
0.13297(5)0.00.0140(5)0.03150(18)0.23287(17)0.1081(2)0.1288(2)0.1366(3)–0.2500
0.01426(5)
0.0
0.25
0.0
0.0342(9)0.1256(2)0.1201(2)–0.1007(4)–0.0912(4)0.1756(4)0.00954(18)0.0120(2)0.0112(3)0.0129(2)0.0301(9)0.0393(19)0.0236(6)0.0218(6)0.0182(8)0.0184(8)0.0250(9)Ce0.44 Ca0.40 Mn0.12 Th0.04
Zr0.937(9) Mn0.063(9)Nb0.608(8) Ti0.392(8)Ti0.74 Nb0.24 Al0.02
Fe0.24 Mn0.04
Fe0.20 Mn0.02
O
O
O
O
O

8
8
4
8
8
8
16
16
8
8
8

VII M(2)VI M(3)
VI M(4)
IV M(5)
V M(6)

O(1)
O(2)
O(3)
O(4)
O(5)

Table 6. Selected interatomic distances (Å) in the structure of nöggerathite-(Ce).
CationLigandDistance

CationLigandDistance

M(1)
M(1)
M(1)
M(1)
M(2)

O(3)
O(4)
O(1)
O(2)
O(4)

2.376(2) × 2
2.441(3) × 2
2.491(3) × 2
2.508(3) × 2
2.091(3)

M(3)
M(4)
M(4)
M(4)
M(5)

O(4)
O(2)
O(1)
O(5)
O(3)

2.044(3) × 2
1.952(2) × 2
1.955(3) × 2
1.9759(16) × 2
1.928(4) × 2

64

Minerals 2018, 8, 449

Table 6. Cont.
CationLigandDistance

CationLigandDistance

M(2)
M(2)
M(2)
M(2)
M(3)

O(2)
O(3)
O(5)
O(2)
O(1)

2.122(3) × 2
2.132(4)2.142(4)2.354(3) × 2
1.961(3) × 4

M(5)
M(6)
M(6)
M(6)
M(6)

O(1)
O(3)
O(3)
O(5)
O(1)

2.134(5) × 2
1.855(8)1.910(9)2.251(9)2.426(5) × 2

Table 7. Cation distribution in structurally investigated samples of zirconolite-3O, laachite, andnöggerathite-(Ce).
Site

Zirconolite-3O

VIII M(1)Ca0.53 Ce0.41 Na0.04 Th0.02

VII M(2)VI M(3)Zr
Ti0.52 Nb0.47 Ta0.01

VI M(4)

Ti0.88 Nb0.12

IV M(5)

Fe0.46
Fe0.03[16]

V M(6)

Reference

Laachite
Ca0.28 Mn0.26 Ln0.26 Th0.14 Y0.06
Ca0.32 Mn0.28 Ln0.24 Th0.14 Y0.02
Zr0.78 Mn0.22
Nb0.82 Ti0.18
Ti0.72 Nb0.28
Nb0.44 Ti0.40 Al0.16
Fe0.34 Mn0.10 Y0.06[9]

Nöggerathite-(Ce)
Ln0.44 Ca0.40 Mn0.12 Th0.04
Zr0.937 Mn0.063
Nb0.608 Ti0.392
Ti0.74 Nb0.24 Al0.02
Fe0.24 Mn0.04
Fe0.20 Mn0.02
This work

Figure 4. Octahedral (A) and large-cation (B) layers and arrangement of Fe-dominant sites M(5) andM(6) inside the octahedral layer (C) in the structure of nöggerathite-(Ce).

65

Minerals 2018, 8, 449

Figure 5. The crystal structure of nöggerathite-(Ce). The unit cell is outlined. For legend see Figure 4.

4. Discussion
Nöggerathite-(Ce) is isostructural with zirconolite-3O and is its REE-dominant analogue, with Nb
prevailing in one of two octahedral sites (see Tables 5–7). Zirconolite-3O was originally described as“polymignite” [17,18] and was later redeﬁned and renamed [16,19–21]. However, “polymignite”
is usually metamict and, as a rule, its powder X-ray diffraction pattern can be obtained only
after calcination.
Unlike most samples of zirconolites, nöggerathite-(Ce) is non-metamict because it was formed
recently: the last eruption of the Laach Lake volcano occurred no later than 13,000 years ago. According
to Schmitt [14], the age of zircon from sanidinitic ejecta of the Laach Lake volcano is between 17,000
and 30,000 years. Consequently, sanidinite could have formed 4000–17,000 years before the eruption.
A non-metamict REE-, Nb-, and Mn-rich variety (or, in accordance with the accepted mineralogicalnomenclature rules, an REE-analogue) of zirconolite-3O from the Laach Lake eruptive center was
described by Della Ventura et al. [7] without structural data. Its chemical composition is close tothat of nöggerathite-(Ce) and varies within the following ranges (calculated on the basis of 7 O apfu):
Ca0.312-0.337 Y0.079-0.093 La0.069-0.086 Ce0.269-0.296 Pr0.020-0.028 Nd0.052-0.062 Sm0.004-0.006 Gd0.005-0.006 Dy0.003-0.006
Er0.003-0.005 Th0.014-0.073 U0.008-0.022 Mn0.345-0.397 Mg0.003-0.005 Al0.020-0.025 Fe0.301-0.339 Zr0.888-0.910 Hf0.006-0.009
Ti0.840-0.888 Nb0.575-0.613 Ta0.007-0.009 Si0.000-0.012 O7.000 .
The empirical formula corresponding to spot analysis number 1 from [7], calculated on 14 O apfu,
is (Ln0.98 Ca0.63 Mn0.17 Y0.16 Th0.04 U0.02 )Σ2.00 [(Fe0.65 Mn0.55 Al0.05 Mg0.01 )(Zr1.79 Hf0.01 )(Ti1.71 Nb1.19 Ta0.02 )Si0.02 ]Σ6.00 O14 . The latter formula may correspond to nöggerathite-(Ce) or its Mn-dominant analogue(with Mn > Fe in the M(5) and M(6) sites).
Another crystalline mineral related to zirconolite-3O and nöggerathite-(Ce) is laachite(Ca2 Zr2 Nb2 TiFeO14 ), which originates from sanidinite of the Laach Lake volcano and demonstrates aperfect crystal structure. Laachite is a monoclinic (pseudo-orthorhombic) analogue of zirconolite-3O,
with Nb prevailing over Ti in two octahedral sites [9]. Chemical compositions of 24 samples of66

Minerals 2018, 8, 449

zirconolite-type minerals from the Laach Lake volcano have been determined by us. Most ofthem correspond to stefanweissite (Ca,REE)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14 (IMA 2018-020), which isan analogue of zirconolite-3O with Nb as a species-deﬁning component. Comparative data for
nöggerathite-(Ce), zirconolite-3O, and laachite are given in Table 8 and in Figures 6–8. The main
substitution scheme following from the compositional data for these minerals is: Ca2+ + Ti4+ + Zr4+ ↔
REE3+ + Nb5+ + Mn2+ . However, as one can see from Figure 7, the substitution Zr4+ for Mn2+ in theM(2) site is to be completed with other (subordinate) schemes involving zirconium, most probably
Zr4+ ↔ Ti4+ and/or REE3+ + Zr4+ ↔ Ca2+ + Nb5+ .
Table 8. Comparative data for nöggerathite-(Ce) and related minerals.
MineralNöggerathite-(Ce)Idealized
formulaCrystal system
Space group
a, Å
b, Å
c, Å
β, ◦
Z

(Ce,Ca)2 Zr2 (Nb,Ti)(Ti,Nb)2 Fe2+ O14
Orthorhombic
Cmca
7.298514.1454
10.160790
4Laachite

Strong lines of the
X-ray powder
diffraction pattern:d, Å (I, %)2.963 (91)2.903 (100)2.540 (39)1.823 (15)1.796 (51)1.543 (20)
1.519 (16)Refractive index

2.267 (mean, calc.)

2.26 (mean, calc.)Density, g cm–3

5.332 (calc.)

5.42 (calc.)

SourcesThis work[9]

Zirconolite-3O

Ca2 Zr2 Nb2 TiFeO14

CaZrTi2 O7

Monoclinic
C2/c
7.311914.1790
10.1700
90.072
4
4.298 (22)2.967 (100)2.901 (59)2.551 (32)1.800 (34)1.541 (24)1.535 (23)1.529 (23)

Orthorhombic
Cmca
7.278–7.284
14.147–14.18
10.145–10.1489083.176 (30)2.914 (100)2.506 (40)1.980 (90)1.792 (90)1.517 (10)2.215 (meas., metamict);2.26–2.31 (calc.)
4.7 (meas.)
4.9 (calc.)[7,8,16,19–22]

Note: For zirconolite-3O the standard space group Cmca is given instead of the space group Acam reported inearlier publications.

Figure 6. Ratios of large cations with coordination number 8 in zirconolite-type minerals from the Laach
Lake volcano: the holotype () and cotype ( ) nöggerathite-(Ce), other nöggerathite-(Ce) samples (×),
the stefanweissite holotype (), other stefanweissite samples (), and the holotype laachite (Δ).

67

Minerals 2018, 8, 449

Figure 7. The relationships between the Zr and Mn contents (apfu) at the M(2) site of zirconolite-typeminerals from the Laach Lake volcano. The correlation coefﬁcient is R = –0.757 for the whole set ofanalyses, but almost no correlation is observed within Zr-rich (i.e., Mn-poor) and Zr-poor (i.e., Mn-rich)groups of samples separated with ellipses. For legend see Figure 6.

Figure 8. Correlation between contents of Ti and Nb (apfu) in zirconolite-type minerals from the Laach
Lake volcano. Correlation coefficient R = –0.932. For legend see Figure 6.

REE-dominant zirconolite-type minerals (with REE > Ca in atomic units and Y prevailing among
rare-earth elements) have been described in gabbro pegmatite in St. Kilda, Scotland, United Kingdom,in nepheline syenite from Tchivera, Angola, in metamorphic rocks of the Vestfold Hills, East Antarctica,
and in lunar rocks [6], but crystal structures of these minerals have not been studied because of their
metamict state.
An important specific feature of nöggerathite-(Ce) is its high content of niobium. A review of
localities of zirconolites worldwide and a compilation of chemical compositions of about 300 samples
were provided by Williams and Gieré [6]. All analyses show significant predominance of Ti over Nb.
For most samples the content of Nb2 O5 is below 10 wt %. The Nb-richest zirconolite from Vuoriyarvi,
Northern Karelia, Russia, was described by Borodin et al. [23] as “niobozirconolite”. It contains 0.798 Nb
atoms and 1.081 Ti atoms as per a formula calculated on the basis of 7 O atoms. High content of Nb was68Minerals 2018, 8, 449

also detected in zirconolite samples from Kovdor, Kola, Russia (0.652 apfu Nb vs. 0.857 apfu Ti), Kaiserstuhl,
Germany (0.687 apfu Nb vs. 0.702 apfu Ti), and Sokli, Finland (0.608 apfu Nb vs. 0.773 apfu Ti) [6].
Author Contributions: N.V.C., N.V.Z., and I.V.P. wrote the paper. C.S. collected the type material. B.T. and W.S.
collected other samples of zirconolite-type minerals. S.N.B. obtained single-crystal X-ray diffraction data. N.V.Z.
and D.Y.P. solved and reﬁned the crystal structure. I.V.P. obtained powder X-ray diffraction data. V.N.E. and
N.V.C. conceived and designed chemical data. M.F.V. obtained Raman spectra. N.V.C. analyzed Raman data. Y.S.P.
measured reﬂectance data and hardness.
Funding: This research was funded by the Russian Science Foundation, under grant no. 14-17-00048 (for partof the chemical study of nöggerathite-(Ce) and other zirconolite-type minerals) and the Russian Foundationfor Basic Research, under grant no. 18-29-12007 (for parts of the mineralogical study, Raman spectroscopy and
X-ray structural analysis). SNB acknowledges St. Petersburg State University for ﬁnancial support, grant no.
3.42.741.2017 (in part of single-crystal measurements).
Acknowledgments: The authors thank the X-ray Diffraction Centre of St. Petersburg State University for
instrumental and computational resources.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Laachite, (Ca,Mn)2 Zr2 Nb2 TiFeO14 , a new zirconolite-related mineral from the Eifel volcanic region, Germany.
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Zubkova, N.V.; Chukanov, N.V.; Pekov, I.V.; Ternes, B.; Schüller, W.; Pushcharovsky, D.Y. The crystal structureof natural nonmetamict Nb-rich zirconolite-3T from the Eifel paleovolcanic region, Germany. Z. Kristallogr.
2018, 233. [CrossRef]
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Negendank, J.F.W. Correlation and synchronisation of Lateglacial continental sequences in northern Central
Europe based on annually laminated lacustrine sediments. Quat. Sci. Rev. 2001, 20, 1233–1249. [CrossRef]
Schmitt, A.K.; Wetzel, F.; Cooper, K.M.; Zou, H.; Wörner, G. Magmatic longevity of Laacher See volcano(Eifel, Germany) indicated by U–Th dating of intrusive carbonatites. J. Petrol. 2010, 51, 1053–1085. [CrossRef]
Frechen, J. Vorgänge der Sanidinit-Bildung im Laacher Seegebiet. Fortschritte der Mineralogie 1947, 26,
147–166. (In German)
Schmitt, A.K. Laacher See revisited: High-spatial-resolution zircon dating indicates rapid formation of azoned magma chamber. Geology 2006, 34, 597–600. [CrossRef]

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diffraction data obtained from the curved image plate detector of Rigaku RAXIS Rapid II diffractometer. Zap. Ross.
Mineral. Obsh. 2017, 146, 104–107. (In Russian)Mazzi, F.; Munno, R. Calciobetafite (new mineral of the pyrochlore group) and related minerals from Campi Flegrei,
Italy; crystal structures of polymignite and zirkelite: Comparison with pyrochlore and zirconolite. Am. Mineral. 1983,
68, 262–276.
Berzelius, J. Undersökning af några Mineralier. 2. Polymignit. Kongl. Svenska Vetensk.-Acad. Handl. 1984,
338–345. (In Swedish)Brøgger, W.C. Die Mineralien der Syenitpegmatitgänge der südnorwegischen Augit und Nephelinsyenite.
Z. Kristallogr. Speziellen Teil 1890, 16, 1–663. (In German)
Bayliss, P.; Mazzi, F.; Munno, R.; White, T.J. Mineral nomenclature: Zirconolite. Mineral. Mag. 1989, 53, 565–569.
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Chukhrov, F.V.; Bonshtedt-Kupletskaya, E.M. Mineraly Vol. II(3); Nauka: Moscow, Russia, 1967; pp. 188–190.
(In Russian)Pudovkina, Z.V.; Chernitzova, N.M.; Pyatenko, Y.A. Crystallographic study of polymignite. Zap. Vses. Mineral. Obshch.
1969, 98, 193–199. (In Russian)
Borodin, L.S.; Nazarenko, I.I.; Richter, T.L. On a new mineral zirconolite–A complex oxide of AB3 O7 type.
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Borodin, L.S.; Bykova, A.V.; Kapitonova, T.A.; Pyatenko, Y.A. New data on zirconolite and its new niobian variety.
Dokl. Akad. Nauk SSSR 1960, 134, 1188–1192. (In Russian)© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleNew Occurrence of Rusinovite, Ca10(Si2O7)3Cl2:
Composition, Structure and Raman Data of
Rusinovite from Shadil-Khokh Volcano,South Ossetia and Bellerberg Volcano, Germany
Dorota Środek 1, *, Rafał Juroszek 1 , Hannes Krüger 2 , Biljana Krüger 2 , Irina Galuskina 1 andViktor Gazeev 3,4
1
2
3
4

*

Department of Geochemistry, Mineralogy and Petrography, Faculty of Earth Sciences, University of Silesia,
B˛edzińska 60, 41-200 Sosnowiec, Poland; rjuroszek@us.edu.pl (R.J.); irina.galuskina@us.edu.pl (I.G.)
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria;
hannes.krueger@uibk.ac.at (H.K.); biljana.krueger@uibk.ac.at (B.K.)
Insitute of Geology of Ore Deposits, Petrography, Mineralogy and Biochemistry, RAS, Staromonetny 35,
119017 Moscow, Russia; gazeev@igem.ru
Vladikavkaz Scientiﬁc Centre of the Russian Academy of Sciences, Markov str. 93a,
362008 Vladikavkaz, Republic of North Ossetia-Alania, Russia
Correspondence: dsrodek@us.edu.pl; Tel.: +48-693-404-979

Received: 28 August 2018; Accepted: 8 September 2018; Published: 10 September 2018

Abstract: Rusinovite, Ca10 (Si2 O7 )3 Cl2 , was found at two new localities, including Shadil-Khokh
volcano, South Ossetia and Bellerberg volcano, Caspar quarry, Germany. At both of these localities,
rusinovite occurs in altered carbonate-silicate xenoliths embedded in volcanic rocks. The occurrenceof this mineral is connected to speciﬁc zones of the xenolith characterized by a deﬁned Ca:Si< 2 ratio. Chemical compositions, as well as the Raman spectra of the investigated rusinovite
samples, correspond to the data from the locality of rusinovite holotype—Upper Chegem Caldera,
Northern Caucasus, Russia. The most intense bands of the Raman spectra are related to vibrationsof (Si2 O7 ) groups. Unit cell parameters of rusinovite from South Ossetia are: a = 3.76330(4) Å,
b = 16.9423(3) Å, c = 17.3325(2) Å, V = 1105.10(4) Å3 , Z = 2. The performed synchrotron radiation
diffraction experiments did not conﬁrm a doubling of c as reported for the synthetic phase,
Ca10 (Si2 O7 )3 Cl2 . However, one-dimensional diffuse scattering parallel to b* has been observed.
This can be interpreted with an ordered arrangement of Si2 O7 groups creating layers with a doubled
a parameter. Consequently, the two different displacements of neighbouring layers allow random
stacking faults to occur.
Keywords: Rusinovite; Raman spectroscopy; pyrometamorphism; stacking faults; Shadil-Khokh volcano;Bellerberg volcano

1. Introduction
Rusinovite, Ca10 (Si2 O7 )3 Cl2 , was found in 2011 in an altered carbonate-silicate xenolith fromthe Upper Chegem Caldera located in Kabardino-Balkaria, Northern Caucasus, Russia. These rocks
are formed at high-temperature and low-pressure conditions (sanidinite facies) and contain different
Cl-bearing minerals, such as rondorﬁte MgCa8 (SiO4 )4 Cl2 , rustumite Ca10 (SiO4 )(Si2 O7 )2 Cl2 (OH)2 ,and minerals of the mayenite supergroup: wadalite Ca12 Al10 Si4 O32 Cl6 and chlorkyuygenite
Ca12 Al14 O32 [(H2 O)4 Cl2 ] [1–3]. Besides their natural occurrence, rusinovite-type synthetic phases
were also described from anthropogenic formations. Firstly, it was reported from the combustion
ashes comprised by Cl-bearing Ca-silicates, which were formed after the incineration of waste [4].
Minerals 2018, 8, 399; doi:10.3390/min8090399

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Secondly, the Ca10 (Si2 O7 )3 Cl2 phase was also described as “chesoﬁite” from the burned spoil heaps ofthe coal mine located near the town of Kopeisk, Chelyabinsk Region, Russia [5]. In addition to thegeological and anthropogenic occurrences, a phase with rusinovite composition and structure was alsosynthesized utilizing the ﬂux method [6]. A new green-emitting phosphor was created by doping thesynthetic analogue of rusinovite with Europium (Ca10 (Si2 O7 )3 Cl2 :Eu2+ ) [7]. Its emission is efﬁciently
excited by the output of near-UV light-emitting diodes (LEDs).
Both rusinovite and the synthetic analogue are reported as strongly elongated grains, which form
needle-like single crystals or spherulites composed of ﬁbrous aggregates [1,6]. The structure of thesynthetic phase was primarily reported as order-disorder (OD) structure with an orthorhombic unit
cell [6], but later its structure was redeﬁned as monoclinic [4]. Galuskin et al. [1] have described thestructure of rusinovite as columns of disordered “face-sharing” disilicate units extending parallel toa. At a local level, only each second Si2 O7 unit (along a) can be occupied in the structure. Due to thesmall dimensions of the grains from the holotype locality, some structural issues were still unresolved(such as possible superstructures due to the ordering of the Si2 O7 groups).
The main goal of this study is to describe rusinovite from two new localities, including chemicalcomposition and Raman spectroscopy. Furthermore, the structure of rusinovite from South Ossetia
is to be re-investigated using synchrotron radiation, to check for superstructures, which have been
observed in synthetic analogues.
2. The Occurrence and Paragenesis of Rusinovite
Rusinovite was found in pyrometamorphic rock at the two localities. The ﬁrst one is Shadil-Khokh
volcano, belonging to the Kel’sky volcanic plateau located at the southern part of the Greater Caucasus
Mountain Range in Southern Ossetia. Small (about 2 m in diameter) altered carbonate-silicate xenolith
was revealed at the north-west slope of the volcano within dacite lava.
This xenolith is composed of minerals characteristic for sanidinite facies, such as spurrite, larnite,
gehlenite, merwinite, bredigite, rondorﬁte, and srebrodolskite [8]. Rusinovite occurs as relatively large(100–200 μm in length) elongated crystals with a characteristic macroscopic brown-orange colour insmall veins (up to 0.5 cm wide) encountered at the exocontact zone of xenolith (Figure 1A,B). In thesezones, another Cl-bearing phase–eltyubyuite, Ca14 Fe3+ 10 Si4 O32 Cl6 , was noted [9]. Rusinovite wasfound also at the endocontact zone of strongly altered dacite (Figure 2A,B).
The presence of microzonation at both endo- and exocontact zones is reﬂected in the zonal
distribution of minerals with the different Ca:Si ratio. At the endocontact, rock is presented bydacite microbreccia and carbonate-silicate microxenoliths (Figure 2A,B). We distinguish the following
zonation (Figure 2C). The ﬁrst zone (the numbering is according to the Ca:Si ratio increasing)–dacite,
consisting of enstatite, albite phenocrysts, and quartz enclosed in the ﬁne-grained aggregate of diopside,
plagioclase, and Na–K feldspar. Visible zonation of enstatite (Figure 2C) is caused by variations ofminor Fe substitutions in composition. Ilmenite occurs as an accessory mineral. The second zone
is composed of massive gehlenite. The third zone is comprised mostly of rusinovite and gehlenite;hydroxylellestadite and wollastonite are noted rarely. The fourth zone is represented by strongly
altered rock, mainly composed of secondary Ca–Si hydrosilicates, rusinovite, cuspidine and rondorﬁte.
Another type of zonation is observed at the dacite-xenolith (exocontact) boundary (Figure 1A,B).
The altered dacite is composed of diopside, enstatite, plagioclase phenocrysts, and quartz. The matrix ispresented by a ﬁne-grained aggregate of plagioclase. Furthermore, ilmenite, titanite, apatite, and pyrite
occur as accessory minerals. The next zone towards xenolith, contains Al–Ca hydrosilicates (Figure 1C).
In the second zone, wollastonite and andradite are the main minerals. The accessory phases are
represented by cuspidine, gehlenite, and wadalite. In the third zone, rusinovite occurs as a major
component together with cuspidine. Wollastonite is present, but not as commonly as in the previous
zone. Cl-bearing hydroxylellestadite, gehlenite, magnesioferrite, and rondorﬁte are also noted. The lastzone is a strongly altered xenolith part composed mostly of secondary Ca-hydrosilicates with the relicsof rondorﬁte, larnite, cuspidine, and merwinite.

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Figure 1. (A,B) Macroscopic image of exocontact; (C) BSE (backscattered electron) image of the
exocontact zone; 1—dacite zone; 2—wollastonite zone; 3—cuspidine—rusinovite zone; 4—alteredxenolith. An—anorthite, Adr—andradite, Cus—cuspidine, Di—diopside, Grs—grossular, HSi—Al–Ca
hydrosilicates, Lrn—larnite, Mw—merwinite, Rnd—rondorﬁte, Rus—rusinovite, Wo—wollastonite.

The Bellerberg volcano (Caspar quarry), located near Mayen in the Eifel district in western
Germany is the second studied rusinovite locality. In the active quarry, within leucite tephrite, small
Ca-rich xenoliths can be found [10]. These xenoliths were formed under similar conditions as the
xenoliths from South Ossetia—at high temperatures and low pressure. Bellerberg volcano is a holotype
locality for a few Cl-bearing minerals, such as rondorﬁte Ca8 Mg[SiO4 ]4 Cl2 [10], chlormayenite
Ca12 Al14 O32 [4 Cl2 ] [11,12] or vondechenite Cu4 CaCl2 (OH)8 × 4H2 O [13].
Macroscopically, the studied xenolith is dark green-grey and contains veins characterized bya colour that is deﬁnitely lighter (Figure 3A).
The main part of the xenolith is composed of larnite, gehlenite, rondorﬁte, Cl-bearinghydroxylellestadite, spurrite, and secondary Ca-hydrosilicates. Perovskite, hematite, cuspidine,Zr-garnet (with a chemical composition close to kerimasite), andradite, and pyrite with chalcopyrite
intergrowths are also identiﬁed here. The lighter part of the rock is characterized by similar paragenesis,
the difference is in the predominance of hydrocalumite among secondary Ca-hydrosilicates. Besides theminerals mentioned above, this zone contains rusinovite, wollastonite, chalcopyrite, baghdadite,and lakargiite (Figure 3B). Aggregates of rusinovite can reach up to 200 μm in size. They contain a lotof mineral inclusions composed of secondary Ca-hydrosilicates, gehlenite, chlorine-bearing ellestadite,perovskite and hematite, mainly, and strongly hydrated zones (Figure 3B).

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Minerals 2018, 8, 399

Figure 2. (A) Macroscopic image of the endocontact zone; (B) BSE image of the endocontact
zone; fragment in the frame is magniﬁed in (C); Zones: 1—altered dacite zone; 2—gehlenite zone;
3—rusinovite zone; 4—altered xenolith. Cus—cuspidine, Di—diopside, En—enstatite, Gh—gehlenite,
Pl—plagioclase, Rus—rusinovite.

Figure 3. (A) Macroscopic photo of vein containing rusinovite from the Bellerberg volcano,
Caspar quarry; (B) BSE image of rusinovite. Ell—ellestadite, Gh—gehlenite, Hem—hematite,
HSi—unidentiﬁed secondary Ca-silicates; Lak—lakargiite; Prv—perovskite, Rus—rusinovite.

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3. Materials and Methods
The chemical composition and morphology of rusinovite and associated minerals were examinedusing a scanning electron microscope, the Phenom XL, PhenomWorld (ThermoFisher Scientiﬁc,
Eindhoven, The Netherlands) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector(Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland), and a CAMECA SX100
electron microprobe (Institute of Geochemistry, Mineralogy and Petrology, University of Warsaw,
Warsaw, Poland). The microprobe analyses were performed at 15 kV and 10 nA using a 1-5 μm beam
spot. The following lines and standards were used for rusinovite analyses: CaKα, SiKα, MgKα-diopside;
FeKα-synthetic Fe2 O3 ; MnKα-rhodonite; TiKα-rutile; ClKα-tugtupite.
The Raman spectra of rusinovite and associated minerals were collected using a WITec confocal
Raman microscope CRM alpha 300, WITec, Ulm, Germany (Institute of Physics, University of Silesia,
Poland) equipped with an air-cooled solid-state laser (λ = 532 nm) and an electron multiplying
charge-coupled device (EMCCD) detector. Raman scattered light was focused onto a multi-mode
ﬁber (50 mm diameter) and monochromator with a 600 line/mm grating. An air Olympus MPLAN
100× objective was used. The spectra were collected with an integration time of 100 ms per spectrum
and accumulation of 10 scans, precision of ±1 cm−1 and a resolution of 3 cm−1 . The laser radiation
power was 10mW at the sample and 1 μm diameter of the incident laser beam. The calibration of themonochromator was held by checking the position of the Si (520.7 cm−1 ). Spectra processing, such asbaseline correction and smoothing, was performed using the Spectracalc software package GRAMS(Galactic Industries Corporation, Salem, NH, USA). The band ﬁtting was done using a Gauss-Lorentz
cross-product function, with the minimum number of component bands used for the ﬁtting process.
Diffraction data were collected at the X06DA beamline at the Swiss Light Source (Paul Scherrer
Institute, Villigen, Switzerland) using a Pilatus 2M-F detector at a distance of 80 mm from the sample.
A single 180◦ omega-scan was performed, divided into frames of 0.1◦ rotation using 0.3 s integration
time. The wavelength was tuned to 0.72931 Å. Data reduction, including Lorentz-Polarisation andabsorption correction, was performed with X-ray detector software (XDS) [14]. The reﬁnement of thelattice parameters was done with CrysAlis (Rigaku Oxford Diffraction, 2015). For the reconstruction ofreciprocal space sections, a modiﬁed version of the software Xcavate [15] was utilised. Further detailsof the intensity data collection and crystal-structure reﬁnement of rusinovite are reported in Table 1.
Data about atom coordinates (x,y,z), occupancies, and equivalent isotropic displacement parameters(Uiso, Å2) (Table S1), as well as, anisotropic displacement parameters (Å2) (Table S2), the selectedinteratomic distances (Å) (Table S3) and cif ﬁle (Scheme S1) for rusinovite from Shadil-Khokh volcano,South Ossetia were reported in Supplementary materials.
Table 1. Parameters for X-ray data collection and crystal-structure reﬁnement.
Crystal Data
Chemical formula
Crystal system
Space group

Ca10 (Si2 O7 )3 Cl2
orthorhombic
Cmcm (No. 63)
a = 3.76330(4) Å
b = 16.9423(3) Åc = 17.3325(2) Å1105.10(4) Å3
976.2
2.934 g/cm3
2
28 × 18 × 15 μm

Unit-cell dimensions
Unit cell volume
Formula weight
Density (calculated)
Z
Crystal size
Data collection

beamline X06DA, Swiss Light Source
single-axis Aerotech goniometer,
PILATUS 2M-F detector

Diffractometer

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Table 1. Cont.
Crystal Data
Radiation wavelength
Detector to sample distanceOscillation range
No. of frames measured
Time of exposure

Reﬂection measured
Rint

0.72931 Å
80 mm
0.1◦
1800
0.3 s
−5 ≤ h ≤ 5;
−25 ≤ k ≤ 23;−25 ≤ l ≤ 23
5015
0.0274Reﬁnement of structure

full matrix least-squares on F

No. of unique reﬂections
No. of observed unique reﬂ. [I > 3s(I)]Final R values [I > 3s(I)]Final R values (all data)
S (all data)
Reﬁned parameters
Weighting schemeΔρmin [e Å−3 ]Δρmax [e Å−3 ]

1087
1032
R = 0.021; wR = 0.035
R = 0.022; wR = 0.036
2.3485
w = 1/(s2 (F) + 0.0001F2 )
−0.73
0.32

Reﬂection ranges

4. Results
4.1. Chemical Composition and Raman Spectroscopy of Rusinovite
The chemical analysis of rusinovite from the two localities is listed in Table 2. A small loss ofthe chlorine content in rusinovite from Shadil-Khokh volcano can be caused by slight hydration or
methodological problems of chlorine measurements [1].
Table 2. Chemical analysis of rusinovite from the Bellerberg and Shadil-Khokh volcanoes.
Bellerberg Volcano

Shadil-Khokh Volcano

Mean 18

Mean 14

Component
SiO2
CaO
FeO
MgO
Cl
H2 O *
–O=Cl
Total

wt %

S.D.

Range

Atom

pfu

wt %

S.D.

Range

Atom

pfu

36.73
56.70
0.39
0.06
7.21
0.00
1.63
99.47

0.23
0.35
0.13
0.03
0.26

36.44–37.33
56.24–57.11
0.26–0.61
0.00–0.09
7.11–7.31

Si
Ca
Fe
Mg
Cl

6.00
9.93
0.05
0.02
2.00

36.89
57.25
0.26
0.09
6.94
0.08
1.57
99.95

0.23
0.35
0.13
0.03
0.26

36.12–36.93
56.79–58.00
0.20–0.32
0.08–0.11
6.76–7.18

Si
Ca
Fe
Mg
Cl
OH

5.99
9.95
0.03
0.02
1.91
0.08

* calculated on charge balance; S.D. = 1σ standard deviation.

The Raman spectra of the investigated rusinovite (Figure 4A,B) are similar to the spectrum
from the holotype specimen. The most intense bands correspond to the vibrations of (Si2 O7 ) group.
The stretching motions of this group generally occur between 800 and 1100 cm−1 , and it can bedifferentiated into vibrations of SiO4 (800–1000 cm−1 ) and stretching vibrations of Si–O–Si bridges(1000–1100 cm−1 ) [16]. A sharp, strong band at 905 cm−1 is related to the ν1 symmetric stretchingvibrations of Si2 O7 . The bands at 1041, 658 and 641, 365 cm−1 are attributed to the ν3 , ν4 , and ν2 ,respectively. Galuskin [1] reported, that the rusinovite spectrum is highly similar to the spectrum of
rankinite Ca3 Si2 O7 [1]. The presence of a band doublet about 640 and 660 cm−1 linked to the symmetricbending ν4 (Si2 O7 ) vibrations is characteristic for rusinovite. Bands occurring below 350 cm−1 areconnected with Ca–O–Ca, O–Ca–O and librational vibrations of the Si2 O7 group [1,17].
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Minerals 2018, 8, 399

Figure 4. Raman spectra of rusinovite from (A) Shadil-Khokh volcano and (B) Bellerberg volcano.

4.2. Crystal Structure of Rusinovite
Rusinovite [1], as well as its synthetic analogue [6], exhibits columns of disordered disilicate
groups. The ﬁrst crystal structure reﬁnement of the average structure of rusinovite from single-crystal
data was reported by Galuskin et al. [1].
As the rusinovite crystals from the new locality (Shadil-Khokh volcano, South Ossetia) showedalmost identical lattice parameters (Table 1) as reported before [1], we started the structure reﬁnementfrom the known model. The reﬁnement proves that the structures are basically the same. Final atom
coordinates, anisotropic displacement parameters, as well as interatomic distances for rusinovite from
Shadil-Khokh volcano can be found in a cif ﬁle (Supplementary Material).
The average crystal structure of Ca10 (Si2 O7 )3 Cl2 consists of the Ca1- and Ca3-polyhedrons andtwo disilicate units, which form undulating layers perpendicular to b (Figure 5). The disilicate units
appear to be “face-sharing” (Figure 6), which is an artifact of the disorder. In the real structure,
only each second Si2 O7 unit is occupied. The single layers are connected by the Ca2-site and Cl atoms.
Chlorine atoms with characteristic distorted tetrahedral coordination are surrounded only by calcium
ions. The bond lengths between Ca1–Cl and Ca2–Cl are within the range from 2.75Å to 2.92Å.

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Figure 5. Crystal structure of rusinovite, projected along [100]. There are two types of Si2 O7 disilicate
units: Si1 and Si2 tetrahedra are light brown and greenish blue, respectively. Calcium atoms are shownas grey spheres and in addition, coordination polyhedra of Ca3 are depicted in grey. Oxygen atoms arepresented as small blue spheres and Cl as green spheres.

Figure 6. “Face-sharing” disilicates units appear in the average structure of rusinovite. The real
structure exhibits ordered columns, where every second Si2 O7 group is present.

In addition to the lattice formed by Bragg reﬂections, the diffraction pattern exhibits diffuse
streaks which are parallel to b* and do not intersect any of the lattice nodes. The streaks are locatedat h = 0.5, 1.5, 2.5 . . . . Reciprocal space sections (hkl) with l = 0, 1, 2, . . . show Bragg reﬂections andlines of diffuse scattering. Sections (hkl) at h = 0.5, 1.5, 2.5, . . . exhibit rods of diffuse scattering only
(Figure 7).

78Minerals 2018, 8, 399

Figure 7. Layers of the reciprocal space of rusinovite as reconstructed from synchrotron diffraction
data. Layer hk1 (top half) exhibits Bragg reﬂections and diffuse lines parallel to b*, whereas the layer(0.5kl) (lower half) shows only diffuse lines. In both images, b* is oriented horizontally parallel to the
diffuse lines. The intensity scale (arb. units) was chosen to range from zero (white) to 60 (black) toshow details of the weak diffuse intensities. For comparison, the intensities of the Bragg reﬂections in
the upper half of the image range up to 12.5 × 103 .

The observed pattern of diffuse lines proves the existence of stacking faults in rusinovite.
The positions of the diffuse lines suggest that layers (parallel to the a, c-plane) are stacked along
b with random disorder. The most likely mechanism is that within each layer the Si2 O7 groups areordered—which results in the doubling of the lattice parameter a—and thus creating two possible
origins (along a) for each layer. The Si2 O7 groups, which in the average structure appear to bedisordered, are indeed ordered. Between neighboring columns of Si2 O7 groups, strong correlations
exist which create ordered layers.
Information on the relative arrangement of Si12 O7 groups may be obtained from bond-valencesum (BVS) calculations of the Ca3 atom, which connects to two neighboring Si12 O7 units (Figure 5).
Ca3 is coordinated by eight oxygen atoms in the average structure. Two of them belong to the Si2
tetrahedra and their BVS contribution is independent of the two possible positions of the tetrahedra.
The other six oxygen atoms are connected to two Si1-columns. These six atoms are split-atoms
belonging to one of two possible positions of the Si12 O7 groups. Looking at the two neighboring
Si12 O7 columns, there may be two different relative arrangements of the Si2 O7 groups: Ladder-like(Si2 O7 groups are side by side along a), or zig-zag-like (Figure 8). Taking these two arrangements
into account, we must consider four different environments for Ca3: (1) includes bridging oxygen
atoms (O3) of both columns (8-fold coordination), (2) no bridging O3 atom is included (6-foldcoordination), (3) and (4) only one bridging O3 is coordinating Ca3 (7-fold coordination). In theladder-like arrangement the environments (1) and (2) occur, whereas in the zig-zag-arrangement only
(3) and (4) are possible. The BVS for (1) and (2) are 1.74 and 2.38, respectively. The zig-zag arrangement
seems more likely as the corresponding BVS are 1.84 and 2.28 for the environments (3) and (4).

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Correlations between the Si1 and Si2-columns are less obvious as neither Ca2 nor Ca1 havebridging oxygen atoms of both columns in their ﬁrst coordination sphere. Additionally, distortions of
the local structure may be involved.
A careful re-investigation of the original data used in [1], revealed that diffuse scattering is
detectable, although very weak. The observation of diffuse streaks in X-ray rotation photographs [6]is consistent with our observation in rusinovite crystals. However, the doubled lattice parameter c(corresponds to b in the setting used by Hermoneit et al. [6]) is not observed in our data. However,this could be explained by a more complex order of disilicate groups within the layers.
A relation to the monoclinic cell (a = 18.66, b = 14.11, and c = 18.14 Å, β = 111.6◦ ) described by
Stemmermann et al. [4] seemingly does not exist. It is possible that the monoclinic cell corresponds toa polymorph, which has not yet been further characterized.

Figure 8. Two possible relative arrangements of neighboring “Si12 O7 -columns” and resulting bonds of
Ca3, projected along [011], the a-direction is vertical (the shown part of the structure is encircled in red
in Figure 5). (a) shows a ladder-like arrangement, whereas in (b) the zig-zag arrangement is depicted.
Bond valence sums for the resulting environments of Ca3 (1–4) are discussed in the text. Bonds from
Ca3 to oxygen atoms of the Si22 O7 -column are omitted.

5. Discussion
Rusinovite was previously found in South Ossetia [9] and at the Bellerberg volcano [18], but thisstudy is a ﬁrst mineralogical description of this mineral from these localities. The obtained chemical
data are in good agreement with the composition of the holotype specimen from Chegem Caldera,
Northern Caucasus, Russia [1]. An insigniﬁcant presence of Mg2+ and Fe2+ impurities at Ca site
is characteristic for the studied samples. The Raman spectra of rusinovite from both localities areanalogous to the one of the holotype rusinovite with a characteristic doublet about 640–660 cm−1
(Figure 4; [1]). In the Raman spectra of disilicates, the split of 640–660 cm−1 band was noted before [19].
It is connected with the change Si–O–Si angle [20] and suggests that two types of (Si2 O7 ) groups withdifferent Si–O–Si angles [1] exist in rusinovite.

80Minerals 2018, 8, 399

One of the goals of this study was to check rusinovite for superstructure reﬂections, which havebeen reported for synthetic material [6]. Additional Bragg reﬂections were not observed, however
diffuse scattering revealed that stacking faults of layers with doubled a are present. Ordered polytypes
may exhibit larger periodicity in b, the doubling in c as suggested by Hermoneit et al. [6] is not
explained. The new data does not show evidence for any signiﬁcant correlation in the stacking
sequence. The diffraction pattern can be fully explained by random stacking faults.
For the samples from South Ossetia, we were enabled to distinguish successive stages ofmineral formation with a decrease of silica content towards the xenolith from volcanic rock.
Minerals occurring in the subsequent zones are characterized by the variable Ca:Si ratio. Provisionally it
is possible to distinguish the following successive metasomatic zones in the Shadil-Khokh xenolith:wollastonite/rankinite zone (also as endocontact zone), Ca:Si ≈ 1–1.5; larnite/cuspidine/rondorﬁtezone, Ca:Si ≈ 2:1; spurrite/Ca-humite zone, Ca:Si ≈ 2.33–2.5:1; and central marble zone
(Figure 1C, [21]). In both, xenolith exo- and endocontact, the ﬁeld of rusinovite prevalence is distinct
and corresponds to a Ca:Si ratio between 1.5:1 and 2:1. This ratio is similar to the Ca:Si ratio of therusinovite-bearing zone from the holotype locality [1,21].
The occurrence of rusinovite in the speciﬁc zones with characteristic Ca:Si ratio proves that its
formation is a result of metasomatic processes, which require Si diffusion to Ca-bearing xenolith and
Ca diffusion through a porous system of volcanic rock to endocontact zones (Figure 2). At the ﬁrst
stage chlorine-free minerals such as wollastonite or rankinite are formed [21]. Later, these minerals
were affected by the ﬂow of Cl-bearing ﬂuids of volcanic origin, and as a result rusinovite was formed.
6. Conclusions
The rusinovite from the two new localities shows similar chemical composition and physicalproperties as the holotype specimen. The single-crystal synchrotron diffraction experiments on
rusinovite from the Shadil-Khokh volcano did not conﬁrm the presence of a superstructure as observedin its synthetic analogue. However, the measured one-dimensional diffuse scattering revealed moredetails of the stacking fault mechanism.
Rusinovite genesis is connected with metasomatic processes, which occurred at the contact zonesof the xenolith and volcanic rock.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/9/399/s1,
Table S1: Atom coordinates (x,y,z), occupancies and equivalent isotropic displacement parameters (Uiso, Å2) for
rusinovite from Shadil-Khokh volcano, South Ossetia, Table S2: Anisotropic displacement parameters (Å2) for
rusinovite from Shadil-Khokh volcano, South Ossetia, Table S3: Selected interatomic distances (Å) for rusinovite
from Shadil-Khokh volcano, South Ossetia, Scheme S1: The Rusinovite_cif.ﬁle.
Author Contributions: D.Ś., R.J., H.K. and I.G. wrote the paper, D.Ś. performed the mineralogical and petrologicalinvestigations and Raman studies, R.J., H.K. and B.K. performed investigation of rusinovite structure, V.G.
collected the samples.
Funding: This work was supported by the National Science Centre (NCN) of Poland, grant no.
2015/17/N/ST10/03141 (D.Ś.). The investigations were partially supported by the National Science Centre(NCN) of Poland, grant no. 2016/23/N/ST10/00142 (R.J.). The research leading to these results has received
funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no.
730872, project CALIPSOplus. The publication has been partially ﬁnanced from the funds of the Leading National
Research Centre (KNOW) received by the Centre for Polar Studies of the University of Silesia, Poland, grant no.
2018/D2/K20-3.
Acknowledgments: The authors would like to thank B.T. for donating the samples from the Bellerberg volcano.
H.K. acknowledges help from A.P., V.K. and T.G. during the synchrotron experiments.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Hermoneit, B.; Ziemer, B.; Malewski, G. Single crystal growth and some properties of the new compound
Ca3 Si2 O7 ·13CaCl2 . J. Cryst. Growth 1981, 52, 660–664. [CrossRef]
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Mayenite supergroup, part IV: Crystal structure and Raman investigation of Al-free eltyubyuite fromthe Shadil-Khokh volcano, Kel’ Plateau, Southern Ossetia, Russia. Eur. J. Mineral. 2015, 27, 137–143.
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Galuskin, E.V.; Gfeller, F.; Galuskina, I.O.; Armbruster, T.; Bailau, R.; Sharygin, V.V. Mayenite supergroup,part I: Recommended nomenclature. Eur. J. Mineral. 2015, 27, 99–111. [CrossRef]
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calcium silica chlorine mineral containing magnesium in tetrahedral position [MgO4 ]6− , with the aid of thevibrational spectroscopies and ﬂuorescence. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2013, 101, 382–388.
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CaFe3+ 2 Mn4+ O6 : The ﬁrst nonstoichiometric post-spinel mineral, from Bellerberg volcano, Eifel, Germany.
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Dzierżanowski, P. Fluorchegemite Ca7 (SiO4 )3 F2 , a new mineral from the edgrewite-bearing endoskarn zoneof an altered zenolith in ignimbrites from Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria,
Russia: Occurrence, crystal structure and new data on the mineral assemblages. Can. Mineral. 2015, 53,
325–344. [CrossRef]© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

83minerals
ArticleNew Mineral with Modular Structure Derived from
Hatrurite from the Pyrometamorphic Rocks of the
Hatrurim Complex: Ariegilatite,
BaCa12(SiO4)4(PO4)2F2O, from Negev Desert, Israel
Evgeny V. Galuskin 1, *, Biljana Krüger 2 , Irina O. Galuskina 1 , Hannes Krüger 2 ,Yevgeny Vapnik 3 , Justyna A. Wojdyla 4 and Mikhail Murashko 5
1
2
3
4
5

*

Faculty of Earth Sciences, Department of Geochemistry, Mineralogy and Petrography, University of Silesia,
B˛edzińska 60, 41-200 Sosnowiec, Poland; irina.galuskina@us.edu.pl
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria;
biljana.kruger@gmail.com (B.K.); hannes.krueger@gmail.com (H.K.)
Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev,
P.O. Box 653, Beer-Sheva 84105, Israel; vapnik@bgu.ac.il
Swiss Light Source, Paul Scherrer Institute, 5232 Villigen, Switzerland; justyna.wojdyla@psi.ch
Saint Petersburg State University, Faculty of Geology, 7-9 Universitetskaya nab.,
St. Petersburg 199034, Russia; mzmurashko@gmail.com
Correspondence: evgeny.galuskin@us.edu.pl; Tel.: +48-32-3689365

Received: 19 February 2018; Accepted: 5 March 2018; Published: 8 March 2018

Abstract: Ariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O (R3m, a = 7.1551(6) Å, c = 41.303(3) Å, V = 1831.2(3) Å3 ,Z = 3), is a new member of the nabimusaite group exhibiting a modular intercalated antiperovskite
structure derived from hatrurite. It was found in a few outcrops of pyrometamorphic
rocks of the Hatrurim Complex located in the territories of Israel, Palestine and Jordan.
The holotype specimen is an altered spurrite marble from the Negev Desert near Aradcity, Israel.
Ariegilatite is associated with spurrite, calcite, brownmillerite, shulamitite,
CO3 -bearing ﬂuorapatite, ﬂuormayenite-ﬂuorkyuygenite and a potentially new mineral,
Ba2 Ca18 (SiO4 )6 (PO4 )3 (CO3 )F3 O. Ariegilatite is overgrown and partially replaced by stracherite,
BaCa6 (SiO4 )2 [(PO4 )(CO3 )]F. The mineral forms ﬂat disc-shaped crystals up to 0.5 mm in size. It is
colorless, transparent, with white steaks and vitreous luster. Optically, ariegilatite is uniaxial,
negative: ω = 1.650(2), ε = 1.647(2) (λ = 589 nm). The mean composition of the holotype ariegilatite,(Ba0.98 K0.01 Na0.01 )Σ1 (Ca11.77 Na0.08 Fe2+ 0.06 Mn2+ 0.05 Mg0.04 )Σ12 (Si3.95 Al0.03 Ti0.02 )Σ4 (P1.70 C0.16 Si0.10 S6+ 0.03
V0.01 )Σ2 F2.04 O0.96 , is close to the end-member formula. The structure of ariegilatite is described as a
stacking of the two modules {F2 OCa12 (SiO4 )4 }4+ and {Ba(PO4 )2 }4− along (001). Ariegilatite, as well as
associated stracherite, are high-temperature alteration products of minerals of an early clinker-likeassociation. These alterations took place under the inﬂuence of pyrometamorphism by-products,
such as gases and ﬂuids generated by closely-spaced combustion foci.
Keywords: ariegilatite; nabimusaite group; new mineral; crystal structure; intercalated hexagonal
antiperovskite; CO3 -group; Raman; pyrometamorphic rocks; Hatrurim Complex

1. Introduction
Ariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O (R3m, a = 7.1551(6) Å, c = 41.303(3) Å, V = 1831.2(3) Å3 ,Z = 3), is the ﬁrst P-bearing mineral in the nabimusaite group, which combines nabimusaite,
KCa12 (SiO4 )4 (SO4 )2 O2 F (R3m, a = 7.1905(4), c = 41.251(3) Å, V = 1847.1(2) Å3 , Z = 3) and dargaite,
BaCa12 (SiO4 )4 (SO4 )2 O3 (R3m, a = 7.1874(4); c = 41.292(3) Å; V = 1847.32(19) Å3 , Z = 3) [1,2]. Ariegilatite
Minerals 2018, 8, 109; doi:10.3390/min8030109

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Minerals 2018, 8, 109

was found in altered spurrite marble belonging to the pyrometamorphic Hatrurim Complex in the
Negev Desert near Arad, Israel. Rocks of the Hatrurim Complex mainly consist of spurrite marbles,
larnite pseudoconglomerates and gehlenite hornfelses. Furthermore, pyrometamorphic rocks andby-products of their low-temperature alterations commonly occur along the Dead Sea Rift in the
territories of Israel, Palestine and Jordan (the “Mottled zone”) [3–5].
Minerals of the nabimusaite group are silicates with additional anions and are isostructural witharctite, a phosphate with an ideal crystal chemical formula of Ba(Ca7 Na5 )(PO4 )6 F3 [6]. Mineralsof the nabimusaite group and arctite can be described as hexagonal intercalated antiperovskites
with the general crystal chemical formula AB12 (TO4 )4 (TO4 )2 W 3 , A = Ba, K, . . . ; B = Ca, Na, . . . ;T = Si, P, S6+ , V5+ , Al, . . . ; W = O2− , F− [2,7]. Nabimusaite group minerals show hatrurite-like
triple antiperovskite layers {[W 3 Ca12 ](SiO4 )4 }e+ , which alternate with A(TO4 )2 e− single layers [1,2].
The same type of layers is found in arctite. However, the composition of the triple antiperovskite
module is {[F3 (Ca7 Na5 )](PO4 )4 }4+ , and the second module is Ba(PO4 )2 4− [6,8].
Minerals of the zadovite group can also be described as intercalated antiperovskites. Theirgeneral formula is AB6 (TO4 )2 [(TO4 )2−x (CO3 )x ]W (A, B, T, W = as above; x ≈ 0 or x ≈ 1): zadovite,
BaCa6 [(SiO4 )(PO4 )](PO4 )2 F; aradite, BaCa6 [(SiO4 )(VO4 )](VO4 )2 F; gazeevite, BaCa6 (SiO4 )2 (SO4 )2 O andstracherite BaCa6 (SiO4 )2 [(PO4 )(CO3 )]F. In these minerals, single antiperovskite layers {[WB6 ](TO4 )2 }e+intercalate with single Ba(TO4 )2 e− layers [7,9]. In stracherite, about half of the (PO4 )3− tetrahedra are
replaced by (CO3 )2− groups [10].
Ariegilatite is named in honor of Dr. Arie Gilat (b. 1939). Arie Gilat is retired from the Geological
Survey of Israel, where he was involved in geological mapping, tectonics and geochemical studies
for more than 30 years. He is the author and co-author of numerous geological papers. At present,
his main interests are related to the study of earthquake physics and processes at the core and mantle
interface. His continuous support, consulting and several new unconventional ideas on the genesis of
the Hatrurim Complex are greatly appreciated by the authors.
Ariegilatite was approved as a new mineral species by Commission on New Minerals,
Nomenclature and Classiﬁcation, International Mineralogical Association (CNMNC IMA) in March2017 (IMA2016-100). The material was deposited in the mineralogical collection of the Fersman
Mineralogical Museum, Leninskiy pr., 18/k2, 115162 Moscow, Russia, Catalogue Number 4956/1.
2. Methods of Investigation
The crystal morphology and chemical composition of ariegilatite and associated minerals
were examined using optical microscopes, as well as the analytical electron scanning microscopes
Philips XL30 and Phenom XL, PhenomWorld, ThermoFisher Scientiﬁc, Eindhoven, The Netherlands(Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland). Chemical analyses of ariegilatitewere performed with a CAMECA SX100 microprobe analyzer (Institute of Geochemistry, Mineralogy
and Petrology, University of Warsaw, Warszawa, Poland) at 15 kV and 10 nA using the following lines
and standards: BaLα, SKα, baryte; PKα, apatite; CaKα, wollastonite; MgKα, SiKα, diopside; FeKα,hematite, AlKα, KKα, orthoclase; TiKα, rutile; NaKα, albite, SrLα, SrTiO3 , FKα, ﬂuorphlogopite.
Raman spectra of ariegilatite were recorded on a WITec alpha 300R Confocal Raman Microscope,
WITec, Ulm, Germany (Department of Earth Science, University of Silesia, Sosnowiec, Poland)
equipped with an air-cooled solid-state laser (532 nm) and a CCD camera operating at −61 ◦ C.
The laser radiation was coupled to a microscope through a single-mode optical ﬁber with a diameter of
3.5 μm. An air Zeiss LD EC Epiplan-Neoﬂuan DIC-100/0.75NA objective was used. Raman scattered
light was focused on a broad band single mode ﬁber with an effective pinhole size of about 30 μm, anda monochromator with a 600-mm−1 grating was used. The power of the laser at the sample position
was ca. 40 mW. Integration times of 5 s with an accumulation of 15–20 scans and a resolution 3 cm−1
were chosen. The monochromator was calibrated using the Raman scattering line of a silicon plate(520.7 cm−1 ). Fitting of spectra was performed with the help of the “GRAMS” program using themixed Lorentz–Gauss function.

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Minerals 2018, 8, 109

Single-crystal X-ray diffraction data were collected from a crystal of ariegilatite (∼38 × 32 × 25 μm)
using synchrotron radiation at the super-bending magnet beamline X06DA at the Swiss Light Source,Paul Scherrer Institute, Villigen, Switzerland. The multi-axis goniometer PRIGo [11] and a PILATUS
2M-F detector, which was placed at a distance of 120 mm from the crystal with a vertical offset
of 67 mm, was used. The wavelength was set to λ = 0.70848 Å. The data were collected in asingle 180◦ omega scan with steps of 0.1◦ and 0.1-s exposures, controlled by the DA + software [12].
Data evaluation and processing was performed using the CrysAlisPro software package [13]. As astarting model, the structure of the isostructural analogue nabimusaite was used, adapted to the
expected composition. With subsequent analyses of difference-Fourier maps, the structure was refined
to R1 = 1.95%. The refinements include anisotropic displacement-parameters and have been carried out
with neutral atom scattering-factors, using the program SHELX97 [14].
3. Results
3.1. Occurrence and Description of Holotype Specimen
Ariegilatite was found in a few outcrops of pyrometamorphic rocks of the Hatrurim Complex
located in the territories of Israel, Palestine and Jordan. Investigations of a new mineral were
performed in samples of spurrite rocks collected in the Negev Desert (Hatrurim Basin, N31◦ 13
E35◦ 16 ) near Arad, Israel. Ariegilatite forms strongly ﬂattened crystals of disc-shaped form.
In thin-sections, its pseudo-aciculate morphology is usually observed (Figure 1A). The sizeof some highly-fractured crystals of ariegilatite reaches 0.5 mm with a thickness of 0.1 mm(Figure 1A). Ariegilatite is associated with spurrite, calcite, brownmillerite, shulamitite, CO3 -bearing
ﬂuorapatite, ﬂuormayenite-ﬂuorkyuygenite, periclase, brucite, barytocalcite, baryte, garnets ofelbrusite-kerimasite series, unidentiﬁed Ca-Fe- and Rb-bearing K-Fe sulﬁdes and a potentially
new mineral, Ba2 Ca18 (SiO4 )6 (PO4 )3 (CO3 )F3 O [15]. Ariegilatite is often overgrown and replaced
by stracherite (Figure 1B). Ariegilatite and stracherite are usually limited to re-crystallization zonesof dark-grey ﬁne-grained spurrite rocks, which differ from the surrounding rocks by discoloration,
development of thin calcite veins and also by local appearance of large spurrite metacrysts (up to 1 cm
in size), as well as the presence of sulﬁde mineralization.

Figure 1. Disc-shaped, ﬂattened ariegilatite crystals in spurrite rock (A), holotype specimen L15;
overgrowing and replacing of ariegilatite by stracherite (B). Arg = ariegilatite, Flm = ﬂuormayenite,
Spu = spurrite, Str = stracherite.

Ariegilatite is a colorless, transparent mineral with white streaks and vitreous luster. It does notshow any ﬂuorescence. The mineral is uniaxial, negative: ω = 1.650(2), ε = 1.647(2) (λ = 589 nm)
and non-pleochroic. The hardness was measured using micro-indentation and determined to
VHN50 = 356(16) kg·mm−2 , as an average of 16 measured spots ranging from 331–378 kg·mm−2 , which
86Minerals 2018, 8, 109

corresponds to 4–4.5 on the Mohs scale. In contrast to minerals of the nabimusaite-dargaite series [1,2],
ariegilatite does not show pronounced cleavage on (001), and the fracture is irregular. The small
size of separated crystal fragments does not allow measuring the density. Therefore, the density wascalculated on the basis of structural data and the mean composition of ariegilatite: 3.329 g·cm−3 .
Compatibility index 1 − (Kp /Kc ) = −0.017 (superior) was calculated for the empirical formula ofariegilatite of the holotype specimen (Table 1).
The Raman spectrum of ariegilatite exhibits the following strong bands (Figure 2; cm−1 ): 129, 179,
229 and 309 (lattice mode, Ba-O, Ca-O vibrations); 403 [υ2 (SiO4 )4− ]; 427 [υ2 (PO4 )2− ]; 520 [υ4 (SiO4 )4− ];569 and 591 [υ4 (PO4 )3− ]; 834 and 874 [υ1 (SiO4 )4− ]; 947 [υ1 (PO4 )3− ]; 993 [υ1 SO4 )2− ]; 1030 [υ3 (PO4 )2− ];1066 [υ1 (CO3 )2− ]. The main strong Raman bands of ariegilatite are related to vibrations in (SiO4 )4−and (PO4 )3− (Figure 2). The band at about 1066 cm−1 can be attributed to υ1 (CO3 )2− . Its positionis very close to the υ1 (CO3 )2− mode in CO3 -bearing hydroxylapatite and hydroxylellestadite [16,17].
Raman spectroscopy data indicate that H2 O is absent in ariegilatite. Direct determination of the CO2
content was not feasible due to abundant microscopic inclusions of other minerals.
The mean composition of the holotype ariegilatite, (Ba0.98 K0.01 Na0.01 )Σ1 (Ca11.77 Na0.08 Fe2+ 0.06 Mn2+ 0.05
Mg0.04 )Σ12 (Si3.95 Al0.03 Ti0.02 )Σ4 (P1.70 C0.16 Si0.10 S6+ 0.03 V0.01 )Σ2 F2.04 O0.96 , is close to the end-member formula
BaCa12 (SiO4 )4 (PO4 )2 F2 O. The amount of CO3 in ariegilatite is calculated assuming charge balance and
13 non-tetrahedral cations (Table 1, L15).
Experimental details and reﬁnement data of ariegilatite are summarized in Tables 2–5.
The structural formula of ariegilatite Ba1Ca16 Ca26 [(Si1O4 )2 (Si2O4 )2 ](P1O4 )~1.8 F12 O2 (Table 4) is
simpliﬁed to BaCa12 (SiO4 )4 (PO4 )2 F2 O according to the general formula of the nabimusaite groupminerals [2]. The ariegilatite structure is most easily described as a 1:1 stacking of the two modules{[F2 OCa12 ](SiO4 )4 }4+ and {Ba(PO4 )2 }4− along (001) (Figure 3). The sites F1 and O7 are coordinated by
six Ca atoms in an octahedral arrangement (Figure 3B). The module {Ba(PO4 )2 }4− is characterized by(P1O4 ) tetrahedra connected to six-coordinated Ba1 (Figure 3C).
Table 1. Chemical composition of ariegilatite, wt %.
L15

YV595

SS20b

Mean

22

s.d.

Range

23

s.d.

Range

14

s.d.

Range

SO3
V2 O 5
P2 O5
TiO2
SiO2
Fe2 O3Al2 O3
BaO
FeO
MnO
CaO
MgO
K2 O
Na2 O
F
CO2 *
–O=F
Total
Ba
K
Na
Ca
A
Ca
Mn2+
Fe2+
Mg
Na

0.17
0.10
9.83
0.12
19.87

0.07
0.06
0.45
0.06
0.26

0.05–0.31
0–0.17
8.96–10.55
0.05–0.25
19.52–20.42

0.03
0.06
0.06
0.08
0.24
0.03
0.03
0.05
0.10

0.07–0.18
12.14–12.41
0.24–0.46
0.09–0.39
53.19–54.40
0.11–0.22
0–0.100.16–0.36
2.96–3.34

0.22
0.08
0.55
0.04
0.36
0.09
0.11
0.35

0.32–1.02
0.13–0.51
9.63–12.53
0.24–0.40
17.70–19.62

0.12
12.26
0.32
0.29
53.84
0.14
0.04
0.22
3.17
0.57
1.33
99.72
0.98
0.01
0.01

0.60
0.32
10.39
0.31
19.21
0.34
0.17
12.01
0.49
0.26
53.77
0.38
n.d.
0.05
3.24
0.00
1.36
100.18
0.96

n.d.
0.41
10.61
0.26
18.64
0.17
0.24
12.29

0.07
0.4
0.16
0.27
0.05
0.06
0.34

0.26–0.52
10.05–11.40
0.10–0.56
18.21–19.15
0.11–0.24
0.17–0.38
11.74–12.94

0.07
0.30
0.05

0.14–0.41
53.13–54.27
0.27–0.53

0.31
0.02

54.30–55.62
0–0.070.02
0.45

0.02–0.09
2.27–3.27

0.05
0.11

0.20–0.38
2.84–3.17

1
11.77
0.05
0.06
0.04
0.08

0.02
0.02
1
11.75
0.05
0.08
0.12

0.10–0.68
10.81–12.49

n.d.
54.24
0.03
n.d.
0.31
3.05
0.62
1.28
99.58
0.99
0.01
1
11.88

0.01
0.11

87

Minerals 2018, 8, 109

Table 1. Cont.
L15

YV595

s.d.

Range

23

22

B
Si
Ti4+
Fe3+
Al
P
T1
P
Si
V5+
S6+
C
T2
O
F
W

12
3.95
0.02

Range

4
1.70
0.10
0.01
0.03
0.16
2
0.96
2.04
2

Mean

29

s.d.

Range

11

s.d.

Range

12

SO3
V2 O5
P2 O5
TiO2
SiO2
Fe2 O3Al2 O3
BaO
FeO
MnO
CaO
MgO
K2 O
Na2 O
F
CO2 *
–O=F
Total
Ba
K
Na
Ca
A
Ca
Mn2+
Fe2+
Mg
Na
B
Si
Ti4+
Fe3+
Al
P
T1
P
Si
V5+
S6+
C
T2
O
F
W

0.05
0.42
10.52
0.27
18.85
n.d.
0.25
12.26

0.08
0.07
0.52
0.16
0.42

0–0.340.24–0.62
9.48–11.65
0.05–0.59
18.25–20.30

1.35
0.18
12.91
0.18
16.46

0.22
0.11
0.43
0.06
0.45

1.10–1.92
0.04–0.41
11.89–13.60
0.09–0.28
15.81–17.16

0.08
0.41

0.14–0.48
11.39–13.15

0.10
0.49
0.11

0.40–0.75
11.42–13.31
0.03–0.38

0.36
0.16

54.00–55.58
0–0.930.08
0.15

0.06–0.37
2.68–3.37

0.13
0.02
0.02
0.04
0.07

54.25–54.68
0.06–0.12
0.02–0.10
0.15–0.32
2.15–2.48

0.02

0.53
12.17
0.17
n.d.
54.49
0.09
0.06
0.23
2.30
0.18
0.97
100.32
0.97
0.02
0.01

n.d.
n.d.
9.02
n.d.
19.83
n.d.
0.15
12.36

1
11.90

1
11.87

0.02
0.08
12
3.86
0.04

0.03
0.03
0.07
12
3.35
0.02

0.06
0.04
4
1.78

0.13
0.50
4
1.72

0.06
0.01
0.15
2
1.04
1.96
2

0.02
0.21
0.05
2
1.52
1.48
2

12
3.86
0.05
0.05
0.04

0.03

14

s.d.

Range

12
3.81
0.04
0.03
0.06
0.07
4
1.77

4
1.80
0.07
0.04
0.09

0.06
0.17
2
1.03
1.97
2

2
0.91
2.09
2
SS27A

n.d.
54.26
0.08
n.d.
0.24
3.02
0.55
1.27
99.50
0.98

s.d.

SS20b

Mean

MA5b

IS129

n.d.
54.78
n.d.
0.04
0.66
2.92
1.68
1.23
100.21
0.97
0.01
0.02

teor

s.d.

Range

0.49

8.31–10.16

11.53

0.27

19.44–20.23

19.53

0.1
0.17

0.09–0.47
12.12–12.66

12.46

0.15

54.56–55.07

54.68

0.02
0.11
0.14

0.02–0.08
0.49–0.82
2.56–3.22

3.08
0.00
1.29
100.00
1.000

1
11.76

12.000

0.24
12
3.97

4.000

0.03
4
1.53
0.01

0.46
2
1.15
1.85
2

* Calculated on charge balance, Fe2+ : calculated as 13 − (A + B) cations, n.d., not detected.

88

2.000

1.000
2.000
2

Minerals 2018, 8, 109

Figure 2. Raman spectra of ariegilatite: holotype specimen, Negev Desert, Israel (Sample Number L15);
gehlenite-larnite rock, Daba-Siwaqa, Jordan (YV595); apatite-spurrite rock, Gurim Anticline,
Negev Desert (IS129); spurrite rock, Daba-Siwaqa, Jordan (SS27A and SS20b); exsolution structures
after ﬂamite (MA5b). Grey vertical bars show the positions of characteristic bands of stretchingvibrations (υ1 ) in tetrahedral anion groups and (CO3 )2− .

89Minerals 2018, 8, 109

Table 2. Parameters for X-ray data collection.
Crystal Data
Crystal system

trigonal
a = 7.1551(6), c = 41.303(3) Å, b = 7.1551(6)c = 41.303(3)α, β = 90◦ γ = 120◦
R3m (no.166)
1831.2(3)3
3.329 g·cm−3~BaCa12 (SiO4 )4 (PO4 )1.8 F2 O
38 × 32 × 25 μm

Unit cell dimensions (Å)
Space group
Volume (Å3 )Z
Density (calculated)
Chemical formula
Crystal size (mm)Data Collection
beamline X06DA, SLS
multi-axis goniometer PRIGoPILATUS 2M-F detector
λ = 0.70848 Å
32.139−10 ≤ h ≤ 6−8 ≤ k ≤ 10
−59 ≤ l ≤ 44
3445
825
822

DiffractometerMax. θ◦ -range for data collection
Index ranges

No. of measured reﬂections
No. of unique reﬂections
No. of observed reﬂections (I > 2σ (I))

Reﬁnement of the Structure
No. of parameters used in reﬁnement
Rint
Rσ
R1, I > 2σ(I)R1 all DatawR2 on (F2)GooF
Δρ min (−e. Å−3 )Δρ max (e. Å−3 )58
0.01990.01050.01950.01950.0555
1.128
−0.54
0.82

The antiperovskite module {[F2 OCa12 ](SiO4 )4 }4+ consists of columns formed by Ca-triplets Ca3 O14 ,rotated relative to each other by 60◦ (Figure 4A,B). Ca-triplets form four layers with (SiO4 )4− tetrahedra
in structural cavities (Figure 4C). The structure of hatrurite (Ca3 SiO5 ) shows similar antiperovskite
modules [1,18,19].
Raman and electron microprobe data (Figure 2, Table 1, L15) indicate the presence of small
amounts of (CO3 )2− in ariegilatite, replacing (PO4 )3− groups. Refinement of the occupancy factorof P1 (Table 3) shows a small reduction from full occupation (89%), which is in agreement with thesubstitution by CO3 . The structural formula of ariegilatite calculated assuming a balanced charge,
BaCa12 (SiO4 )4 {[(PO4 )0.890.11 ](CO3 )0.11 }2 F1.78 O1.22 , is close to the empirical formula obtained frommicroprobe analyses (Table 1, L15). However, the exact location and orientation of the planar CO3 groupcannot be determined from diffraction data. Most likely, replacement of (PO4 )3− by planar (CO3 )2−groups takes place the same way as in stracherite: CO3 triangles are randomly located along one of
the faces of the replaced tetrahedra excluding the face parallel to (001) [10]. Replacement of (PO4 )3−by (CO3 )2− in ariegilatite takes place according to the schemes: Ca2+ + (PO4 )3− ↔ Na+ + (CO3 )2− ,2(PO4 )3− ↔ (SiO4 )4− + (CO3 )2− and (PO4 )3− + F− ↔ (CO3 ) 2− + O2− .
As ariegilatite occurs only in tiny amounts, and the crystals contain numerous inclusions of otherphases, useful X-ray powder diffraction data could not be collected. However, a powder pattern was
calculated (Table S1) using the model derived from the single-crystal structure reﬁnements.

90Minerals 2018, 8, 109

Table 3. Atom coordinates, Ueq (Å2 ) values for ariegilatite.
Atom

x/a

y/b

z/c

sof

Ueq

Ba1
Ca1
Ca2
P1
Si1
Si2
O1
O2
O3
O4
O5
O6
O7
F1

0
0.16368(3)0.15534(3)0000.55031(11)0.12532(10)0.12681(11)
0
0
0
0
0

0
0.83632(3)0.84466(3)0000.44969(11)0.87468(10)0.87319(11)
0
0
0
0
0

0
0.39407(2)0.53179(2)0.67473(2)0.20615(2)0.08404(2)0.64495(3)0.19359(3)0.07376(3)0.36244(5)0.75407(5)0.12420(5)0.50.43179(4)11
10.892(4)
1
1
1
1
1
1
1
1
1
1

0.01731(9)0.01059(12)0.01036(12)0.0085(2)0.00650(17)0.00723(17)0.0148(4)0.0130(4)0.0140(4)0.0125(5)0.0132(5)0.0113(5)0.0057(6)0.0134(4)Table 4. Anisotropic displacement parameters Uij for ariegilatite.
Atom

U 11

U 22

U 33

U 23

U 13

U 12

Ba1
Ca1
Ca2
P1
Si1
Si2
O1
O2
O3
O4
O5
O6
O7
F1

0.01604(12)0.00850(14)0.00855(14)0.0087(3)0.0053(2)0.0063(2)0.0156(4)0.0113(4)0.0107(4)0.0136(6)0.0147(6)0.0114(6)0.0059(7)0.0131(5)0.01604(12)0.00850(14)0.00855(14)0.0087(3)0.0053(2)0.0063(2)0.0156(4)0.0113(4)0.0107(4)0.0136(6)0.0147(6)0.0114(6)0.0059(7)0.0131(5)0.01985(15)0.01499(17)0.01337(17)0.0082(4)0.0089(3)0.0091(3)0.0144(5)0.0194(6)0.0220(6)0.0103(8)0.0102(8)0.0110(8)0.0054(10)0.0140(8)0.00802(6)0.00441(12)0.00382(12)0.00437(15)0.00264(11)0.00316(11)0.0088(5)0.0079(5)0.0065(5)0.0068(3)0.0074(3)0.0057(3)0.0030(4)0.0066(3)00.00013(5)−0.00066(5)0000.0010(2)0.0009(2)0.0015(2)
0
0
0
0
0

0
−0.00013(5)0.00066(5)000−0.0010(2)−0.0009(2)−0.0015(2)
0
0
0
0
0

Table 5. Selected interatomic distances (Å) for ariegilatite.
Atom

-Atom

Distance

Ba1

-O1
mean

2.8346(10)2.8346

Ca1

-O5
-O3
-O4
-O1
F1
mean
bvs

2.3735(11)2.4006(9)2.4128(12)2.4628(12)2.5577(12)2.43871.921(2)

-O6
-F1
-O2
-O7
-O2
-O3
mean
bvs
bvs

2.2495(5)2.4431(12)2.3301(8)2.3303(5)2.5114(15)2.6028(15)2.3996
2.196(2)1.024(1)

Ca2

Atom

-Atom

Distance

P1

-O4
-O1

1.535(2)1.5484(11)

mean
bvs
-O2
-O5
mean
bvs
-O3

1.54514.859(9)1.6374(10)1.643(2)1.63883.843(7)1.6279(10)

-O6
mean
bvs
-Ca2
mean
bvs
-Ca1
-Ca2
mean

1.659(2)1.6356
3.879(7)2.3303(5)2.33032.248(1)2.5577(12)2.4431(12)2.5004

×6

×2
×2

Si1

Si2

×2
O7

F1

91

×3

×3

×3

×6
×3
×3

Minerals 2018, 8, 109

Figure 3. (A) The structure of ariegilatite, BaCa12 (SiO4 )4 (PO4 )2 F2 O, described as a 1:1 stacking of thetwo modules {[F2 OCa12 ](SiO4 )4 }4+ and {Ba(PO4 )2 }4− along (001). Atoms F1 and O7 are surroundedby six Ca atoms and form face-sharing octahedra (green and blue). (B) Module {[F2 OCa12 ](SiO4 )4 }4+consisting of close-packed seven-fold coordinated Ca (dark yellow ﬁlled circles) with (SiO4 ) ﬁlling thegaps (red and purple tetrahedra). (C) The {Ba(PO4 )2 }4− module is characterized by (PO4 ) tetrahedra(lilac) connected to six-fold coordinated Ba (purple translucent octahedra).

Figure 4. Using a cation-centered representation of the ariegilatite structure (A–C), the antiperovskite
module can be decomposed into four layers assembled of triplets of Ca coordination polyhedra.
Structural voids within each layer accommodate the silicate tetrahedra. Perpendicular through thelayers, columns of four Ca-triplets can be identiﬁed. These are shown in a projection along c and[010] in (B). The layers of Ca-triplets are connected by common anions in octahedral coordination:
Ca1 and Ca2 layers are sharing F1; Ca2 and Ca2 are connected by O7. In (C), two layers of Ca-triplets
are removed at the top half of the image.

92

Minerals 2018, 8, 109

3.2. Occurrences of Ariegilatite in Other Localities of the Hatrurim Complex Rocks
Ariegilatite occurs both in larnite and spurrite rocks of the Hatrurim Complex, whereas
minerals of the nabimusaite-dargaite series are found in larnite rocks only [1,2]. The first samples
of ariegilatite were found in 2011 by M. Murashko in larnite pebble, in the northern part of
the Siwaqa pyrometamorphic rock area, 80 km south of Amman, Jordan. Daba-Siwaga is thelargest area of the Hatrurim Complex within the Dead Sea rift region [20–22]. In this samples,ariegilatite is associated with larnite, gehlenite, spinel, fluormayenite, fluorapatite, perovskite anda bredigite-like Ba-bearing mineral (Figure 5A). It forms poikilitic crystals up to 0.25 mm in size andfine reaction rims on fluorapatite (Figure 5A). The mean ariegilatite composition from larnite rockis (Ba0.96 K0.02 Na0.02 )Σ1 (Ca11.75 Mg0.12 Fe2+ 0.08 Mn2+ 0.05 )Σ12 (Si3.86 Ti4+ 0.05 Fe3+ 0.05 Al0.04 )Σ4 (P1.80 S6+ 0.09 Si0.07
V5+ 0.04 )Σ2 F2.09 O0.91 (Table 1, YV595); here, Mg is slightly increased in comparison to the holotype
specimen (Table 1, L15). The Raman spectrum of ariegilatite from larnite rocks in Jordan shows
no characteristic bands of (CO3 )2− groups (Figure 2, YV595). Furthermore, the calculation of the
stoichiometric formula does not indicate the presence of (CO3 )2− (Table 1, YV595).
Investigation of spurrite rocks (+calcite, ﬂuorapatite, ﬂuormayenite-ﬂuorkyuygenite,
brownmillerite and periclase) occurring within a 1–2-km radius from the place, where thelarnite pebble was found at the location in Siwaqa, revealed ariegilatite metacrysts 30–50 μm in size
(Figure 3B). These rocks are locally enriched in sphalerite and oldhamite and are interspersed withcalcite veins of 1–2 mm in thickness. As well as the holotype material, ariegilatite from spurrite rocksof Jordan contains about 0.15–0.17 (CO3 )2− pfu (Table 1, SS20b and SS27A). The presence of (CO3 )2−
is also conﬁrmed by Raman spectroscopy (Figure 2, SS20b and SS27A).

Figure 5. Cont.

93

Minerals 2018, 8, 109

Figure 5. Poikilitic crystals of ariegilatite in gehlenite-larnite rock (A), where it also forms reaction rims
on ﬂuorapatite, Daba-Siwaqa, Jordan (Sample No. YV595); poikilitic crystals of ariegilatite in spurriterock (B), Daba-Siwaqa, Jordan (SS20b); replacement of ﬂamite by ariegilatite with exsolution structures(C), Ma’ale Adummim, Palestinian Autonomy (MA5b); apatite-spurrite rock with ariegilatite andchannels ﬁlled with calcite (D). The fragment magniﬁed in (E) is shown in the frame; Gurim Anticline,
Negev Desert, Israel (IS129); xenomorphic aggregates of ariegilatite with spurrite inclusions (E);cathodoluminescence image of the area shown in (D,F), round shape of the channel, the walls of
which are incrusted by oldhamite, having intensive luminescence. Arg = ariegilatite, Ap = ﬂuorapatite,
Brm = brownmillerite, Cal = calcite, Etr = ettringite, Flm = ﬂamite, Fl = ﬂuorite, Ghl = gehlenite,
Lrn = larnite; May = ﬂuormayenite-ﬂuorkyuygenite, Old = oldhamite, Sp = spinel, Spu = spurrite.

Ariegilatite was also found in the territory of Palestinian Autonomy in an outcrop near theJerusalem-Dead Sea highway close to Ma’ale Adummim. At this location, the mineral was detected
in altered flamite rocks (+fluormayenite-fluorkyuygenite, brownmillerite, fluorapatite, gehlenite)with heterogeneous development of jasmundite. Flamite is an analog of the synthetic phase
α-Ca2 SiO4 stabilized by K, Na and P impurities [23,24]. Flamite usually shows exsolution structureswith individual lamellas enriched in P [24]. Ariegilatite replaces flamite lamellas with increased
phosphorus content (Figure 5C) and exhibits a mean composition of (Ba0.97 K0.02 Na0.01 )Σ1 (Ca11.87 Na0.07
Mg0.03 Fe2+ 0.03 )Σ12 (Si3.35 P0.50 Al0.13 Ti4+ 0.02 )Σ4 (P1.72 S6+ 0.21 C0.05 V5+ 0.02 )Σ2 F1.48 O1.52 , and it is characterized
by increased (SO4 )2− content (Table 1, MA5b); accordingly, its Raman spectrum shows a strong
υ1 (SO4 )2− band near 995 cm−1 (Figure 2, MA5b).
Ariegilatite samples showing the highest (CO3 )2− contents were detected influorapatite-calcite-spurrite rocks in the Negev Desert near the junction of Hatrurim (Gurim Anticline,
Hatrurim Basin), Israel (Figure 5D,E). In these rocks, also cuspidine, fluorite, brownmillerite,
chlormayenite, periclase, sphalerite, millerite, U-bearing lakargiite, vorlanite and oldhamite
are identified. The empirical formula of ariegilatite from this locality is (Ba0.97 K0.01 Na0.02 )Σ1(Ca11.76 Na0.24 )Σ12 (Si3.97 Al0.03 )Σ4 (P1.53 C0.46 Si0.01 )Σ2 F1.85 O1.15 (Table 1, IS129). Its Raman spectrum
reveals a characteristic band of (CO3 )2− groups (Figure 2, IS129). The (CO3 )2− groups can beincorporated according to two schemes: (PO4 )3− + Ca2+ ↔ (CO3 )2− + Na+ and (PO4 )3− + F− ↔
(CO3 )2− + O2− . Interestingly, in spurrite rock, an ariegilatite relic net of dendrite channels (with an oval
cross-section of 0.2–0.4 mm in diameter) filled with coarse-crystalline calcite was observed. Walls of thechannels are incrusted by small oldhamite crystals up to 1–2 μm in size. Thanks to intensive oldhamite
cathodoluminescence, these channels are well visible within the background of the weakly-luminescentminerals of spurrite rocks (Figure 5F).

94

Minerals 2018, 8, 109

4. Discussion
Ariegilatite exhibits an ordered anion distribution. Fluorine and oxygen were assigned to theanion sites utilizing the results of bond valence sum (BVS, Table 5) calculations. Oxygen resides
in the center (Figure 3A, blue) and ﬂuorine on the two outer octahedral sites (Figure 3A, green)of the antiperovskite module. New BVS calculations revealed the same preference in nabimusaite,where one ﬂuorine atom seems to be distributed over the two outer octahedral sites, in contrast towhat has been reported before [1,2,25]. Fluorine may enter the central octahedra when the charge
is compensated as, for example, in arctite BaCa7 Na5 (PO4 )6 F3 [6,8], where Ca atoms (of the central
octahedra) are substituted by sodium. Similar behavior is observed in the isostructural synthetic
compound Ca5.45 Li3.55 [SiO4 ]3 O0.45 F1.55 [26], where the outer octahedral sites are occupied by ﬂuorine,
whereas the inner site seems to be shared by oxygen and ﬂuorine. For charge compensation,
the Ca-sites coordinating the inner octahedral site are partly substituted by lithium.
The main schemes of isomorphism, unifying minerals of the nabimusaite group and arctite are
Ca2+ + SiO4 4− = Na+ + PO4 3− and Ca2+ + O2− = Na+ + F− . Arctite was the ﬁrst mineral with ahexagonal intercalated antiperovskite structure to be discovered. We propose to distinguish an arctite
super group (hexagonal intercalated antiperovskites), which at present combines two silicate groups:
nabimusaite (nabimusaite, dargaite, ariegilatite, with triple-layer antiperovskite modules) and zadovite(zadovite, aradite, gazeevite, stracherite, with single-layer antiperovskite modules) and arctite, as anungrouped phosphate.
According to Krivovichev [19], polyphite, Na6 (Na4 Ca2 )2 Na2 Ti2 Na2 Ti2 (Si2 O7 )2 (PO4 )6 O4 F4 [27],also belongs to the hexagonal intercalated antiperovskites. However, we propose that polyphite,
the structurally-related sobolevite, Na12 Ca(NaCaMn)Ti2 (Ti,Mn)[Si2 O7 ]2 (PO4 )4 O3 F3 , and quadruphite,
Na14 Ca2 Ti4 [Si2 O7 ]2 (PO4 )4 O4 F2 [27], should not be considered in the arctite super group. In thesestructures, columns of anion-centered octahedra form continuous layers parallel to titanium-silicate
sheets. The proposed arctite super group exhibits A(TO4 ) layers, which are perpendicular to the“antiperovskite columns” (Figure 3A).
So far, ariegilatite shows the highest fluorine content (>3 wt %, Table 1) of all members of thenabimusaite group. It occurs in spurrite, as well as in larnite rocks (Figures 1 and 5). Ariegilatite fromspurrite rocks contain significant amounts of (CO3 )2− (Table 1). It forms relatively large poikilitic crystals
standing out against a fine-grained matrix and grows after fluorapatite (Figures 1 and 5). Minerals of
the dargaite-nabimusaite series (and also gazeevite), as well as minerals of the ternesite-silicocarnotite
series and jasmundite are high-temperature alteration products of minerals of an early clinker-likeassociation of larnite rocks. These alterations took place under the influence of pyrometamorphism
by-products, such as gases and fluids generated by closely-spaced combustion foci [1,28,29].
In our understanding, ariegilatite, as well as associated stracherite are the products ofhigh-temperature alteration of early pyrometamorphic rocks with increased phosphorus (ﬂuorapatite)and decreased sulfur (absence of ﬂuorellestadite and ye’elimite) contents under the inﬂuenceof high-ﬂuorine, CO2 -bearing ﬂuids/gases. Transport of these ﬂuids/gases is realized throughcracks and micro-channel nets (Figure 5F), which may have existed for a long time in heatedpyrometamorphic rocks.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/3/19/s1,
Table S1: Calculated powder pattern for ariegilatite. Intensities were calculated using the software JANA2006 [30].
Acknowledgments: The investigations were partially supported by the National Science Centre of Poland,
Grant No. 2016/23/B/ST10/00869. We would like to thank Jakub Kaminski for important technical assistanceduring the diffraction experiments at X06DA.

95

Minerals 2018, 8, 109

Author Contributions: Evgeny V. Galuskin, Biljana Krüger, Irina O. Galuskina and Hannes Krüger wrote
the paper. Evgeny V. Galuskin, Irina O. Galuskina and Yevgeny Vapnik took part in the ﬁeld works, by the
results of which ariegilatite was discovered. Mikhail Murashko found the ﬁrst specimen with ariegilatite in
Jordan. Evgeny V. Galuskin and Irina O. Galuskina performed petrological investigations, measurement ofariegilatite composition from different localities, Raman and optical studies and also selected grains for structuralinvestigations. Biljana Krüger and Hannes Krüger performed SC XRD investigation and reﬁned ariegilatite
structure. Justyna A. Wojdyla helped in single-crystal structural investigations using synchrotron radiation at thesuper-bending magnet beamline X06DA at the Swiss Light Source.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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minerals
ArticleDynamic Disorder of Fe3+ Ions in the Crystal
Structure of Natural Barioferrite
1, *, Taras L. Panikorovskii 2 , Irina O. Galuskina 1 and Evgeny V. Galuskin 1
Arkadiusz Krzatała
˛
1
2

*

Faculty of Earth Sciences, University of Silesia, B˛edzińska 60, 41-200 Sosnowiec, Poland;
irina.galuskina@us.edu.pl (I.O.G.); evgeny.galuskin@us.edu.pl (E.V.G.)
Kola Science Centre, Russian Academy of Sciences, 14 Fersman street, Apatity 184200, Russia;
taras.panikorovsky@spbu.ru
Correspondence: akrzatala@us.edu.pl; Tel.: +48-32-368-9689

Received: 13 July 2018; Accepted: 6 August 2018; Published: 8 August 2018

Abstract: A natural barioferrite, BaFe3+ 12 O19 , from a larnite–schorlomite–gehlenite vein ofparalava within gehlenite hornfels of the Hatrurim Complex at Har Parsa, Negev Desert,
Israel, was investigated by Raman spectroscopy, electron probe microanalysis, and single-crystal
X-ray analyses acquired over the temperature range of 100–400 K. The crystals are up
to 0.3 mm × 0.1 mm in size and form intergrowths with hematite, magnesioferrite,khesinite, and harmunite. The empirical formula of the barioferrite investigated is asfollows: (Ba0.85 Ca0.12 Sr0.03 )∑1 (Fe3+ 10.72 Al0.46 Ti4+ 0.41 Mg0.15 Cu2+ 0.09 Ca0.08 Zn0.04 Mn2+ 0.03 Si0.01 )∑11.99 O19 .
The strongest bands in the Raman spectrum are as follows: 712, 682, 617, 515, 406, and 328 cm−1 .
The structure of natural barioferrite (P63 /mmc, a = 5.8901(2) Å, c = 23.1235(6) Å, V = 694.75(4) Å3 , Z = 2)
is identical with the structure of synthetic barium ferrite and can be described as an interstratiﬁcationof two fundamental blocks: spinel-like S-modules with a cubic stacking sequence and R-modules
that have hexagonal stacking. The displacement ellipsoids of the trigonal bipyramidal site show
elongation along the [001] direction during heating. As a function of temperature, the mean apical
Fe–O bond lengths increase, whereas the equatorial bond lengths decrease, which indicates dynamic
disorder at the Fe2 site.
Keywords: barioferrite; crystal structure; single-crystal investigation; Raman; Hatrurim Complex

1. Introduction
Barioferrite, BaFe12 O19 , P63 /mmc, a = 5.8921(2) Å, c = 23.1092(8) Å was described as a newmineral species from a metamorphosed baryte nodule, which was found on the southern slope of
Har Ye’elim, a mountain in the Negev Desert, Israel. In this nodule, barioferrite occurs as small platy
crystals up to 3 × 15 × 15 μm3 , as well as their aggregates. Therefore, the unit cell dimensions of
the holotype specimen were obtained only from powder X-ray diffraction data [1]. There are several
descriptions of synthetic BaFe12 O19 [2–5], whereas there are no previous single-crystal structure studies
of natural BaFe12 O19 .
Barioferrite belongs to the magnetoplumbite group. This group is characterized by the generalcrystal chemical formula AM12 O19 , where A = Ba, Pb, K, Ca, Fe2+ , Mn, Mg, REE and M = Fe2+,3+ , Mg,
Al, Ti4+ , Cr3+ , Mn2+,4+ , Zn [6]. Minerals of the magnetoplumbite group are characterized by a modular
structure composed of three-layered spinel blocks (S) interstratiﬁed with R-blocks, in which the larger
A-site cations are located [7].
A natural occurrence of BaFe12 O19 was ﬁrst reported from Si-undersaturated Ca-Fe-rich buchitein porcelanite deposits at Želénky in the North Bohemian Brown Coal Basin, Czech Republic [8].
Our investigations have shown that barioferrite, occurring in small segregations intergrownwith other ferrites, is a common accessory mineral of rankinite-bearing paralavas that are hosted byMinerals 2018, 8, 340; doi:10.3390/min8080340

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gehlenite hornfelses that belong to the pyrometamorphic Hatrurim Complex [9–11]. Pyrometamorphic
rocks of the Hatrurim Complex are distributed in the territory of Israel, the Palestinian Autonomy,
and Jordan along the rift of the Dead Sea [12–15].
Synthetic BaFe12 O19 is well known under such names as barium ferrite, hexaferrite,
barium hexaferrite, ferroxdure, M ferrite, and BaM [7]. An X-ray single-crystal study of synthetic
BaFe12 O19 was ﬁrst undertaken by Townes [16], and the structure was reinvestigated in detail by
Obradors [2]. They found that it is isostructural with magnetoplumbite, PbFe3+ 12 O19 , which was
described by Aminoff (1925) [17] and structurally characterized by Adelsköld in 1938 [7]. Many authors
have tried to reﬁne the crystal structure of synthetic BaFe12 O19 in different space groups (e.g., P-62c andP63 mc), and all of them considered space group P63 /mmc as the most probable one [2–5]. In space group
P63 /mmc, the Fe2 site has trigonal bipyramid coordination [18]; this site demonstrates splitting or
extreme elongation of the displacement ellipsoid along the [001] direction [2]. The relative displacementof iron from the center of the FeO5 bipyramid induces a local electric and dipole moment, which affects
the paraelectric/ferroelectric properties of hexaferrites [19].
In this paper, we present the results of the single-crystal structural investigation of natural
barioferrite for the ﬁrst time. Also, powder diffraction data, chemical composition, and single-crystal
Raman spectroscopic data are provided for the studied mineral. We pay attention to the structuraldeformations of the trigonal bipyramidal site at different temperatures and consider mechanisms forthe incorporation of Al, Ti, and Mg into the structure of natural barioferrite.
2. Methods and Description
2.1. Geological Setting and Mineral Description
The barioferrite investigated was found in the outcrops at Har Parsa, which is located in theeastern part of the Hatrurim Basin, about 10 km southeast of Arad, Negev Desert, Israel. The Hatrurim
Basin is the one of the largest units in the Hatrurim pyrometamorphic complex. It is a generally
recognized fact that the terrigenous-carbonate protolith of the Hatrurim Complex was driven by
combustion processes [20–23]. The geology and origin of the Hatrurim Complex have been discussed
previously in some detail [21–26]. Many new minerals including aradite, zadovite [9], gurimite,hexacelsian [10], ariegilatite [27], ﬂamite [28], shulamitite [29], barioferrite [1], silicocarnotite [30],and gazeevite [31] have been described from this locality.
Barioferrite forms platy submetallic crystals in a small paralava vein up to 1.5 cm thickin gehlenite hornfels at Har Parsa. Some barioferrite crystals reach 0.3 mm × 0.1 mm in size
(Figure 1b,c). Minerals of the larnite-flamite and andradite-schorlomite series, Fe3+ -bearing gehlenite,magnesioferrite, and khesinite, are rock-forming minerals in this paralava (Figure 1a). Barioferrite forms
intergrowths with hematite, magnesioferrite MgFe2 O4 , khesinite Ca4 Mg2 Fe3+ 10 O4 (Fe3+ 10 Si2 )O36
(often with Ca > Mg at the Mg site [11]), and harmunite CaFe2 O4 (Figure 1d) and is also present as small
inclusions in gehlenite (Figure 1a). Minor and accessory minerals, in addition to the above-listed ferrites,
are represented by vorlanite CaUO4 , Si–Fe-bearing perovskite, P-bearing fluorellestadite, and baryte.
2.2. Methods of Investigation
The morphology and composition of barioferrite and associated minerals were studied using
optical microscopy, scanning electron microscopy (Phenom-World, Eindhoven, The Netherlands))(Phenom XL, Faculty of Earth Sciences, University of Silesia, Sosonowiec, Poland), and an electronprobe microanalyzer (Cameca, Gennevilliers, France) (Cameca SX100, Institute of Geochemistry,
Mineralogy and Petrology, University of Warsaw, Warsaw, Poland). Elemental analyses were carriedout (WDS-mode, 15 kV, 20 nA, ~1 μm beam diameter) using the following X-ray lines (detection limit,
wt %) and standards: Si Kα (0.03), Ca Kα (0.04), Mg Kα (0.03)—diopside; Al Kα (0.03)—orthoclase;
Ti Kα (0.05)—TiO2 ; Mn Kα (0.10)—rhodonite; Fe Kα (0.13)—Fe2 O3 ; Ba Lα (0.11)—baryte; Sr Lα99

Minerals 2018, 8, 340(0.09)—celestine; Cu Kα (0.17)—cuprite; and Zn Kα (0.23)—sphalerite. Corrections were calculatedusing a PAP procedure provided by CAMECA [32].

Figure 1. (a) Fragments of larnite-gehlenite paralava from Har Parsa with barioferrite aggregatesand gehlenite porphyroblasts (megacrysts); ﬁne-grained greenish gehlenite rock occurs in the center.
The minerals can be distinguished by color: garnet—dark-brown; gehlenite—orange to yellow-brown;
larnite—light-grey to colorless; barioferrite—black; secondary hydrous Ca-silicates—chalk white tolight-grey. The area shown in the red frame is magniﬁed in Figure 1b; (b) backscattered-electron(BSE) image of larnite-gehlenite paralava with barioferrite crystals. The framed areas are magniﬁed
in Figure 1c,d; (c) subhedral barioferrite crystals with harmunite; (d) aggregate composed ofmagnesioferrite, hematite, and khesinite with lamellar barioferrite crystals. Labels: Bfr—barioferrite,
Ghl—gehlenite, Hem—hematite, Hrm—harmunite, HSi—hydrous silicate, Khs—khesinite, Ln—larnite,Mgf—magnesioferrite, Schr—schorlomite, and Vorl—vorlanite.

The Raman spectra of barioferrite were recorded using a WITec alpha 300R Confocal Raman
Microscope (WITec, Ulm, Germany) (Faculty of Earth Sciences, University of Silesia, Sosnowiec,
Poland) equipped with an air-cooled solid-state laser (532 nm) and a CCD camera operating at −61 ◦ C.
The laser radiation was coupled to a microscope through a single-mode optical ﬁber with a diameter of
3.5 μm. A Zeiss LD EC Epiplan-Neoﬂuar DIC-100/0.75NA objective was used. Raman scattered light
was focused on a broadband single-mode ﬁber with an effective pinhole size of about 30 μm, and a
monochromator with a 600 groove/mm−1 grating was used. The power of the laser at the sample
position was ca. 25–30 mW. Integration times of 10 s with an accumulation of 20 scans and a resolution
3 cm−1 were chosen. The monochromator was calibrated using the Raman scattering line of a siliconplate (520.7 cm−1 ). Spectra processing, such as baseline correction and smoothing, was performed
using the SpectraCalc software package GRAMS (Galactic Industries Corporation, Salem, NH, USA).
Band-ﬁtting was performed using a Gauss–Lorentz cross-product function, with a minimum numberof component bands used for the ﬁtting process.
Powder X-ray diffraction patterns of barioferrite were obtained using a Rigaku R-AXIS RAPID II
diffractometer (Rigaku Co., Tokyo, Japan) (Saint Petersburg State University, Saint Petersburg, Russia)
equipped with a cylindrical image plate detector using the Debye-Scherrer geometry (d = 127.4 mm;100

Minerals 2018, 8, 340

Co Kα radiation) and measured at room temperature. The data were integrated using the software
package Osc2Tab/SQRay [33].
The single-crystal X-ray diffraction experiments were carried out at 100, 200, 300, and 400 K
using an Agilent Technologies Xcalibur Eos diffractometer (Agilent Technologies, Santa Clara, CA,
USA ) (Saint Petersburg State University, Saint Petersburg, Russia) operated at 50 kV and 40 mA andequipped with an Oxford Cobra Plus system (Table 1). More than a hemisphere of three-dimensional
data was collected using monochromatic Mo Kα X-radiation with frame widths of 1◦ and 40–80 s counttime for each frame. The systematic absences observed are consistent with space group P63 /mmc.
Table 1. Crystal data and structure reﬁnement for barioferrite at 100, 200, 300, and 400 K.
Identiﬁcation Code

100 K

200 K

300 K

400 K

Temperature (K)100(2)

200(2)

300(2)

400(2)

Crystal system, Z

Hexagonal

Space group

P63 /mmc

a (Å)

5.8920(2)5.8901(2)5.8901(2)5.8992(2)c (Å)

23.1092(7)23.1171(7)23.1235(6)23.1891(9)698.87(6)Volume (Å3 )694.78(5)694.57(4)694.75(4)ρcalc (g/cm3 )

5.183

5.184

5.183

5.152

μ (mm−1 )14.13114.13614.13214.049F(000)

1017.01017.01017.01017.00.20 × 0.09 × 0.07

Crystal size (mm3 )Radiation

Mo Kα (λ = 0.71073)

2θ range (◦ )7.054–64.038

7.05–63.804

7.048–61.54

7.028–64.436

hkl range−8 ≤ h ≤ 8, −6 ≤ k≤ 6, −33 ≤ l ≤ 33

−8 ≤ h ≤ 8, −8 ≤k ≤ 8, −32 ≤ l ≤ 32

−7 ≤ h ≤ 7, −8 ≤ k≤ 8, −32 ≤ l ≤ 28

−8 ≤ h ≤ 8, −7 ≤ k≤ 8, −32 ≤ l ≤ 32

Reﬂections collected

3907

6363

6065

6804

Independent
reﬂections

501 [Rint = 0.0176,
Rsigma = 0.0106]

505 [Rint = 0.0300,
Rsigma = 0.0148]

461 [Rint = 0.0281,
Rsigma = 0.0121]

519 [Rint = 0.0279,
Rsigma = 0.0118]

Data/restraints/
parameters501/0/42

508/0/42

461/0/42

519/0/42

GooF

1.229

1.270

1.288

1.268

Final R indexes[I ≥ 2σ (I)]R1 = 0.0142,
wR2 = 0.0364

R1 = 0.0207,
wR2 = 0.0493

R1 = 0.0161,
wR2 = 0.0425

R1 = 0.0173,
wR2 = 0.0433

Final R indexes[all data]

R1 = 0.0164,
wR2 = 0.0372

R1 = 0.0234,
wR2 = 0.0506

R1 = 0.0179,
wR2 = 0.0431

R1 = 0.0193,
wR2 = 0.0440

Largest difference
peak/hole (e Å−3 )0/53/−0.56

1.13/−0.99

0.43/−0.89

0.79/−1.16

2.3. Chemical Composition and Raman Spectroscopy
To identify barioferrite, we performed electron probe microanalysis and Raman spectroscopy.
Barioferrite from the paralava of Har Parsa has a relatively constant composition and contains
substantial substitutional elements, including Ti4+ , Al, and Ca (Table 2). The presence of Cu
and Zn and also the interpreted incorporation of small amounts of Ca in the spinel module
is a speciﬁc feature not only of barioferrite from Har Parsa paralava, but also barioferrite andmagnesioferrite from other localities of pyrometamorphic rocks of the Hatrurim Complex [11,29,34].
The positive charge caused by substituted trivalent iron by tetravalent titanium is compensatedby the presence of divalent elements. The empirical crystal chemical formula of barioferrite is as101

Minerals 2018, 8, 340

follows: (Ba0.85 Ca0.12 Sr0.03 )∑1 (Fe3+ 10.72 Al0.46 Ti4+ 0.41 Mg0.15 Cu2+ 0.09 Ca0.08 Zn0.04 Mn2+ 0.03 Si0.01 )∑11.99 O19 .
This composition of barioferrite is close to the end-member BaFe12 O19 .
In the barioferrite structure, the spinel blocks are composed of Fe(3) O4 tetrahedra and Fe(1,4) O6
octahedra, whereas R-blocks are formed by Fe(2) O5 trigonal bipyramids and Fe(5) O6 octahedra withdodecacoordinated Ba in structural cages. The main bands observed in the Raman spectra of thestudied barioferrite are associated with Fe3+ O vibrations and shifted towards the higher Raman shift
frequency in comparison with analogous bands of synthetic barioferrite (our data, Figure 2/synthetic
barioferrite [35], cm−1 ): 714/713 [A1g , tetrahedron Fe(3) O4 ]; 689/684 [A1g , bipyramid Fe(2) O5 ];624/614 [A1g , octahedron Fe(5) O6 ]; 532/527, 520/512 [A1g , octahedron Fe(1) O6 and Fe(4) O6 ]; 412/409[A1g , Fe(4) O6 octahedron dominated]; 335/335 + 317 [E1g + E2g ]. Similar band shifts were observedin Raman spectrum of synthetic barioferrite Ba1−x Cax Fe12 O19 , with x = 0.1 [36]. In the studied
barioferrite, 0.12 Ca pfu is present at the Ba site (Table 2), which most probably determines the band
shifts. Also, we cannot exclude an inﬂuence of Al and Ti4+ impurities substituting for Fe3+ (Table 2) on
bands position in the Raman spectrum. A similar effect was described before for Ti-bearing synthetic
barioferrite [37]. Moreover, wide band centered about 1359 cm−1 is overtone (Figure 2). The bands
at 164 and 178 cm−1 are assigned to the motion of the spinel block as a whole [35]. The band near
673 cm−1 is not usually noted in barioferrite spectra [35,37]; it was observed once in the spectrum ofpure barioferrite [38], and it is possible, that this band is related to Fe3+ in the bipyramid.
Table 2. Chemical composition of barioferrite from Har Parsa.
Component (wt %)Mean (n = 17)

Range

TiO2
SiO2
Fe2 O3Al2 O3
BaO
MgO
CaO
MnO
SrO
CuO
ZnO

3.05
0.05
80.47
2.22
12.32
0.58
1.03
0.18
0.32
0.65
0.33
101.2

2.90–3.27
0.00–0.10
78.89–81.53
2.11–2.38
11.56–13.41
0.51–0.68
0.83–1.32
0.11–0.23
0.14–0.43
0.48–0.84
0.19–0.47
100.09–101.69

Standard
Deviation.
0.09
0.02
0.72
0.06
0.46
0.05
0.17
0.04
0.07
0.11
0.08

Calculated on
19 O
Ba
Ca
Sr
A site
Fe3+
Al
Ti
Mg
Cu2+
Ca
Zn
Mn2+
Si
M sites

0.85
0.12
0.03
1
10.72
0.46
0.41
0.15
0.09
0.08
0.04
0.03
0.01
11.99

Figure 2. Raman spectrum of barioferrite from Har Parsa; grain is shown in Figure 1c.

102Minerals 2018, 8, 340

3. X-ray Diffraction Studies
3.1. Powder Diffraction Data
All peaks in the powder spectra (Figure 3) of barioferrite correspond to the synthetic BaFe12 O0.19(PDF-2, card number 01-084-0757). The unit-cell parameters reﬁned from these data are as follows:
P63 /mmc, a = 5.8908(1) Å, c = 23.1334(6) Å, V = 694.91(3) Å3 , and Z = 2, which are in good agreementwith the single-crystal data a = 5.8901(6) Å, c = 23.1235(6) Å, and V = 694.75(4) Å3 obtained at 300 K.
There are no additional superstructure reﬂections, as observed in [39].

Figure 3. Powder diffraction spectra of barioferrite.

3.2. Single-Crystal X-ray Diffraction Data
A fragment of the barioferrite crystal shown in Figure 1c was used for the single-crystal
structural investigations. The experimental details and crystallographic parameters are given in
Table 1. The crystal structure of barioferrite was reﬁned with the use of the SHELX program [40].
Empirical absorption corrections were applied in the CrysAlisPro [41] suite of programs using sphericalharmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The volumes of coordination
polyhedra were calculated using the VESTA 3 program [42]. Atomic labels were assigned following
the ﬁrst crystal structure description [3]. The ﬁnal atomic coordinates and isotropic displacement
parameters are reported in Table 3, selected interatomic distances are in Table 4, and anisotropic
displacement parameters are given in the Table 5. Table 6 provides information on the polyhedral
volumes of cations.

103

104

z/c
1/4
0
1/4
0.0272(3)0.10789(2)0.18975(3)0.15107(13)0.0557(1)0.14958(7)0.05206(8)
1/4

z/c
1/4
0
1/4
0.02730(3)0.10792(2)0.18977(3)0.1514(1)0.0555(1)0.1496(1)0.05198(5)1/4
Ueq
0.0082(1)0.0049(2)0.0184(3)0.0052(2)0.00563(14)0.0055(1)0.0063(6)0.0063(6)0.0064(4)0.0073(4)0.0080(5)Ueq
0.0038(1)0.0020(3)0.0116(2)0.0025(1)0.0026(1)0.0031(1)0.0043(5)0.0037(5)0.0035(3)0.0050(3)0.0051(4)population
Ba0.89 Ca0.011
Fe0.69 Al0.31
Fe0.96 Ti0.04
Fe0.94 Mg0.06
Fe0.94 Ti0.06
Fe
O
O
O
O
O

Population
Ba0.89 Ca0.011
Fe0.69 Al0.31
Fe0.96 Ti0.04
Fe0.94 Mg0.06
Fe0.94 Ti0.06
Fe
O
O
O
O
O
x/a
2/3
0
0
1/3
0.33624(7)
1/3
0
2/3
0.5024(2)0.1542(2)0.1820(3)

x/a
2/3
0
0
1/3
0.33622(7)
1/3
0
2/3
0.5022(2)0.15399(19)0.1825(3)The reﬁned formula is: (Ba0.89 Ca0.11 )1.00 (Fe11.17 Ti0.40 Al0.31 Mg0.12 )12.00 O19 .

y/b
1/3
0
0
2/3
0.16811(4)
2/3
0
1/3
0.0044(3)0.3082(4)0.3643(5)

x/a
2/3
0
0
1/3
0.33623(7)
1/3
0
2/3
0.5022(2)0.1541(2)0.1821(3)300 K

population
Ba0.89 Ca0.011
Fe0.69 Al0.31
Fe0.96 Ti0.04
Fe0.94 Mg0.06
Fe0.94 Ti0.06
Fe
O
O
O
O
O

Site
Ba1
Fe1
Fe2
Fe3
Fe4
Fe5
O1
O2
O3
O4
O5

y/b
1/3
0
0
2/3
0.16809(4)
2/3
0
1/3
0.0043(3)0.3086(3)0.3651(5)Temperature:

100 K

x/a
2/3
0
0
1/3
0.33617(8)
1/3
0
2/3
0.5022(2)0.1543(2)0.1825(2)

Site
Ba1
Fe1
Fe2
Fe3
Fe4
Fe5
O1
O2
O3
O4
O5

population
Ba0.89 Ca0.011
Fe0.69 Al0.31
Fe0.96 Ti0.04
Fe0.94 Mg0.06
Fe0.94 Ti0.06
Fe
O
O
O
O
O

Temperature:

y/b
1/3
0
0
2/3
0.16812(4)
2/3
0
1/3
0.0048(4)0.3084(4)0.3639(5)

400 K

y/b
1/3
0
0
2/3
0.16811(4)
2/3
0
1/3
0.0044(4)0.3080(4)0.3649(5)

200 K

z/c
1/4
0
1/4
0.02729(3)0.10789(2)0.18972(3)0.1509(1)0.0557(1)0.14951(8)0.05187(8)
1/4

z/c
1/4
0
1/4
0.02729(3)0.10789(2)0.18974(3)0.15126(14)0.05564(15)0.14967(8)0.05197(9)1/4Table 3. Atomic coordinates, displacement parameters (Å2 ), and site occupancies for barioferrite at 100, 200, 300, and 400 K.

Ueq
0.0104(1)0.0063(2)0.0222(3)0.0066(1)0.00720(11)0.00744(14)0.0079(6)0.0076(6)0.0083(3)0.0088(3)0.0098(5)Ueq0.0063(1)0.0038(2)0.0154(2)0.0043(2)0.0044(1)0.0047(2)0.0057(6)0.0054(6)0.0056(4)0.0062(4)0.0066(5)Minerals 2018, 8, 340

x3
x3

x2
x2

x3

x6
x2
x3

2.8624(16)2.9505(2)2.90611.980(2)2.278(3)1.863(2)2.028
1.914(3)1.914(2)1.914
1.9886(5)2.0757(17)1.9399(10)2.0956(12)2.022
1.9562(16)2.0749(18)2.016

Ba1-O3
Ba1-O5<Ba1-O>
Fe1-O4Fe2-O1Fe2-O5<Fe2-O>
Fe3-O2Fe3-O4<Fe3-O>
Fe4-O1Fe4-O2Fe4-O3Fe4-O4<Fe4-O>
Fe5-O3Fe5-O5<Fe5-O>

x6
x6

100 K
Mean

Temperature
Bond
Ba1-O3
Ba1-O5<Ba1-O>
Fe1-O4Fe2-O1Fe2-O5<Fe2-O>
Fe3-O2Fe3-O4<Fe3-O>
Fe4-O1Fe4-O2Fe4-O3Fe4-O4<Fe4-O>
Fe5-O3Fe5-O5<Fe5-O>
x3
x3

x2
x2

x3

x6
x2
x3

x6
x6

Temperature Bond2.8624(19)2.9495(2)2.906
1.9782(19)2.283(3)1.862(3)2.030
1.917(4)1.917(2)1.917
1.9867(17)2.074(2)1.9394(12)2.0943(15)2.021
1.9560(19)2.076(2)
2.016

200 K
Mean
Ba1-O3
Ba1-O5<Ba1-O>
Fe1-O4Fe2-O1Fe2-O5<Fe2-O>
Fe3-O2Fe3-O4<Fe3-O>
Fe4-O1Fe4-O2Fe4-O3Fe4-O4<Fe4-O>
Fe5-O3Fe5-O5<Fe5-O>

Temperature
Bond

x3
x3

x2
x2

x3

x6
x2
x3

x6
x6

2.8646(18)2.9493(16)2.907
1.9796(19)2.288(3)1.858(3)2.041
1.916(2)1.915(4)1.917
1.9847(16)2.0750(19)1.9385(12)2.0949(13)2.020
1.9574(19)2.078(12)
2.018

300 K
Mean
Ba1-O3
Ba1-O5<Ba1-O>
Fe1-O4Fe2-O1Fe2-O5<Fe2-O>
Fe3-O2Fe3-O4<Fe3-O>
Fe4-O1Fe4-O2Fe4-O3Fe4-O4<Fe4-O>
Fe5-O3Fe5-O5<Fe5-O>

Temperature
Bond

Table 4. Selected bond distances (Å) for the crystal structures of barioferrite at 100, 200, 300, and 400 K.

x3
x3

x2
x2

x3

x6
x2
x3

x6
x6

2.8711(18)2.9537(2)2.913
1.9822(18)2.298(3)1.859(3)2.034
1.922(3)1.9167(19)1.919
1.9866(17)2.0783(19)1.9406(12)2.1013(14)2.025
1.9629(18)2.0847(19)2.024

400 K Mean

Minerals 2018, 8, 340105

0.00371(12)0.0024(2)0.0012(2)0.00195(17)0.00214(13)0.00272(17)0.0032(7)0.0030(7)0.0034(7)0.0053(7)0.0033(10)U 22
0.00855(16)0.0060(3)0.0036(3)0.0050(2)0.00538(17)0.0062(2)0.0054(9)0.0066(9)0.0060(9)0.0078(9)0.0054(13)0.00371(12)0.0024(2)0.0012(2)0.00195(17)0.00195(16)0.00272(17)0.0032(7)0.0030(7)0.0022(5)0.0049(5)0.0052(8)U 11
0.00855(16)0.0060(3)0.0036(3)0.0050(2)0.0048(2)0.0062(2)0.0054(9)0.0066(9)0.0060(6)0.0077(6)0.0104(10)

Ba1
Fe1
Fe2
Fe3
Fe4
Fe5
O1
O2
O3
O4
O5

Atom
Ba1
Fe1
Fe2
Fe3
Fe4
Fe5
O1
O2
O3
O4
O5

U 22

U 11

Atom

U 33
0.0075(2)0.0026(4)0.0480(8)0.0056(3)0.00608(19)0.0056(3)0.0083(13)0.0057(14)0.0074(8)0.0065(8)0.0066(11)300 K

0.00389(17)0.0010(4)0.0323(6)0.0037(3)0.00356(16)0.0038(3)0.0064(12)0.0051(12)0.0054(7)0.0050(7)0.0062(10)

U 33

100 K

U 23
0.000
0.000
0.000
0.000
−0.00019(7)
0.000
0.000
0.000
0.0020(7)0.0005(7)
0.000

0.000
0.000
0.000
0.000
−0.00001(6)
0.000
0.000
0.000
0.0009(6)0.0010(6)
0.000

U 23

U 13
0.000
0.000
0.000
0.000
−0.00037(13)
0.000
0.000
0.000
0.0010(3)0.0003(3)
0.000

0.000
0.000
0.000
0.000
−0.00003(11)
0.000
0.000
0.000
0.0004(3)0.0005(3)
0.000

U 13

U 12
0.00428(8)0.00300(15)0.00179(14)0.00251(11)0.00239(10)0.00312(11)0.0027(5)0.0033(5)0.0030(4)0.0039(5)0.0027(6)0.00186(6)0.00121(12)0.00062(11)0.00097(8)0.00098(8)0.00136(8)0.0016(4)0.0015(4)0.0017(3)0.0026(4)0.0016(5)

U 12

U 11
0.01059(14)0.0074(3)0.0044(3)0.00611(19)0.00589(18)0.00755(18)0.0068(8)0.0075(8)0.0068(5)0.0091(6)0.0125(9)0.00695(15)0.0051(3)0.0032(3)0.0042(2)0.0042(2)0.0052(2)0.0054(9)0.0061(9)0.0051(6)0.0068(6)0.0092(10)

U 11

U 22
0.01059(14)0.0074(3)0.0044(3)0.00611(19)0.00685(14)0.00755(18)0.0068(8)0.0075(8)0.0078(8)0.0093(8)0.0062(12)0.00695(15)0.0051(3)0.0032(3)0.0042(2)0.00473(17)0.0052(2)0.0054(9)0.0061(9)0.0059(8)0.0073(9)0.0055(13)

U 22

U 33
0.01000(19)0.0041(4)0.0579(8)0.0076(3)0.00853(18)0.0072(3)0.0100(13)0.0079(13)0.0107(8)0.0081(8)0.0085(11)0.0050(2)0.0014(4)0.0400(7)0.0043(3)0.0041(2)0.0039(3)0.0064(14)0.0042(14)0.0059(9)0.0047(9)0.0039(12)U 33

Table 5. Anisotropic displacement parameters (Å2 ) for the structure of barioferrite.

U 23

U 23
0.000
0.000
0.000
0.000
−0.00031(6)
0.000
0.000
0.000
0.0025(6)0.0006(6)
0.000

400 K

0.000
0.000
0.000
0.000
−0.00010(6)
0.000
0.000
0.000
0.0011(6)0.0008(7)
0.000

200 K
U 13

U 13
0.000
0.000
0.000
0.000
−0.00062(12)
0.000
0.000
0.000
0.0012(3)0.0003(3)
0.000

0.000
0.000
0.000
0.000
−0.00021(12)
0.000
0.000
0.000
0.0005(3)0.0004(3)
0.000

U 12

U 12
0.00529(7)0.00371(14)0.00219(13)0.00305(9)0.00295(9)0.00377(9)0.0034(4)0.0038(4)0.0039(4)0.0047(4)0.0031(6)0.00348(8)0.00253(15)0.00159(14)0.00212(11)0.00211(10)0.00258(10)0.0027(4)0.0030(5)0.0030(4)0.0036(5)0.0028(6)Minerals 2018, 8, 340106Minerals 2018, 8, 340

Table 6. Data of polyhedral volumes in the crystal structure of barioferrite at 100, 200, 300, and 400 K.
PolyhedronVol. (Å3 ) 100 K

Vol. (Å3 ) 200 K

Vol. (Å3 ) 300 K

Vol. (Å3 ) 400 K

Ba1
Fe1
Fe2
Fe3
Fe4
Fe5

57.06010.3186.847
3.591
10.90810.68057.04510.2736.849
3.606
10.88610.68757.12410.3046.841
3.596
10.88210.72057.46310.340
6.877
3.615
10.94110.815The site labels and atomic positions were taken from [3], and the crystal structure was reﬁned
anisotropically to R1 = 0.014, 0.021, 0.016, and 0.017 for 501, 508, 461, and 519 unique observed
reﬂections with |Fo | ≥ 4σF at 100, 200, 300, and 400 K, respectively (Supplementary materials).
The thermal ellipsoid of the Fe2 site shows elongation along the [001] direction (Table 5) at 100 K,
and additional experiments at 200, 300, and 400 K were performed in order to check the thermal
behavior of this site. In contrast to hibonite [43], the model where Fe2 split into two 4e sites does not
improve R1 values. In the split model, during reﬁnement, both 4e sites shift into the central 2b position.
In this model, the anisotropic displacement parameters will be more reasonable in character, but due
to the short Fe2–Fe2 distance of 0.277(2) Å, their thermal ellipsoids overlapped. Because modern
reﬁnements of magnetoplumbite-type structures use a split model, we choose the model with the one
central Fe2 atom for convenience of calculation of the polyhedral volumes.
The crystal structure of barioferrite belongs to the magnetoplumbite-type structure (Figure 4).
In the terms of close packing, it can be described as the stacking of cubic and hexagonal pack layers
in the sequence . . . BAB’ABCAC’AC . . . along the [001] direction [5]. The layers A, B, and C consist
exclusively of oxide ions, while the layers B and C are represented by the (BaO3 )4- layers [5,44]. In theview of modular crystallography, the crystal structure of barioferrite consist of two fundamental blocks(Figure 4): spinel-like S-modules with a cubic stacking sequence of close packed layers and R-modules
with a hexagonal stacking sequence [2]. S-modules consist of a spinel structure fragment, which isbased on the two identical sheets of edge-shared Fe4 octahedra and one octahedral (Fe1)/tetrahedral(Fe3) layer embedded between two octahedral layers. The S-module contains three independent ironFe1 (2a), Fe3 (4f ), and Fe4 (12k) sites. The Al atoms are incorporated onto the Fe1 site, for which thereﬁned occupancy at 100 K is Fe0.69 Al0.31 . The polyhedral volume of the Fe1 octahedron is signiﬁcantly
smaller than the other octahedral sites (Figure 5), and the Fe1–O bond length is 1.980 Å (100 K),
also consistent with the admixture of aluminum. The tetrahedral Fe3 and octahedral Fe4 sites haveadmixtures of Mg and Ti, respectively. Their mean Fe–O bond lengths are 1.915 and 2.022 Å (at 100 K)
and are consistent with the reﬁned occupancies of Fe0.94 Mg0.06 and Fe0.94 Ti0.06 . These bond lengths areclose to those theoretically expected, 1.87 and 2.05 Å [45], respectively, and close to those observed insynthetic barioferrite: 1.888 and 2.007 Å, respectively [2].

107

Minerals 2018, 8, 340

Figure 4. Crystal structure of barioferrite at 100 K. The arrangement of iron-centered polyhedra:
octahedral—light green, tetrahedra—dark green, and trigonal bipyramid—orange, and the blue spheres
represent Ba.

Figure 5. Polyhedral volumes of Fe1, Fe4, and Fe5 octahedra in the crystal structure of barioferrite at100, 200, 300, and 400 K, respectively.

Each R-module contains layers formed by face-shared (Fe2 O9 ) octahedra (Fe5 site) that areconnected by their apical vertexes with (FeO5 ) trigonal bipyramids (Fe2 site). These layers include
cavities 5.89 × 5.89 Å ﬁlled by Ba (Ba1) cations. The twelve-coordinated Ba1 site (2d) has an admixtureof Ca, its reﬁned occupancy is Ba0.904 Ca0.096 at 100 K. The reﬁned occupancy of the Fe2 site is Fe0.96 Ti0.04 .
The Fe5 (4f ) and Fe2 (2b) sites have mean Fe–O bond lengths of 2.016 and 2.028 Å, which are consistent108

Minerals 2018, 8, 340

with magnetoplumbite-type compounds [5,18,46]. The admixtures of Sr, Cu, Ca, Zn, Mn, and Si
with an amount lower than 0.1 apfu were not determined during reﬁnement; their incorporation is
discussed below.
The reﬁned formula of barioferrite can be written as Ba1 (Ba0.89 Ca0.11 )Fe1 (Fe0.69 Al0.31 )Fe2 (Fe0.96 Ti0.04 )Fe3 (Fe
Fe4 (Fe
Fe5 (Fe1.88 Mg0.12 )5.64 Ti0.36 )2.00 )O19 or simpliﬁed as (Ba0.89 Ca0.11 )1.00 (Fe11.17 Ti0.40 Al0.31
Mg0.12 )12.00 O19 .
4. Discussion
The natural samples of hexaferrites are of interest to chemists and mineralogists due to the wide
range of substitutions, which affect the physical-chemical properties of the titled compounds [43,47].
The present study conﬁrms the incorporation of Mg into the tetrahedral Fe3 site as suggested in [43,48]
The mean Fe3–O bond length of 1.914 Å was slightly higher than that in the synthetic barioferrite of
1.888 Å [2]. The incorporation of Ti into magnetoplumbite-like compounds are widely discussed in theliterature; according to this data Ti orders into M2 and M4 [49,50] or Fe4 > Fe3 > Fe2 [5]. In our case,Ti is allocated in the Fe2 and Fe4 sites. The polyhedral volume of Fe4 octahedra is signiﬁcantly larger
than other octahedral sites (Figure 5) because of the presence of admixtures of divalent cations, such as
Cu, Ca, Zn, and Mn.
With the increase of temperature over the range of 100–300 K, most polyhedra in the structure ofnatural barioferrite do not signiﬁcantly change in volume (Tables 5 and 6). That is in agreement with theobservations of the magnetoelectric effect in some magnetoplumbite-like compounds at temperatures
up to ambient [51]. However, the local negative thermal expansion (NTE) of Fe1 and Fe4 octahedra in
the S-block of the structure at the temperatures 100–200 and 100–300 K, respectively—probably, due tolocal structure deformations—requires further investigation.
The most difﬁcult question is the character of the disorder in the Fe2 site (Figure 6).
According to [2,5,52], there are three different models, which can be considered for the structuraland dynamic characteristics of the bipyramidal Fe3+ ions: (1) a non-disordered conﬁguration, (2) astatic disorder between two adjacent pseudotetrahedral sites, and (3) a dynamical disorder between
these sites (Figure 7).

Figure 6. Fe2 trigonal bipyramid and Fe–O bond lengths at 100, 200, 300, and 400 K, respectively.
Displacement ellipsoids are drawn at the 50% probability level.

It seems that both reﬁnement models support a dynamic disorder character (Figure 8). In the
single-atom model, the elongation of the displacement ellipsoid of the iron atom in the trigonal
bipyramid along the [001] direction increased with temperature (Table 5). The decrease of the Fe2–O5
distances from 1.862(3) at 100 K to 1.857(3) Å at 400 K was correlated to the increase of the Fe2–O1
distances from 2.279(3) at 100 K to 2.300(3) Å at 400 K. According to our data, the iron atoms wererandomly displaced from the center of the trigonal bipyramid. In this case, both structural models
were appropriate (Figure 7). In the split-atom model, the Fe2–Fe2 distance increased from 0.277(2)at 100K to 0.358(7) Å at 400 K. In the static model of disorder, the site on the one side of the triangle(O5–O5–O5) plane is occupied by tetrahedrally coordinated Fe, whereas the site on the other side of109Minerals 2018, 8, 340

the plane is vacant. In our case, the changes of the bond lengths of the FeO5 polyhedra are signiﬁcantly
smaller for the static model of splitting the Fe2 site, and all attempts to reﬁne the model with a split Fe2
site (situated on the center of the tetrahedra) leads to a signiﬁcant increase of the R-values. We consider
that the disorder at the trigonal bipyramid in the barioferrite crystal structure has an exclusively
dynamic disorder character.

Figure 7. Theoretical models of arrangement of the Fe2 site in the barioferrite crystal structure.
Arrows shows direction of the bond elongation.

Figure 8. Thermal ellipsoids of Fe2, O1, and O5 atoms at 100 and 400 K. Thermal ellipsoids are drawn
at the 90% probability level.

110Minerals 2018, 8, 340

In conclusion, it should be noted that although barioferrite was ﬁrst discovered in an alteredbaryte nodule and considered to be a very rare mineral [1], it turned out to be a common accessory
mineral of high-calcium paralavas and gehlenite hornfelses [9,11]. The natural occurrence of theCa-analogue of barioferrite (or Fe-analogue of hibonite) CaFe12 O19 has been recently reported [11],and in addition to barioferrite with a S/R = 3:1 module ratio, a barium ferrite with the S/R = 5:1 ratio
Ba(Fe2+ ,Mg)2 Fe16 O27 also exists [53].
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/8/340/s1,
barioferrite cif ﬁles for 100 K, 200 K, 300 K, and 400 K.
Author Contributions: A.K. and T.L.P. wrote the paper. A.K. performed the Raman study and also selected thegrains for the structural investigations. A.K., I.O.G., and E.V.G. collected samples for investigation during the ﬁeld
work, performed petrological investigations, and measured the composition of the barioferrite from Har Parsa.
T.L.P. performed the SC-XRD investigation, reﬁned the barioferrite structure, and interpreted the structural data.
Funding: Grant Preludium of the National Science Centre of Poland No. 2016/21/N/ST10/00463.
Acknowledgments: The investigations were partially supported by the National Science Centre of Poland,
Grant Preludium No. 2016/21/N/ST10/00463. X-ray diffraction studies were performed at the X-ray Diffraction
Centre of Saint Petersburg State University. The authors thank the anonymous reviewers for their careful reviews,
which helped to improve a previous version of the manuscript.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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minerals
ArticleParaﬁniukite, Ca2Mn3(PO4)3Cl, a New Member of theApatite Supergroup from the Szklary Pegmatite,
Lower Silesia, Poland: Description and
Crystal Structure
Adam Pieczka 1, *, Cristian Biagioni 2 , Bożena Goł˛ebiowska 1 , Piotr Jeleń 3 , Marco Pasero 2 andMaciej Sitarz 3
1
2
3

*

Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and Technology,
30-059 Kraków, Poland; goleb@agh.edu.pl
Dipartimento di Scienze Della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy;
cristian.biagioni@unipi.it (C.B.); marco.pasero@unipi.it (M.P.)
Faculty of Materials Science and Ceramics, AGH University of Science and Technology,
30-059 Kraków, Poland; pjelen@agh.edu.pl (P.J.); msitarz@agh.edu.pl (M.S.)
Correspondence: pieczka@agh.edu.pl

Received: 29 September 2018; Accepted: 22 October 2018; Published: 26 October 2018

Abstract: Paraﬁniukite, ideally Ca2 Mn3 (PO4 )3 Cl, is a new apatite-supergroup mineral from the
Szklary pegmatite, Lower Silesia, Poland. It occurs as anhedral grains, up to 250 μm in size, dark olive
green in colour, embedded in a mixture of Mn-oxides and smectites around beusite. It has a vitreous
luster, and it is brittle with irregular, uneven fracture. The calculated density is 3.614 g·cm−3 .
Paraﬁniukite is hexagonal, space group P63 /m, with unit-cell parameters a = 9.4900(6), c = 6.4777(5) Å,V = 505.22(5) Å3 , Z = 2. The eight strongest reﬂections in the calculated X-ray powder diffraction
pattern of paraﬁniukite are [d in Å (I) hkl]: 3.239 (39) 002; 2.801 (55) 211; 2.801 (76) 121; 2.740 (100)300; 2.675 (50) 112; 2.544 (69) 202; 1.914 (31) 222; and 1.864 (22) 132. Chemical analysis by an electron
microprobe gave (in wt%) P2 O5 39.20, MgO 0.19, CaO 24.14, MnO 31.19, FeO 2.95, Na2 O 0.05, F 0.39,
Cl 3.13, H2 O(calc) 0.68, O=(Cl,F) −0.87, sum 101.05. The resulting empirical formula on the basisof 13 anions per formula unit is (Mn2.39 Ca2.34 Fe0.22 Mg0.03 Na0.01 )Σ4.99 P3.00 O12 [Cl0.48 (OH)0.41 F0.11 ].
The crystal structure of paraﬁniukite was reﬁned to an R1 = 0.0463 for 320 independent reﬂections
with Fo > 4σ(Fo ) and 41 reﬁned parameters. Paraﬁniukite is isotypic with apatites. Manganese is thedominant cation at the M(2) site, and Ca is the dominant cation at the M(1) site.
Keywords: paraﬁniukite; apatite supergroup; hedyphane group; manganese; calcium; phosphorus;
Szklary pegmatite; Lower Silesia; Poland

1. Introduction
Paraﬁniukite, ideally Ca2 Mn3 (PO4 )3 Cl, was discovered in an LCT (Li–Cs–Ta) granitic pegmatite
hosted by serpentinites of the Szklary massif, Lower Silesia, SW Poland. The mineral occurs as small
relict grains, not exceeding 250 μm in size, embedded in a Mn-oxide, smectite mixture overgrowing
partly altered beusite, the most frequent mineral component of phosphate aggregates occurring ingraphic zone of the pegmatite. The new mineral and its name have been approved by the Commission
on New Minerals, Nomenclature and Classiﬁcation (CNMNC) of the International Mineralogical
Association (IMA 2018-047). The name of the mineral is after Jan Paraﬁniuk (b. 1954), Professor ofmineralogy at the Institute of Geochemistry, Mineralogy and Petrology of the University of Warsaw,
Poland. The paraﬁniukite holotype (specimen Sz 31) is deposited in the collection of the MineralogicalMuseum of the University of Wrocław, Faculty of Earth Science and Environmental Management,
Minerals 2018, 8, 485; doi:10.3390/min8110485

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Institute of Geological Sciences, 50-205 Wrocław, Cybulskiego 30, Poland, with the catalogue number
MMWr IV8024. The crystal used for the collection of single-crystal data is deposited in the mineralogicalcollection of the Museo di Storia Naturale, Università di Pisa, Via Roma 79, Calci (Pisa), Italy, catalogue
number 19902. Paraﬁniukite was previously reported as “Mn-rich apatite” [1], and codiﬁed by the IMA
Subcommittee on Unnamed Minerals as UM2007-18:PO:CaClMn mineral, Mn3 Ca2 (PO4 )3 (Cl,F,OH),corresponding to the Mn-dominant analogue of apatite. The present study indicates that the correct
end-member formula of this mineral is Ca2 Mn3 (PO4 )3 Cl. The aim of this paper is to describe thismember of the apatite supergroup.
2. Occurrence
Paraﬁniukite, was discovered in the Szklary LCT (Li–Cs–Ta) pegmatite (50◦ 39.068 N, 16◦ 49.932 E),~6 km N of the Zabkowice˛Ślaskie˛
town, ~60 km south of Wrocław, in Lower Silesia, SW Poland,
outcropped in the northern part of the Szklary serpentinite massif. The massif is considered as a partof the tectonically dismembered Central-Sudetic Ophiolite. It adjoins the Góry Sowie Block (GSB)
unit on the east and is enclosed as a mega-boudin in the mylonitized Góry Sowie gneisses of the
Early Carboniferous Niemcza Shear Zone. The Szklary pegmatite, now completely excavated bymineral collectors, formed a NNE-SSW elongated lens or a boudin ~4 × 1 m large in planar section,
which was in primary intrusive contact with an altered aplitic gneiss, up to 2 m thick, to the southwest,
and both rocks were surrounded by tectonized serpentinite. A vermiculite-chlorite-talc zone was
locally present along the pegmatite-serpentinite contact (for more information about the geologicalsetting of the pegmatite see [2]). The pegmatite represents the beryl–columbite–phosphate subtypeof the rare element (REL)–Li pegmatite class sensu Černý and Ercit [3]. The age of the pegmatite,
estimated at 383 ± 2 Ma (CHIME dating on monazite-(Ce) [4]), is signiﬁcantly older than the age~335–340 Ma of the neighbouring small late-syntectonic dioritic, syenitic and granodioritic intrusions
occurring in the Niemcza Shear Zone (e.g., [5]). It is rather contemporaneous with the anatectic eventin the adjacent GSB [6–8]. The Szklary pegmatite is built mostly of sodic plagioclase (oligoclase-albite),
microcline perthite, quartz and biotite, with minor Fe3+ -bearing schorl-dravite tourmaline, spessartine
garnet and muscovite. It is relatively poorly zoned with: (1) a marginal graphic zone of albite + quartz± minor-to-accessory biotite commonly altered to clinochlore + black tourmaline; (2) a coarser-grainedintermediate graphic zone of microcline perthite + quartz + small quartz-tourmaline nests, with smaller
amounts of albite and biotite and, simultaneously, increased abundance of muscovite and spessartine;
accessory chrysoberyl is locally present in muscovite aggregates; (3) a central zone of graphic
microcline + quartz, in places developed as blocky microcline with interstitial albite, rare muscovite,
and no black tourmaline or biotite [4,9]. The zones (2) and (3) contain numerous accessory minerals
disseminated as inclusions mainly in quartz, microcline, albite, and muscovite. They include native
elements and alloys (arsenic, antimony, bismuth, gold, stibarsen, and paradocrasite), Nb-Ta oxides(columbite-group minerals, stibiocolumbite and stibiotantalite, fersmite, pyrochlore-, microlite- andbetaﬁte-group minerals), Mn-oxides (ernienickelite, jianshuiite, cesàrolite, cryptomelane, and others),
As and Sb oxides, lepageite (type locality, TL) and other arsenites and antimonites, beryllium
minerals (chrysoberyl, beryl, and bertrandite), phosphate and arsenate minerals (monazite-(Ce),cheralite, xenotime-(Y), Mn-bearing ﬂuor-, hydroxyl- and chlorapatite, pieczkaite, paraﬁniukite,beusite and beusite-(Ca), bobfergusonite, ﬁllowite, lithiophilite grading to sicklerite, simferite andpurpurite, natrophilite, Pb- and Ba-bearing dickinsonite, triploidite, fairﬁeldite, phosphohedyphane,plumbogummite, mitridatite, pararobertsite, gorceixite, arsenogorceixite), and a number of silicates,
such as e.g., (Be,Mn,Na,Cs)-bearing cordierite, tourmaline-supergroup minerals (dravite/schorl
and foitite/oxy-schorl), dumortierite-supergroup minerals (dumortierite, holtite, nioboholtite (TL),titanoholtite (TL) and szklaryite (TL)), spessartine, zircon, harmotome and pollucite [4,10–12].
Paraﬁniukite is closely associated with small aggregates of beusite, up to ~1 cm in size,
which underwent intense alteration into a secondary assemblage of Mn-oxides and smectites.

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Pieczkaite and rarer paraﬁniukite usually survived only as small relicts, not exceeding 250 μm in size,
surrounded by the mixture (Figure 1).

Figure 1. Paraﬁniukite as relict grains within the Mn-oxide; smectite matrix, around beusite. Holotype
specimen MMWrIV8024. Labels: Beu = beusite; Mn-ox. = Mn-oxides; Prf = paraﬁniukite.

3. Experimental Data
3.1. Mineral Description and Physical Properties
Paraﬁniukite occurs only as small anhedral grains, up to 250 μm in size (Figure 1). The mineralis transparent with dark olive-green colour sometimes masked by the Mn-oxides. It has a vitreous
luster, and it is brittle with irregular, uneven fracture. No forms and twinning were observed. Due tothe very small amount of available material and its possible contamination by the surrounding
Mn-oxides, streak, hardness, density as well as main optical properties were not determined. Cleavage,
parting, and ﬂuorescence were not observed. By analogy with pieczkaite [13], the Mohs hardness of
paraﬁniukite could be estimated at 4–5. The density calculated on the basis of the empirical formulaand the reﬁned unit-cell volume of type paraﬁniukite is 3.614 g·cm−3 . The mean refractive index,
obtained from the Gladstone–Dale relation [14,15], using the empirical formula and calculated density,is 1.731.
3.2. Chemical Data
Preliminary EDS chemical analysis showed Ca, Mn, P, and Cl as the only elements with
Z > 8. Quantitative chemical analyses were carried out using a Cameca SX 100 electron microprobe(WDS mode, 15 kV, 20 nA, 3 μm beam diameter) at the Inter-Institute Analytical Complex for Minerals
and Synthetic Substances at the University of Warsaw. Standard materials (element, emission line)
were: ﬂuorophlogopite (F Kα), YbPO4 (P Kα), hematite (Fe Kα), rhodonite (Mn Kα), diopside (Mg Kα,
Si Kα, Ca Kα), albite (Na Kα), tugtupite (Cl Kα), orthoclase (Al Kα, K Kα), celestine (Sr Lα), baryte(S Kα, Ba Lα), crocoite (Pb Mα) and sphalerite (Zn Kα) and GaAs (As Lα). The following diffracting
crystals were used: PC0 for F; TAP for Na, Mg, Al, Si and As; LPET for P, S, Cl, K, Ca, Sr and Pb;
LLIF for Mn, Fe, Zn and Ba. Aluminium, Si, S, K, Zn, Sr, As, Ba and Pb were sought but found below

116Minerals 2018, 8, 485

the detection limits. The raw data were reduced with the PAP routine [16]. Direct H2 O determination
was not performed owing to scarcity of material. However, the occurrence of H2 O was conﬁrmed
by micro-Raman spectroscopy (see below). The analytical data on the holotype material are given in
Table 1.
Table 1. Chemical composition of paraﬁunikite (in wt%).
Constituent

Mean (n = 10)

Rangee.s.d.

P2 O 5
MgO
CaO
MnO
FeO
Na2 O
F
Cl
H2 O(calc)
O=(F + Cl)
Total

39.20
0.19
24.14
31.19
2.95
0.05
0.39
3.13
0.68
−0.87
101.0538.98–39.44
0.12–0.27
23.66–24.64
30.04–31.78
2.72–3.15
0.01–0.07
0.29–0.46
3.00–3.29
0.61–0.71

0.14
0.05
0.39
0.62
0.16
0.02
0.05
0.09
0.03

Note: H2 O was calculated according to stoichiometry, in order to have 1 (OH + F + Cl) per formula unit (pfu); e.s.d.,estimated standard deviation.

The empirical formula of parafiniukite (with rounding errors) calculated in relation to 12 O2− and 1(F,Cl,OH)− anion per formula unit (apfu) is (Mn2.39Ca2.34Fe0.22Mg0.03Na0.01)Σ4.99P3.00O12[Cl0.48(OH)0.41F0.11].
Taking into account the results of the crystal-structure investigation (see below), the ideal formula ofparaﬁniukite should be written as Ca2 Mn3 (PO4 )3 Cl, corresponding to (in wt%): P2 O5 37.14, CaO 19.56,
MnO 37.12, Cl 6.18, sum 100.00.
3.3. Micro-Raman Spectroscopy
The unpolarized micro-Raman spectra in the range 100–4000 cm−1 (Figure 2) were collected ona polished sample of paraﬁniukite with randomly oriented crystals in nearly back-scattered geometryat the Faculty of Materials Science and Ceramics, AGH UST, Cracow, Poland, using a Horiba Labram
integrated with an Olympus BX40 confocal microscope with a 100× objective. The 532 nm line ofa solid-state Nd-YAG laser (10 mW) was used. The minimum lateral and depth resolution was set
to ~1 μm. The system was calibrated using the 520.7 cm−1 Raman band of silicon. The spectra werecollected through two acquisitions with single counting times of 600 s. The backscattered radiation
was analyzed with an 1800 mm−1 grating monochromator.

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Figure 2. Raman spectra of paraﬁniukite in the spectral ranges 100–1200 cm−1 and 3400–3600 cm−1 .

In Figure 2, the Raman spectra are shown, as recorded in various relicts of the holotype
paraﬁniukite. The bands at ~955 (strong), 1019 (medium) and 1105 (weak) cm−1 may be assigned
to stretching vibrations of the PO4 groups, in agreement with [13]. The peaks at ~615 (weak),593 (medium), 575 (medium) and 425 cm−1 (medium) are due to the bending vibrations of PO4
groups. The weaker peaks below ~300 cm−1 are due to deformations of the Ca and Mn polyhedra,
whereas the peak at ~3485 cm−1 is from stretching vibrations of O–H bonds.
3.4. Crystallography
A crystal fragment (70 × 40 × 30 μm) was extracted from polished section Sz31 and mounted
on a carbon ﬁbre. Intensity data were collected using a Bruker Smart Breeze diffractometer (50 kV,
30 mA) equipped with an air-cooled CCD detector, and graphite-monochromatized Mo Kα radiation.
The detector-to-crystal working distance was 50 mm. A total of 1418 frames was collected in ω andϕ scan modes. The exposure time was 120 s per frame. The data were integrated and corrected forLorentz and polarization, background effects, and absorption using the package of software Apex3 [17],resulting in a set of 422 independent reﬂections (see Supplementary Materials). The reﬁnementof unit-cell parameters constrained to hexagonal symmetry gave a = 9.4900(6), c = 6.4777(5) Å,V = 505.22(5) Å3 . The c:a ratio calculated from the unit-cell parameters is 0.6826. The statisticaltests on the distribution of |E| values (|E2 − 1| = 0.784) suggested an acentric structure; taking
into account also the systematic absences, the space group P63 could be proposed. However, any
trial to reﬁne the structure in that space group symmetry was not successful and the space group
P63 /m was eventually chosen. The crystal structure was reﬁned starting from the atomic coordinatesof turneaureite [18] using Shelxl-2014 [19]. Scattering curves for neutral atoms were taken from theInternational Tables for Crystallography [20]. The site occupancy factors (s.o.f.) of the three cationsites were initially reﬁned using the following scattering curves: Ca vs. Mn at the M(1) and M(2) sites,
and P vs.  (vacancy) at the T site. The three O sites (O(1), O(2), and O(3)) were reﬁned assuming thefull occupancy of O. Owing to the complex chemistry of paraﬁniukite, the position of the X anions
was found through successive difference-Fourier maps, showing the occurrence of an electron density
maximum at (0, 0, 1/4). The site scattering of this position was reﬁned using the curves of Cl vs. .
After several cycles of isotropic reﬁnement, the R1 index converged to 0.092. The s.o.f. at M(1) andM(2) sites showed mixed (Ca,Mn) occupancies, whereas the T site was found to be fully occupied by P
only. Its s.o.f. was then ﬁxed to 1.

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After the introduction of the anisotropic displacement parameters for cations, the R1 value
converged to 0.067. Assuming an anisotropic model also for the O(1), O(2), and O(3) positions, the R1value converged to 0.056. However, the anisotropic displacement parameter of O(2) was negatively
deﬁned. The displacement parameter of the X anion was reﬁned isotropically, as discussed in [18].
A residual of ~2.5 e/Å3 at (0, 0, 0.1667) was found. By adding this additional position (named Xb),
constraining the isotropic U value of Xa and Xb to be equal and the sum of their s.o.f. to be 1, the R1value converged to 0.058. Finally, we tried to reﬁne the anisotropic displacement parameters of O(2),resulting in positive deﬁned values. The Xb position was assumed to be occupied by Cl only, owing to
the longer M(2)–Xb distance, whereas Xa was assumed to have a mixed (OH, F, Cl) occupancy. The site
occupancies at the X sub-positions were ﬁxed, taking into account the observed site scattering and theelectron microprobe data, to (OH0.40 Cl0.30 F0.10 ) and Cl0.20 at Xa and Xb, respectively.
The reﬁnement converged to R1 = 0.0463 for 320 unique reﬂections with Fo > 4 σ(Fo ) (0.0676 for all
422 reﬂections) and 41 reﬁned parameters. The chemical formula derived from the structure reﬁnement(SREF) is (Ca2.56 Mn2.44 )(PO4 )3 [Cl0.50 (OH)0.40 F0.10 ]. The details of data collection and reﬁnement are
given in Table 2.
Table 2. Crystal and experimental details for paraﬁniukite.
Crystal Data
0.07 × 0.04 × 0.03
Hexagonal, P63 /m
9.4900(6)6.4777(5)505.22(5)2Crystal size (mm)Cell setting, space group
a (Å)
c (Å)
V (Å3 )Z

Data Collection and Reﬁnement
Radiation, wavelength (Å)
Temperature (K)2θmax (◦ )Measured reﬂections
Unique reﬂections
Reﬂections with Fo > 4σ(Fo )Rint
RσRange of h, k, l
R [Fo > 4σ(Fo )]R (all data)wR (on Fo 2 )GoofNumber of least-squares parameters
Maximum and minimum residual peak (e Å−3 )Mo Kα, λ = 0.71073
293
54.89
6047
422
320
0.10080.0422−12 ≤ h ≤ 12, −12 ≤ k ≤ 12, −8 ≤ l ≤ 8
0.04630.0676
0.09331.163
41
0.77 (at 0.98 Å from Xb)−0.80 (at 0.76 Å from T)

The fractional atom coordinates and isotropic or equivalent isotropic displacement parameters
are reported in Table 3. Table 4 reports the selected bond distances, whereas Tables 5 and 6 show
the proposed site populations and the bond-valence calculations obtained using the bond-valence
parameters of Brese and O’Keeffe [21].

119Minerals 2018, 8, 485

Table 3. Site labels, Wyckoff positions, atom coordinates, and equivalent isotropic or isotropic (*)
displacement parameters (Å2 ) for paraﬁniukite.
Site

Wyckoff

x/a

y/b

z/c

U eq

M(1)
M(2)
T
O(1)
O(2)
O(3)
Xa
Xb

4f
6h
6h
6h
6h
12i
2a
4e

2/3
0.0177(2)0.3752(2)0.5008(7)0.4621(7)0.2629(5)00

1/3
0.2645(2)0.4034(2)0.3465(6)0.5920(7)0.3424(5)000.0054(3)
1/4
1/4
1/4
1/4
0.4403(7)1/40.186(4)0.0082(5)0.0206(5)0.0103(6)0.0119(13)0.0247(18)0.0199(12)0.0132(17) *0.0132(17) *Table 4. Selected bond lengths (in Å) for paraﬁniukite.
M(1)–O(1) 2.280(4) × 3–O(2) 2.366(4) × 3–O(3) 2.835(4) × 3

2.204(5) × 2
2.259(6)2.401(4) × 2
2.431(2)2.466(4)3.070(6)

–O(3)
–O(2)
–O(3)
–Xa
–Xb
–O(1)

M(2)

T

–O(1)
–O(3)
–O(2)

1.535(6)1.540(5) × 2
1.552(6)Table 5. Reﬁned site scattering vs. calculated site scattering (in electrons) and proposed site occupancy
for M sites in paraﬁniukite.
Site

Reﬁned Site Scattering

Proposed Site Population

Calculated Site Scattering

M(1)
M(2)

43.4
69.0

Ca1.25 Mna 0.74 Na0.01
Mna 1.90 Ca1.06 Mg0.04

43.6
69.2

Note: a Includes 0.22 Fe.

Table 6. Bond-valence sums (in valence unit) in paraﬁniukite.
Site

M(1)

M(2)

T

ΣAnionsO(1)O(2)

2×→ 0.37↓× 3

1.21
1.15

1.99
2.07

O(3)

0.08↓× 3

0.04
0.34
0.40↓× 2
0.23↓× 2
3×→ 0.26
3×→ 0.08
1.98

1.19↓× 2

1.90

Xa
Xb
ΣCations

2×→ 0.29↓× 3

2.22

0.78
0.24
4.74

Note: In mixed or partially occupied sites, the BVS has been weighted taking into account the site occupancy.

Owing to the small crystal size, powder X-ray diffraction data were not collected. Table 7 reports
the X-ray powder diffraction data (for Cu Kα) calculated on the basis of the reﬁned structural model.

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Table 7. Calculated X-ray powder diffraction data (d in Å) for paraﬁniukite. Intensity and dhkl were
calculated using the software PowderCell 2.3 [22] on the basis of the structural model given in Table 3.
Only reﬂections with Icalc > 5 are listed, if not observed. The eight strongest reﬂections are given
in bold.
Icalc

dcalc

hkl

Icalc

dcalc

hkl

13
7
6
16
39
14
16
55
76
100
50
69
7
13
10

8.22
5.09
4.109
3.470
3.239
3.106
3.013
2.801
2.801
2.740
2.675
2.544
2.523
2.279
2.242

100
101
200
201
002
120
102
211
121
300
112
202
301
310
122

10
6
31
8
22
6
15
13
17
16
17
14
17
12
7

2.150
1.965
1.914
1.885
1.864
1.864
1.810
1.793
1.773
1.773
1.735
1.629
1.619
1.466
1.439

311
113
222
320
132
312
321
140
123
213
402
232
004
502
151

4. Discussion
4.1. Crystal Structure Description
The crystal structure of paraﬁniukite is topologically similar to those of the other members of theapatite supergroup (e.g., [23]). The reﬁned site scattering values, as given in Table 5, agree with thechemical formula calculated from the electron microprobe data. The M(1) and M(2) sites are Ca- andMn-dominant, respectively, whereas Cl is the dominant X anion. Hence, the end-member compositionof paraﬁniukite is Ca2 Mn3 (PO4 )3 Cl. A strong partitioning of Mn2+ at the M(2) site was previously
reported in pieczkaite, ideally Mn5 (PO4 )3 Cl [13].
The T site is occupied by tetrahedrally-coordinated P5+ with distances ranging between 1.535 and1.556 Å.
The nine-fold coordinated M(1) site has a mixed (Ca,Mn) occupancy. This occupancy can beidealized as Ca0.625 Mn0.375 . Taking into account the ionic radii of [9] Ca2+ (1.18 Å), [9] Mn2+ (1.04 Å),and [3] O2− = 1.36 Å (after [13,24]), a <M(1)–O> distance of 2.49 Å can be predicted, in agreement withthe observed <M(1)–O> distance, i.e., 2.494 Å. This average value is slightly larger than that observedin pieczkaite (2.473 Å), where the M(1) site is Mn-dominant, with a site occupancy Mn0.575 Ca0.425 [13],and smaller <M(1)–O> distance than in Ca-apatites, e.g., hydroxylapatite (2.55 Å [25]).
The M(2) site has a seven-fold coordination and a mixed (Mn,Ca) occupancy. The site
occupancy at M(2) may be idealized as Mn0.63 Ca0.37 , to be compared with that reported inpieczkaite, Mn0.82 Ca0.18 [13]. The lower Mn/(Mn + Ca) atomic ratio in paraﬁniukite results inslightly longer M(2)–O bond distances. M(2) is coordinated also by the monovalent anions at the
X site. In paraﬁunikite, two sub-sites, labelled Xa and Xb, were located; the former on the mirror
plane, at z = 1/4, and the latter slightly displaced from the mirror plane. In pieczkaite, the X anion(X = Cl0.62 OH0.38 ) is only slightly displaced from the mirror plane at z = 1/4. Since paraﬁniukite isa ternary apatite, containing also minor F, the situation is more complicated. Indeed, in order to avoid
short anion–anion contacts, only some conﬁgurations are possible, conﬁrming that local order within
individual [001] anion columns should be present (e.g., [18,26]). In paraﬁniukite, the anion columns
are occupied by (Cl0.50 OH0.40 F0.10 ).
According to previous studies [27,28], Mn tends to order at the M(1) site. However, as discussed
above and observed in pieczkaite [13], Mn is more strongly ordered at the M(2) site than at M(1),with Mn/(Mn + Ca) atomic ratios of 0.375 and 0.63 at M(1) and M(2), respectively, to be compared with
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0.575 and 0.82 in pieczkaite [13]. This preferential distribution seems to be favoured by the occurrenceof Cl anion at the X site, whereas Mn tends to be ordered at M(1) when X = F, indicating that the natureof the monovalent anion strongly affects the ordering of Mn and Ca in mixed Ca-Mn apatites [13].
Following [13], the hypothetical short-range ordered distributions could be proposed. The M(2)site population can be simpliﬁed as Mn1.9 Ca1.1 . All Ca must be locally associated with an OH group
and F at the neighbouring X sites. In the crystal structure of paraﬁniukite, there is 0.50 (OH + F) at
X, and this is associated with 0.50 × 3 = 1.50 M(2) cations. This means that OH and F are bonded to1.10 Ca + 0.40 Mn, and all Cl is bonded to Mn.
Paraﬁniukite corresponds to the end-member composition Ca2 Mn3 (PO4 )3 Cl hypothesized by
Tait et al. [13]. It belongs to class “08.BN Phosphates with only large cations, (OH, etc.): RO4 = 0.33:1”
in the Strunz-Nickel classiﬁcation [29] and to the class “41.08 Anhydrous Phosphates, etc. containinghydroxyl or halogen where (A)5 (XO4 )3 Zq, 41.08.01 Apatite group” of the Dana classiﬁcation [30].
Following [31], paraﬁniukite is a new member of the Hedyphane Group in the Apatite Supergroup.
4.2. Crystallization of Paraﬁniukite
Paraﬁniukite is a primary phosphate mineral that crystallized from a highly fractionatedmelt. The Mn-Fe fractionation index, expressed as Mn/(Mn + Fe) atomic ratio, indicates beusite +beusite-(Ca) as the earliest crystallized phosphates (Mn/(Mn + Fe) = 0.71–0.92), followed by pieczkaite(0.89–0.96), paraﬁniukite (0.91–0.95), Mn-bearing chlorapatite (0.95–0.99), and Cl-free, (Mn,OH)-bearing
ﬂuorapatite (0.97–0.99), determining the position of paraﬁniukite in the crystallization sequenceof apatite-supergroup minerals in the Szklary pegmatite. Szuszkiewicz et al. [2] noticed that thiscrystallization sequence is reversed in relation to that observed in the Cross-Lake pegmatite, the type
locality for pieczkaite [13], where early crystallized manganoan ﬂuorapatite is succeeded by lateMn-rich chlorapatite and pieczkaite. The latter sequence reﬂects progressive enrichment in Mn and F,
and next also in Cl with progress in crystallization, corresponding to the general observation that the
Cl-enrichment in the apatite structure is typically characteristic of late to secondary stages of evolution(e.g., [32]). Studies of the (Mn,Cl)-rich apatites from the Szklary pegmatite [2] showed that, contrary to
the Cross Lake pieczkaite, they are primary magmatic phases that formed together or slightly after
beusites, and their crystallization predates manganoan ﬂuor- to hydroxylapatites. Therefore, the early
crystallization of pieczkaite and paraﬁniukite in the Szklary pegmatite was probably constrained byanother process, which may be related to the interaction of the evolved pegmatite-forming melt withthe ultramaﬁc wall-rock (serpentinite), and its contamination by serpentinite-related ﬂuid-mobile
elements, particularly Cl [33]. This enabled precipitation of the small-volume droplets of a hydrous
melt extremely enriched in Mn, Cl and P, as a precursor for beusites, pieczkaite and paraﬁniukite,
immixing with normal aluminosilicate melt from which Mn-rich ﬂuorapatite to hydroxylapatite devoidof Cl crystallized in the ﬁnal stages of the pegmatite formation. Metasomatic interaction between the
Szklary pegmatite and the host serpentinite is also marked by intensive Mg contamination, appearedby the occurrence of dravite, and the enrichment of the pegmatite in some other rare components
like As and Sb (native elements and alloys, oxides, arsenites and antimonites, arsenates, As- andSb-bearing dumortierite-supergroup minerals) [11,12], Pb (pyrochlore-, microlite- and betaﬁte groupminerals), Bi (native element, pyrochlore-, microlite and betaﬁte group minerals), Ba (K-feldspars,harmotome, baryte), Li and Cs (some phosphates, (Mn,Be,Na,Cs)-bearing cordierite, (Cs,Mg)-bearing
beryl, (Cs,Mg)-bearing muscovite, Cs-bearing phlogophite and Cs-bearing annite) identiﬁed asserpentinite-related ﬂuid-mobile elements [33], playing important role in zones (1) and (2).
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/11/485/
s1, CIF: paraﬁniukite.
Author Contributions: A.P. found paraﬁniukite and made its microprobe studies; B.G., P.J. and M.S. performed
micro-Raman spectroscopic studies of the mineral, and C.B. and M.P. collected single-crystal X-ray diffraction
data and reﬁned the crystal structure; A.P. and C.B. prepared the manuscript.
Funding: This study was supported by the National Science Centre (Poland) grant 2015/17/B/ST10/03231 to A.P.

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Conﬂicts of Interest: The authors declare no conﬂict of interest.

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Symposium on Granitic Pegmatites, Ksiaż,˛ Poland, 17–19 June 2015; PEG2015: Ksia˛ż, Poland, 2015; pp. 73–103.
Oliver, G.J.H.; Corfu, F.; Krogh, T.E. U–Pb ages from SW Poland: Evidence for a Caledonian suture zone
between Baltica and Gondwana. J. Geol. Soc. Lond. 1993, 150, 355–369. [CrossRef]
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age of the Julianna pegmatite system (Góry Sowie Block, NE margin of the Bohemian massif): Evidence
from U-Th-Pb SHRIMP monazite and CHIME uraninite studies. In Book of Abstracts, Proceedings of the 7thInternational Symposium on Granitic Pegmatites, Ksiaż,˛ Poland, 17–19 June 2015; PEG2015: Ksia˛ż, Poland, 2015;
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Pieczka, A. A rare mineral-bearing pegmatite from the Szklary serpentynite massif, the Fore-Sudetic Block,
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new minerals from the Szklary pegmatite, SW Poland: Nioboholtite, (Nb0.6 0.4 )Al6 BSi3 O18 , titanoholtite,(Ti0.75 0.25 )Al6 BSi3 O18 , and szklaryite, Al6 BAs3+ 3 O15 . Mineral. Mag. 2013, 77, 2841–2856. [CrossRef]
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minerals
ArticleAurihydrargyrumite, a Natural Au6Hg5 Phasefrom Japan
Daisuke Nishio-Hamane 1, *, Takahiro Tanaka 2 and Tetsuo Minakawa 3
1
2
3

*

Institute for Solid State Physics, the University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Sunagawa cho, Tachikawa, Tokyo 190-0031, Japan; penta@kuh.biglobe.ne.jp
Ehime University Museum, Ehime University, Matsuyama, Ehime 790-8577, Japan;
minagawa@sci.ehime-u.ac.jp
Correspondence: hamane@issp.u-tokyo.ac.jp; Tel.: +81-4-7136-3462

Received: 28 August 2018; Accepted: 17 September 2018; Published: 19 September 2018

Abstract: Aurihydrargyrumite, a natural Au6 Hg5 phase, was found in Iyoki, Uchiko, Ehime Prefecture,
Shikoku Island, Japan. Aurihydrargyrumite with a metallic silver luster occurs as a submicron- to
2 μm-thick layer on the outermost surface of the placer gold. A prismatic face may be formedby {001} and {100} or {110}. The streak is also silver white and its Mohs hardness value is ca. 2.5.
Its tenacity is ductile and malleable, and its density, as calculated based on the empirical formula andpowder unit-cell data, is 16.86 g·cm−3 . The empirical formula of aurihydrargyrumite, on the basis of
11 Au + Hg, is Au5.95 Hg5.05 . Aurihydrargyrumite is hexagonal, P63 /mcm, with the lattice parameters
a = 6.9960(10) Å, c = 10.154(2) Å and V = 430.40(15) Å3 , which is identical with the synthetic Au6 Hg5phase. The seven strongest lines in the powder X-ray diffraction (XRD) pattern [d in Å(I/I0 )(hkl)]
were 2.877(29)(112), 2.434(42)(113), 2.337(100)(104), 2.234(87)(211), 1.401(39)(314), 1.301(41)(404), and1.225(65)(217). Aurihydrargyrumite forms through the weathering of mercury-bearing placer goldby involvement of self-electroreﬁning. This new mineral has been approved by the IMA-CNMNC(2017-003) and it is named for its composition, being a natural amalgam of gold (Latin: aurum) andmercury (Latin: hydrargyrum).
Keywords: aurihydrargyrumite; Au6 Hg5 phase; gold; placer; self-electroreﬁning; Ehime; Japan

1. Introduction
The investigation of the phase relations in the Au–Hg system has a long history. In the latest
phase diagram [1], three amalgam solid phases appear in the system: Au4 Hg, Au3 Hg, and Au2 Hg.
Additionally, Lindahl [2] reported a metastable Au6 Hg5 phase synthesized by reducing gold andmercury ions. Au6 Hg5 phase is stable around room temperature, while it alters by leaching the
mercury component at temperatures of above 70 ◦ C [2].
In nature, several amalgam phases without structural identiﬁcation have been reported from theplacer deposit from the Palakharya River, Bulgaria [3,4]. They are polymineral aggregates consistingof stoichiometric compounds among gold, silver, and mercury: (Au,Ag,Hg) phases with mercury
contents of up to 3 wt.%, (Au,Ag)3 Hg, (Au,Ag)2 Hg, (Au,Ag)3 Hg2 , and AuAgHg phases. A mineralwith (Au,Ag)1.2 Hg0.8 (=Au3 Hg2 ) composition was described as the new mineral weishanite from the
Weishancheng ore ﬁeld, China [5]. Although the stoichiometric compound (Au,Ag)3 Hg2 does not exist
on the phase diagram, the symmetry and unit-cell parameters for weishanite correspond to those ofa synthetic Au3 Hg phase [5]. Amalgam grains with Au94-88 Hg6-12 composition were also found in
Pleistocene alluvial deposits along the Snake River near Blackfoot, Idaho [6]. The X-ray diffraction(XRD) pattern of this amalgam was indexed as monoclinic symmetry, however this phase also does
not appear in the phase diagram.

Minerals 2018, 8, 415; doi:10.3390/min8090415

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Minerals 2018, 8, 415

The anthropogenic mercury that was used to collect gold in the past affects the composition andtexture of present gold grains through weathering processes. Barkov et al. [7] reported zoned amalgamof composition (Au1.5-1.9 Ag1.1-1.4 )Σ2.8-3.0 Hg1.0-1.2 from a placer deposit in the Tulameen–Similkameen
river system, British Columbia, Canada. They discussed the concentration of the mercury component
at the grain rim by some electrochemical factors that are related to the process of self-electroreﬁning,although they considered the amalgam grain to be anthropogenic in origin [7]. Recently, Svetlitskaya
et al. [8] described native gold from Inagli Pt–Au placer deposit, in the Aldan Shield, Russia. Most gold
placer was determined to be natural in origin, while some gold grains with a brain-like appearance
were rich in a mercury component brought by humans to recover gold in this region [8].
Many gold deposits, including placer deposits, have been reported in the Japanese Islands [9,10].
For example, the gold deposits of Shikoku Island, Ehime Prefecture, were summarized by Miyahisa
and Higaki [11]. Although gold was recovered as a by-product of the Kieslarger-type copper deposit,
there are no deposits that are mined primarily for gold in Ehime [11]. Additionally, a small amount ofplacer gold was reported in the basin of the Dozan River in eastern Ehime. In the present study, we
conducted a survey in an area where gold has not been known to occur to date, and discovered a newplacer locality in southwest Ehime. There, the surface of some gold particles is partly or completely
coated by metal with a silver luster. We subsequently studied this unidentiﬁed silver metal and foundthat it corresponds to a new mineral, a natural Au6 Hg5 phase.
The new mineral, which we name aurihydrargyrumite, is named after its chemical composition,
being a natural amalgam of gold (Latin: aurum) and mercury (Latin: hydrargyrum). Both the newmineral and its name have been approved by the International Mineralogical Association, Commission
on New Minerals, Nomenclature and Classiﬁcation (no. 2017-003). The type specimen has been
deposited in the collections of the National Museum of Nature and Science, Japan (specimen number
NSM-M45047). We herein describe this new mineral.
2. Occurrence
Aurihydrargyrumite, which is a natural Au6 Hg5 phase, was found in a placer from Iyoki, Uchiko,
Ehime Prefecture, Shikoku Island, Japan. The discovery site is located in the middle of the Oda river;
Figure 1 shows the geological map for the investigated area. The placer was carried by the Oda
and Tamatani rivers, which ﬂow through areas where Sanbagawa metamorphic rocks and Mikabu
greenstone are distributed. Sanbagawa metamorphic rocks mainly consist of maﬁc, pelitic, andpsammitic schists. Mikabu greenstone consists of metamorphosed basaltic tuff and lava, metagabbro,
and metadolerite.

Figure 1. Geological map and water system of the investigated area, southwest Ehime Prefecture,
Shikoku Island, Japan.

126

Minerals 2018, 8, 415

These units belong to the Sanbagawa metamorphic rocks. Mikabu greenstone contains signiﬁcantamounts of Kieslarger-type copper deposits and manganese deposits. Although no gold deposit
has yet been reported in the investigated area [11], we found a small quartz vein that included thegold- and mercury-bearing mineral from the Sanbagawa metamorphic rocks (Figure 1). Subsequently,
we investigated the placer of rivers and found a small amount of gold placer from Iyoki.
The placer collected from Iyoki consists of ilmenite, magnetite, chromite, zircon, scheelite, gold,
and the platinum-group minerals (PGMs) iridium, osmium, and irarsite. Aurihydrargyrumite is closely
associated with gold and is formed through the weathering of mercury-bearing gold, as discussed later.
3. Appearance and Physical Properties
The surface of some gold particles is partly or completely coated by aurihydrargyrumitewith a metallic silver luster (Figure 2a). Such particles show a zoned internal texture (Figure 2b).
Aurihydrargyrumite occurs as a thin layer on the outermost surface of the gold particles. Beneath theaurihydrargyrumite layer, a gold-rich zone containing mercury is distributed. The core of the particle
consists of silver- and mercury-bearing gold.

Figure 2. (a) Photographic image of aurihydrargyrumite (silver material); (b) backscattered electron
image of a grain cross section; and (c) backscattered electron image of the surface of the grain.

Aurihydrargyrumite with a metallic silver luster occurs as a thin, submicron- to 2 μm-thick layeron the outermost surface of the gold particles. The mineral is commonly anhedral, and occasionallyshows subhedral hexagonal-like crystals up to 2 μm in width (Figure 2c). Prismatic crystal may beformed by {001} and {100} or {110}. Its streak is silver white, its Mohs hardness value is ca. 2.5, and its
tenacity is ductile and malleable. The density, as calculated based on the empirical formula and powder127Minerals 2018, 8, 415

unit-cell data, is 16.86 g·cm−3 . Other physical and optical data were not determined due to the small
size of the crystals.
4. Chemical Composition
Table 1 summarizes the chemical analysis of aurihydrargyrumite. The data were obtained
from the natural surface, because the aurihydrargyrumite layer on the thin section is too small toanalyze. Attempts to use WDS could not be used reliably for analysis due to the non-horizontal
surface. Therefore, chemical analyses were carried out while using a JEOL JSM-5600 scanning electron
microprobe (EDS mode, 15 kV, 0.4 nA, 1 μm beam diameter), and the ZAF method was used fordata correction. The slightly high analytical total obtained is probably due to the irregular surface
topography. The empirical formula of aurihydrargyrumite, on the basis of 11 Au + Hg, is Au5.95 Hg5.05 .
The ideal formula is Au6 Hg5 , which requires Au 54.09 and Hg 45.91 for a total of 100 wt.%.
Table 1 also shows the chemical compositions inside the particles on which the aurihydrargyrumite
layer was observed. The gold-rich zone consists of a gold–mercury alloy, while the silver components
are generally absent. The core of the particle consists mainly of a gold–silver–mercury alloy.
Table 1. Analytical data for aurihydrargyrumite and the inside of the particles.
Aurihydrargyrumite

Gold-Rich ZoneCore

Mean 5 (Range) wt.%

Ideal

Mean 5 (Range) wt.%

Mean 5 (Range) wt.%

Au
Ag
Hg
Total wt.%

54.92 (54.26–55.76)
47.50 (46.54–48.91)
102.42

54.09
45.91
100

96.82 (95.47–98.73)
2.96 (1.41–4.60)
99.78

88.20 (88.15–88.87)
9.90 (9.83–10.04)
1.69 (1.28–2.17)
99.79

Au
Ag
Hg
Σ

pfu
5.95
5.05
11

6
5
11

pfu
97.09
2.91
100

pfu
81.72
16.75
1.53
100

5. X-ray Crystallography
The micro-XRD measurement technique was applied for the powder XRD, which was carriedout using a Rigaku ultrax18 (CrKα radiation, 40 kV, 200 mA, 100 μm collimator) instrument equippedwith a curved position sensitive proportional counter and an oscillation sample stage. The powder
XRD spectrum of aurihydrargyrumite was successfully obtained from the surface of the particle while
using this method. The XRD spectrum is shown in Figure 3, and the resulting data are summarized inTable 2. The seven strongest lines in the powder XRD spectrum [d in Å(I/I0 )(hkl)] are 2.877(29)(112),
2.434(42)(113), 2.337(100)(104), 2.234(87)(211), 1.401(39)(314), 1.301(41)(404) and 1.225(65)(217). Basedon these data, aurihydrargyrumite can be indexed to the hexagonal P63 /mcm space group. The unit-cellparameters as reﬁned from the powder data are a = 6.9960(10) Å, c = 10.154(2) Å and V = 430.40(15) Å3 .
The c/a ratio calculated from the unit-cell parameters is 1.451.
Aurihydrargyrumite contains one Au site and two Hg sites. Each distinct site forms a sheet in
the ab plane (Figure 4). Gold atoms form triangular trimers, which are arranged in a triangular net
in the Au sheets. The Hg atoms form a ditrigonally distorted Kagome net in the Hg1 sheets, but ahoneycomb net in the Hg2 sheets. Two Au sheets and one Hg1 sheet form a compound Au–Hg1–Au
layer and the next such layer is rotated 60◦ around the c-axis. The Hg2 sheets occur between the layers.

128

Minerals 2018, 8, 415

Figure 3. X-ray diffraction spectrum (CrKα radiation) of aurihydrargyrumite. The bars below thediffraction proﬁle are the simulation calculated using the atomic position by Lindahl [2]. All of thediffraction peaks can be indexed as aurihydrargyrumite.

Figure 4. Crystal structure of aurihydrargyrumite.

Table 2. Powder X-ray diffraction data (CrKα radiation) for aurihydrargyrumite.
Aurihydrargyrumite
hkl
100
002
102
110
111
200
112

dobs. (Å)

dcalc. (Å)

3.502
3.309
3.025
2.877

6.059
5.077
3.891
3.498
3.307
3.029
2.881

Synthetic Au6 Hg5 [2]

129

I/I0

dcalc. (Å)

Icalc.

1
1
2
29

6.057
5.074
3.889
3.497
3.306
3.028
2.879

4
<1
<1
5
2
4
52

Minerals 2018, 8, 415

Table 2. Cont.
Aurihydrargyrumite

Synthetic Au6 Hg5 [2]hkl

dobs. (Å)

dcalc. (Å)

I/I0

dcalc. (Å)

Icalc.

202
004
113
104
210
211
212
114
300
204
213
302
115
220
221
214
006
310
311
222
106
312
304
223
116
215
400
313
206
402
224
314
320
321
216
322
117
225
410
411
404
306
315
323
412
008
108
413
217
324
226
500
118

2.597

2.602
2.539
2.432
2.341
2.290
2.234
2.088
2.055
2.020
1.946
1.897
1.877
1.756
1.749
1.724
1.700
1.692
1.680
1.658
1.654
1.630
1.595
1.580
1.554
1.523
1.519
1.515
1.505
1.477
1.452
1.440
1.401
1.390
1.377
1.361
1.341
1.340
1.325
1.322
0.131
1.301
1.297
1.295
1.286
1.280
1.269
1.242
1.232
1.225
1.219
1.216
1.212
1.193

23

2.600
2.537
2.431
2.340
2.289
2.233
2.087
2.053
2.019
1.945
1.896
1.876
1.755
1.748
1.723
1.700
1.691
1.680
1.657
1.653
1.629
1.595
1.580
1.553
1.523
1.519
1.514
1.505
1.477
1.451
1.440
1.401
1.390
1.377
1.360
1.340
1.339
1.325
1.322
1.311
1.300
1.296
1.294
1.285
1.279
1.269
1.242
1.231
1.225
1.219
1.216
1.211
1.192

49
<1
100
46
38
91
5
12
21
14
18
2
13
<1
3
1
4
<1
10
2
3
1
4
<1
18
<1
12
15
3
<1
18
31
2
8
1
16
4
1
18
1
49
<1
14
29
21
18
<1
3
89
13
11
19
1

2.434
2.337
2.290
2.234
2.087
2.053
2.019
1.947
1.895
1.876
1.758

1.658

1.581
1.525
1.516
1.506
1.478
1.441
1.401
1.378
1.341

1.321
1.301
1.294
1.286
1.280
1.269

1.225
1.219
1.216
1.211

42
100
13
87
3
22
15
13
21
2
13

8

5
23
8
15
5
11
39
7
16

16
41
11
29
18
15

65
15
14
11

Figure 4 shows a crystal structure of aurihydrargyrumite based on the result of Lindahl [2].
Aurihydrargyrumite is identical to the synthetic Au6 Hg5 phase.
130Minerals 2018, 8, 415

6. Relation to Other Species
Minerals and synthetic amalgam phases are summarized in Table 3. In the Au–Hg system [1],three amalgam phases were observed: Au4 Hg, Au3 Hg, and Au2 Hg. Above the amalgam phases,
the structures have been solved for Au3 Hg [12]. Hexagonal symmetry was reported in Au4 Hg
and Au2 Hg phases, while their structures are unsolved [12]. Additionally, Lindahl [2] reported ametastable phase with an Au6 Hg5 composition that was synthesized by reducing ions in a solution
using hydrazine sulfate and electrolytic reﬁning while using a gold anode and a mercury cathode.
The Au6 Hg5 phase is stable at around room temperature, while it alters by leaching the mercury
component at above 70 ◦ C [2].
Among natural amalgams, the unit-cell parameters are known for aurihydrargyrumite, weishanite,
and UM192-08-EAuHg. In this study, Aurihydrargyrumite is equivalent to the synthetic Au6 Hg5phase. Although the composition of weishanite is (Au,Ag)3 Hg2 , the symmetry and unit-cell parameters
can be indexed as the synthetic Au3 Hg phase [5]. There are no synthetic phases corresponding toUM192-08-EAuHg [6].
Table 3. Comparable data for minerals and synthetic phases in Au–Hg system.
MineralsName

Composition

Symmetry

a (Å)

b (Å)

c (Å)

α (◦ )β (◦ )γ (◦ )

V (Å3 )

Reference

Aurihydrargyrumite
Weishanite
UM1992-08-E:AuHg

Au6 Hg5(Au,Ag)3 Hg2
Au94-88 Hg6-12

P63 /mcm
P63 /mmc
Monoclinic

6.996
2.9265
4.729

=a
=a
5.243

10.1544.8176
4.546

90
90
90

90
90
90.9

120
120
90

430.4
35.7
112.7

This study
[5]
[6]

Name

Composition

Symmetry

a (Å)

b (Å)

c (Å)

α (◦ )β (◦ )γ (◦ )

V (Å3 )

Reference-

Au6 Hg5
Au4 HgAu3 HgAu2 Hg

P63 /mcm
Hexagonal
P63 /mmc
Hexagonal

6.9937
8.736
2.918
13.98

=a
=a
=a
=a

10.1489.577
4.8113
17.2

90
90
90
90

90
90
90
90

120
120
120
120

430.4
633
35.7
2911.2

[2]
[12]
[12]
[12]

Synthetic Phases

7. Discussion
The Au6 Hg5 phase (=aurihydrargyrumite) could not be formed by the direct reaction between
gold and mercury [1], and the special synthetic methods [2] cannot be applied directly to the naturalenvironment. However, aurihydrargyrumite certainly occurs in nature, as demonstrated in this study.
The formation of this mineral is discussed in the following.
Aurihydrargyrumite occurs as a thin layer on the outermost surface of the placer gold.
This texture is clearly of secondary origin, since the thin surface layer would probably peel off
if the placer gold were transported. Therefore, aurihydrargyrumite likely occurred after the placer gold
was stably deposited. The accretion of a mercury component from the river water is unlikely, sinceno mercury was detected in the Oda River in the ofﬁcial water quality survey that was conducted by
the Ehime prefectural ofﬁce. Aurihydrargyrumite is probably formed from an Hg component alreadypresent in the placer gold; indeed, the gold grain includes a mercury component in the core. In thesynthetic experiment, the Au6 Hg5 phase (aurihydrargyrumite) was formed by reduction from ions andby electrolytic synthesis. Therefore, in nature, ionization and precipitation may interact in a complex
manner on the surface of the placer gold to form aurihydrargyrumite. When considering the situation,
self-electroreﬁning is probably involved in the migration of the gold and mercury components to thesurface layer, which is analogous to the process of forming a gold-rich rim in the placer gold [13].
We suggest that the placer gold dissolves at the interface, and that subsequently the gold and mercury
components immediately precipitate back onto the surface. The gold and mercury component thatwas concentrated through this process ﬁrst formed the gold-rich zone, and aurihydrargyrumite wassubsequently crystallized by further self-electroreﬁning based on the interface of the gold-rich zone.
In this way, aurihydrargyrumite probably occurs in the process of weathering of the mercury-bearing
placer gold.
131

Minerals 2018, 8, 415

These formation processes were also suggested to explain the zoned texture of anthropogenic
amalgam grains from the Tulameen–Similkameen river system, Canada [7]. The zoning texture
in amalgam phases was also reported in placer gold from the Palakharya River, Bulgaria [3,4],
and Inagli Pt–Au placer deposit, Russia [8]. In natural weathering environments, the gold andmercury components in placer gold seem to concentrate on the surface by repeated leaching anddeposition. However, the mechanism of leaching and deposition is still controversial, and various
mechanisms might be closely involved. Although self-electroreﬁning is one of the most probablemechanisms, there is no evidence to discount other mechanisms, such as the involvement of microbiallyassisted reactions in nugget and layer alloy formations. Ionization and re-metallization of gold andmercury components must be a contributory factor in the formation of aurihydrargyrumite.
8. Conclusions
Aurihydrargyrumite is found in placer gold from Iyoki, Uchiko, Ehime Prefecture, Japan.
The chemical composition and X-ray diffraction proﬁle conﬁrmed that aurihydrargyrumite and the
Au6 Hg5 phase were identical. Aurihydrargyrumite occurs as thin layers on the outermost surfaceof the placer gold and its formation relates to the natural weathering environment, which involves
ionization and re-metallization of gold and mercury components in the placer gold.
Author Contributions: D.N.-H. wrote the paper. D.N.-H., T.T. and T.M. took part in the ﬁeld works. T.M. foundthe ﬁrst specimen, and subsequently T.T. and D.N.-H. collected the placer samples. D.N.-H. performed theobservation, chemical analysis, and XRD study.
Acknowledgments: We would like to thank Hirotada Gotou for technical assistance during the X-ray diffraction
experiments at ISSP. The investigations were partly supported by a Grant-in-Aid for Young Scientists B (Grant
No. 15K17785).
Conﬂicts of Interest: The authors declare no conﬂict of interest.

References
1.
2.
3.
4.

5.
6.

7.

8.

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Okamoto, H.; Massalski, T.B. Au-Hg (Gold-Mercury). In Binary Alloy Phase Diagrams, 2nd ed.; Massalski, T.B.,
Ed.; ASM International: Materials Park, OH, USA, 1990; Volume 1, pp. 376–379.
Lindahl, T. Crystal structure of Au6 Hg5 . Acta Chem. Scand. 1970, 24, 946–952. [CrossRef]
Atanasov, V.; Iordanov, I. Amalgams of gold from the Palakharya River alluvial sands, Distinct of Soﬁa.
Doklady Bolgarskoj Akademii Nauk. 1983, 36, 465–468.
Atanasov, V.; Jordanov, J.A.; Vitov, O.C.; Atanasov, A.V. Part I: Geology. In Amalgams of Gold in Some of
the Bulgarian Alluvial Sands; Higher Institute of Mining and Geology: Soﬁa, Bulgaria, 1988; Volume 34,
pp. 227–238.
Li, Y.; Ouyang, S.; Tian, P. Weishanite—A new gold-bearing mineral. Acta Mineral. Sin. 1984, 4, 102–105.
Desborough, G.A.; Foord, E.E. A monoclinic, pseudo-orthorhombic Au-Hg mineral of potential economic
signiﬁcance in Pleistocene Snake River alluvial deposits of southeastern Idaho. Can. Mineral. 1992, 30,1033–1038.
Barkov, A.Y.; Nixon, G.T.; Levson, V.M.; Martin, R.F. A cryptically zoned amalgam(Au1.5-1.9Ag1.1-1.4)Σ2.8-3.0Hg1.0-1.2 from a placer deposit in the Tulameen-Similkameen river
system, British Columbia, Canada: Natural or Man-made? Can. Mineral. 2009, 47, 433–440. [CrossRef]
Svetlitskaya, T.V.; Nevolko, P.A.; Kolpakov, V.V.; Tolstykh, N.D. Native gold from the Inagli Pt-Au placer
deposit (the Aldan Shield, Russia): Geochemical characteristics and implications for possible bedrock sources.
Miner. Depoz. 2018, 53, 323–338. [CrossRef]
Shikazono, N.; Shimizu, M. Electrum: Chemical Composition, Mode of Occurrence, and Depositional Environment;
The University Museum, University of Tokyo: Tokyo, Japan, 1988; Volume 32, pp. 1–81.
Yokoyama, K.; Takeuchi, S.; Nakai, I.; Tsutsumi, Y.; Sano, T.; Shigeoka, M.; Miyawaki, M.; Matsubara, M.
Chemical Compositions of Electrum Grains in Ore and Placer Deposits in the Japanese Islands; National Museum of
Nature and Science Monographs: Tokyo, Japan, 2011; Volume 42, pp. 1–80.
Miyahisa, M.; Higaki, J. Gold-Silver Mineral Resource of Ehime Prefecture. Ehime-Ken. Chika. Shigen. Shiryo.
1981, 10, 11–24.

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12.
13.

Rolfe, C.; Hume-Rothery, W. The constitution of alloys of gold and mercury. J. Less-Common Met. 1967, 13,
1–10. [CrossRef]
Groen, J.C.; Craig, J.R.; Rimstidt, J.D. Gold-rich rim formation on electrum grains in placers. Can. Mineral.
1990, 28, 207–228.
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

133minerals
ArticleOyonite, Ag3Mn2Pb4Sb7As4S24, a New Member of the
Lillianite Homologous Series from the Uchucchacua
Base-Metal Deposit, Oyon District, Peru
Luca Bindi 1, *, Cristian Biagioni 2 and Frank N. Keutsch 3
1
2
3

*

Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy
Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy;
cristian.biagioni@unipi.it
Paulson School of Engineering and Applied Sciences and Department of Chemistry & Chemical Biology,
Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA; keutsch@seas.harvard.edu
Correspondence: luca.bindi@uniﬁ.it; Tel.: +39-055-275-7532

Received: 18 April 2018; Accepted: 1 May 2018; Published: 2 May 2018

Abstract: The new mineral species oyonite, ideally Ag3 Mn2 Pb4 Sb7 As4 S24 , has been discovered in
the Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, as veryrare black metallic subhedral to anhedral crystals, up to 100 μm in length, associated with orpiment,
tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S,in calcite matrix. Its Vickers hardness (VHN100 ) is 137 kg/mm2 (range 132–147). In reﬂected light,
oyonite is weakly to moderately bireﬂectant and weakly pleochroic from dark grey to a dark green.
Internal reﬂections are absent. Reﬂectance values for the four COM wavelengths [Rmin , Rmax (%)(λ in nm)] are: 33.9, 40.2 (471.1); 32.5, 38.9 (548.3), 31.6, 38.0 (586.6); and 29.8, 36.5 (652.3). Electron
microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 0.76 (2), Ag 8.39 (10), Mn 3.02(7), Pb 24.70 (25), As 9.54 (12), Sb 28.87 (21), S 24.30 (18), total 99.58 (23). Based on 20 cations per
formula unit, the chemical formula of oyonite is Cu0.38 Ag2.48 Mn1.75 Pb3.79 Sb7.55 As4.05 S24.12 . The main
diffraction lines are (d in Å, hkl and relative intensity): 3.34 (−312; 40), 3.29 (−520; 100), 2.920 (−132;40), 2.821 (−232; 70), 2.045 (004; 50). The crystal structure study revealed oyonite to be monoclinic,
space group P21 /n, with unit-cell parameters a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å,
β = 90.471 (11)◦ , V = 2004.1 (4) Å3 , Z = 2. The crystal structure was reﬁned to a ﬁnal R1 = 0.032 for
6272 independent reﬂections. Oyonite belongs to the Sb-rich members of the andorite homeotypic
sub-series within the lillianite homologous series. The name oyonite is after the Oyon district,
Lima Department, Peru, the district where the type locality (Uchucchacua mine) is located.
Keywords: oyonite; lillianite homologous series; sulfosalt; copper; antimony; arsenic; Oyon district;
Lima department; Peru

1. Introduction
Oyonite has been found during a mineralogical investigation of the ore minerals from the
Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru. This deposit is thetype locality for other four Mn-bearing sulfosalts: uchucchacuaite, AgMnPb3 Sb5 S12 [1], benavidesite,
MnPb4 Sb6 S14 [2], manganoquadratite, AgMnAsS3 [3], and menchettiite, AgMn1.60 Pb2.40 Sb3 As2 S12 [4].
The sample containing oyonite was not found in situ but originates from a sample given to oneof the authors (F.N.K.) by the mineral dealer John Veevaert. The material was found in October 2010
from the Nivel 890, Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department,
Peru, and it is the same material where menchettiite [4] was found. Geological and metallogenic data
concerning this mining district have been reported by [2].

Minerals 2018, 8, 192; doi:10.3390/min8050192

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Minerals 2018, 8, 192

Oyonite is associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed
minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix.
The new mineral was named oyonite, after the Oyon district, Lima Department, Peru, where thetype locality (Uchucchacua mine) is located. The mineral and its name have been approved by the
IMA CNMNC, under the number 2018-002. The holotype specimen of oyonite is deposited in themineralogical collections of the Museo di Storia Naturale, Università degli Studi di Firenze, Via G.
La Pira 4, Florence, Italy, under catalogue number 3283/I.
The mineralogical description of oyonite, as well as its crystal structure, are described in this paper.
2. Mineral Description and Physical Properties
Oyonite (Figure 1) occurs as black subhedral to anhedral crystals, up to 100 μm in length. Streak isblack, and the luster is metallic.

Figure 1. Andorite-like minerals occurring with orpiment on calcite. The crystals are not a single phase
but a mixture of mainly different andorite-group minerals (i.e., oyonite, menchettiite, . . . ). The widthis 1 mm. Photo by Christian Rewitzer.

In plane-polarized incident light, oyonite is weakly to moderately bireﬂectant and weakly
pleochroic from dark grey to dark green. Internal reﬂections are absent. Between crossed polars,
the mineral is anisotropic, without characteristic rotation tints.
The reﬂectance was measured in air by means of an MPM-200 Zeiss microphotometer equippedwith an MSP-20 system processor on a Zeiss Axioplan ore microscope. Filament temperature wasapproximately 3350 K. Readings were taken for specimen and standard (SiC) maintained under
the same focus conditions. The diameter of the circular measuring area was 0.1 mm. Reﬂectancepercentages in the form [Rmin , Rmax (%) (λ in nm)] are: 33.9, 40.2 (471.1); 32.5, 38.9 (548.3), 31.6, 38.0
(586.6); and 29.8, 36.5 (652.3).
Oyonite is brittle. Its Vickers hardness (VHN100 ) is 137 kg/mm2 (range 132–147), corresponding
to a Mohs hardness of ~3–3.5. Based on the empirical formula, the calculated density is 5.237 g/cm3 .
The density, calculated based on the ideal chemical formula (see below), is 5.275 g/cm3 .

135

Minerals 2018, 8, 192

3. Chemical Data
A preliminary EDS (Energy Dispersive Spectrometry) analysis performed on the crystal grainused for the structural study did not indicate the presence of elements (Z > 9) other than Cu, Ag, Mn,
Pb, As, Sb and S.
Quantitative chemical analyses were carried out using a JEOL JXA-8200 electron-microprobe,
operating in WDS (Wavelength Dispersive Spectrometry) mode. Major and minor elements were
determined at 15 kV accelerating voltage and 30 nA beam current, with 10 s as counting time.
For the WDS analyses, the following lines were used: PbMα, AgLα, CuKα, MnKα, SbLβ, AsLα,SKα. The standards employed were: galena (Pb), Ag- pure element (Ag), Cu- pure element (Cu),synthetic MnS (Mn), pyrite (Fe,S), synthetic ZnS (Zn), synthetic Sb2 S3 (Sb), synthetic As2 S3 (As),synthetic Bi2 S3 (Bi), and synthetic PtSe2 (Se). Iron, Zn, Bi, and Se were found below detection limit.
The crystal fragment was found to be homogeneous within analytical error. Table 1 gives analytical
data (average of 5 spot analyses).
Table 1. Chemical data of oyonite.
Elementwt (%) (n = 5)

Range

Estimated Standard Deviation

Cu
Ag
Mn
Pb
As
Sb
S
Total

0.76
8.39
3.02
24.70
9.54
28.87
24.30
99.58

0.50–1.05
8.08–8.91
2.89–3.22
24.55–25.81
9.11–9.82
28.24–29.61
23.63–24.71
99.18–100.41

0.02
0.10
0.07
0.25
0.12
0.21
0.18
0.23

The empirical formula, based on 20 cations per formula unit, is Cu0.38 Ag2.48 Mn1.75 Pb3.79 Sb7.55
As4.05 S24.12 , with rounding errors. The Ev(%) value, deﬁned as [Ev(+) − Ev(−)] × 100/Ev(−), is +1.0.
After subtracting minor Cu according to the substitution Cu+ → Ag+ , the formula can be written
as Ag2.86 Mn1.75 Pb3.79 Sb7.55 As4.05 S24.12 . The ideal formula is Ag3 Mn2 Pb4 Sb7 As4 S24 , which requires(in wt %) Ag 10.17, Mn 3.45, Pb 26.03, As 9.41, Sb 26.77, S 24.17, sum 100.00.
4. Crystallography
For the X-ray single-crystal diffraction study, the intensity data were collected using an Oxford
Diffraction Xcalibur 3 diffractometer, equipped with a Sapphire 2 CCD area detector, with MoKα
radiation. The detector to crystal working distance was 6 cm. Intensity integration and standard
Lorentz-polarization corrections were performed with the CrysAlis RED [5] software package.
The program ABSPACK in CrysAlis RED [5] was used for the absorption correction. Tests on the
distribution of |E| values agree with the occurrence of an inversion center (|E2 − 1| = 0.895).
This information, together with the systematic absences, suggested the space group P21 /n. We decided
to keep this non-standard setting of the space group to make easier the comparison with the othermembers of the lillianite homologous series [6]. The reﬁned unit-cell parameters are a = 19.1806 (18),b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11)◦ , V = 2004.1 (4) Å3 .
The crystal structure was reﬁned with Shelxl-97 [7] starting from the atomic coordinates ofmenchettiite [4]. The occurrence of twinning on {100} was considered. The site occupancy factors(s.o.f.) were reﬁned using the scattering curves for neutral atoms given in the International Tables for
Crystallography [8]. After several cycles of anisotropic reﬁnement, a ﬁnal R1 = 0.0227 for 2199 reﬂections
with Fo > 4σ(Fo ) was achieved (0.0317 for all 6272 reﬂections). Crystal data and details of the intensity
data collection and reﬁnement are reported in Table 2.

136

Minerals 2018, 8, 192

Table 2. Crystal and experimental details for oyonite.
Crystal Data
0.045 × 0.055 × 0.070
Monoclinic, P21 /n
19.1806 (18)12.7755 (14)8.1789 (10)90.471 (11)2004.1 (4)2Crystal size (mm3 )Cell setting, space group
a (Å)
b (Å)
c (Å)
β (◦ )
V (Å3 )Z

Data collection and Reﬁnement
Radiation, wavelength (Å)
Temperature (K)2θmax (◦ )Measured reﬂections
Unique reﬂections
Reﬂections with Fo > 4σ(Fo )Rint
RσRange of h, k, l
R [Fo > 4σ(Fo )]R (all data)wR (on Fo 2 )Number of least-squares parameters
Maximum and minimum residual peak (e Å−3 )MoKα, λ = 0.71073
293
64.63
27568
6272
2199
5.01
6.16
−27 ≤ h ≤ 28,
0 ≤ k ≤ 18,
0 ≤ l ≤ 12
0.02270.03170.0756206
1.22 (at 1.26 Å from S9) − 2.98 (at 1.00 Åfrom M8)

Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters are given
in Table 3. Selected bond distances are given in Table 4 and bond valence sums are given in Table 5.
The crystallographic information ﬁle (CIF) is given as Supplementary Material.
Table 3. Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters(in Å2 ) for oyonite.
Sites.o.f.

x

y

z

U eq

M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
S11
S12

Sb0.69(1) Mn0.31(1)As0.96(1) Sb0.04(1)Pb0.91(1) Sb0.09(1)Ag0.80(1) Cu0.20(1)Sb1.00
Sb1.00Sb0.92(1) As0.08(1)Pb1.00Mn0.55(1) Ag0.45(1)As1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00
S1.00

0.13588 (7)0.06326 (7)0.24633 (3)0.35946 (6)0.45771 (5)0.11431 (5)0.05163 (5)0.24437 (3)0.36399 (6)0.43438 (6)0.10164 (17)−0.00527 (16)0.25202 (18)0.16552 (16)0.3869 (2)0.3468 (15)0.09078 (17)0.48785 (19)0.23509 (15)0.16473 (15)0.39479 (17)0.34339 (15)0.33007 (10)0.62044 (9)0.58190 (5)0.32999 (10)0.60464 (6)0.33212 (7)0.63732 (8)0.57781 (4)0.34426 (9)0.59947 (9)0.5080 (3)0.2329 (2)0.4302 (2)0.7182 (2)0.4998 (3)0.7112 (2)0.5105 (3)0.2893 (3)0.4183 (2)0.7199 (2)0.4896 (2)0.7228 (2)0.12257 (19)0.1240 (2)0.38468 (9)0.1290 (2)0.12818 (14)0.61891 (14)0.64699 (15)0.88178 (8)0.61981 (19)0.6332 (2)0.3380 (5)0.1480 (5)0.1411 (4)0.1134 (5)0.4219 (6)0.1078 (6)0.8998 (6)0.0896 (4)0.6086 (5)0.6453 (6)0.8338 (5)0.6462 (5)0.0516 (6)0.0285 (4)0.0473 (2)0.0526 (6)0.0335 (2)0.0379 (3)0.0378 (4)0.0376 (2)0.0206 (4)0.0254 (3)0.0387 (10)0.0307 (8)0.0360 (8)0.0294 (8)0.0470 (12)0.0302 (9)0.0446 (12)0.0410 (10)0.0327 (9)0.0325 (9)0.0397 (11)0.0283 (8)137Minerals 2018, 8, 192

Table 4. Selected bond distances (in Å) in the crystal structure of oyonite.
M1

−S3
−S12
−S6
−S1
−S2
−S7

2.575 (4)2.624 (4)2.698 (4)2.952 (4)2.985 (3)3.056 (4)

M2

−S4
−S7
−S1
−S2
−S8
−S7

2.327 (3)2.372 (4)2.376 (4)3.108 (4)3.337 (4)3.400 (3)

M3

−S3
−S9
−S5
−S1
−S10
−S4
−S12
−S6

2.781 (4)2.789 (4)2.907 (4)2.953 (3)3.187 (4)3.208 (4)3.350 (3)3.411 (4)

M4

−S3
−S8
−S4
−S10
−S11
−S5

2.429 (3)2.540 (4)2.594 (4)2.686 (4)3.236 (4)3.271 (4)

M5

−S8
−S6
−S2
−S11
−S5
−S11

2.476 (4)2.531 (3)2.615 (3)3.061 (4)3.076 (4)3.088 (3)

M6

−S12
−S6
−S9
−S8
−S1
−S7

2.505 (4)2.531 (4)2.568 (3)2.887 (4)3.222 (4)3.271 (5)

M7

−S10
−S2
−S7
−S8
−S1
−S1

2.412 (3)2.525 (4)2.727 (5)2.840 (4)3.175 (5)3.480 (4)

M8

−S3
−S4
−S9
−S10
−S7
−S11
−S6
−S12

2.840 (3)3.023 (4)3.028 (4)3.053 (4)3.074 (4)3.125 (4)3.181 (4)3.288 (4)

M9

−S4
−S10
−S5
−S11
−S9
−S2

2.556 (4)2.557 (4)2.603 (4)2.616 (4)2.648 (3)2.703 (3)

M10

−S11
−S5
−S12
−S8
−S2
−S5

2.293 (4)2.326 (4)2.354 (3)3.054 (4)3.178 (4)3.687 (4)Table 5. Bond valence sums (in valence unit) in oyonite.
Site

M1

M2

S1

0.22

0.75

S2

0.20

0.10

S3

0.61

S4

0.86

S5
S6
S7

0.43
0.16

S8

M3

M4

M5

0.33

M6

M7

0.12

0.14
0.06

0.64
0.52

0.42

0.17

0.27

0.37

0.04

0.10

0.06

S10

0.17

S11

M10

0.24

0.08

0.46
0.28
0.18
0.80

0.93

0.11

0.46

0.31

0.34

0.73
0.21

1.07

0.05

0.19
0.18

0.11

0.86

Σanion

2.05
2.01

0.36
0.32

0.80

0.31
0.51

M9

1.62

0.79

0.76
0.05

S9

M8

1.94
0.84
0.02

1.77

0.18

2.31

0.24

1.78
0.12

2.07

0.27

0.28

1.79

0.26

0.36

2.07

0.21

0.31

0.14

0.92

1.86

0.78

2.42

S12

0.53

Σcation

2.15

2.58

2.17

1.30

2.92

2.93

2.86

2.04

1.87

2.64

Theor.

2.69

3.00

2.09

1.00

3.00

3.00

3.00

2.00

1.55

3.00

X-ray powder diffraction data (Table 6) were obtained on the same fragment used for the
single-crystal study with an Oxford Diffraction Excalibur PX Ultra diffractometer ﬁtted with a
165 mm diagonal Onyx CCD detector and using copper radiation (CuKα, λ = 1.54138 Å). The working
conditions were 40 kV and 40 nA with 1 hour of exposure; the detector-to-sample distance was 7 cm.
The program Crysalis RED was used to convert the observed diffraction rings to a conventional powder
diffraction pattern. The least squares reﬁnement gave the following unit-cell values: a = 19.175 (1),b = 12.7775 (9), c = 8.1817 (8) Å, β = 90.26 (1)◦ , V = 2004.6 (2) Å3 .

138

Minerals 2018, 8, 192

Table 6. Observed and calculated X-ray powder diffraction data (d in Å) for oyonite. The ﬁve strongest
reﬂections are given in bold.
Iobs

dobs

Icalc

dcalc

h

k

l

15
10
25
20
40
100
10
30
40
30
70
35
20
50
20
10
25
10
5
15
-

6.05
5.32
3.83
3.62
3.34
3.29
3.20
3.15
2.920
2.860
2.821
2.678
2.241
2.045
1.980
1.971
1.945
1.919
1.863
1.740
-

13
21
14
12
25
21
16
40
42
100
12
14
30
25
28
30
55
54
20
24
11
12
20
35
20
16
14
25
9
9
9
14
14

9.59006.06055.31643.89473.83483.61833.5986
3.33773.31463.2886
3.19673.19393.15052.91792.91272.85872.82402.81462.68432.67232.24932.23282.04882.04472.0346
1.97791.9714
1.94601.91801.86591.85511.74191.7309

2
1
2
0
4
−2
2
−3
3
5
6
0
1
−1
1
6
−2
2
−3
3
−7
7
−8
0
8
−4
4
4
10
−8
8
−5
5

0
2
2
1
2
1
1
1
1
2
0
4
4
3
3
2
3
3
3
3
1
1
1
0
1
5
5
6
0
3
3
2
2

0
0
0
2
0
2
2
2
2
0
0
0
0
2
2
0
2
2
2
2
2
2
2
4
2
2
2
0
0
2
2
4
4

5. Results and Discussion
5.1. Crystal Structure Description
The crystal structure of oyonite (Figure 2) agrees with those of the 4,4 L homologue in the lillianite
homologous series [6]. There are 10 metal sites and 12 S sites in the unit cell.

Figure 2. Unit-cell content of oyonite as seen down c. Circles: dark grey = Pb sites; light grey = Agsites; orange = Sb sites; violet = As site; purple = Mn site; yellow = S sites. Atom labels as in Table 3(for S sites, only the number is reported).

139

Minerals 2018, 8, 192

The general organization of oyonite, as seen down c, is shown in Figure 3. The crystal structure isformed by the alternation of (311)PbS slabs, four octahedra thick along (100)PbS and unit-cell twinned
by reﬂection on (311)PbS planes. The crystal structure of oyonite shows a two-fold superstructure withrespect to the short 4 Å axis of the substructure.

Figure 3. General organization of oyonite, as seen down c. Within each slab, separated by the(100) composition plane, (100)PbS planes occur (one of them is shown by the black dashed line).
Lone electron-pair micelles are shown as grey ellipses.

Two independent sites are located on the composition planes (100) of the unit-cell twinning, i.e.,
M3 and M8. They show a bicapped trigonal prismatic coordination, with a pure Pb occupancy (at M8)
or a mixed (Pb,Sb) occupancy (at M3), with a Pb:Sb atomic ratio of 91:9. The chemical composition ofthe (100) composition plane is [(Pb1.91 Sb0.09 )S2 ]+0.09 , that can be simpliﬁed as (Pb2 S2 ).
Only one kind of (311)PbS slab occurs in the crystal structure of oyonite, formed by one kind of
diagonal (100)PbS plane (Figure 4).

Figure 4. Organization of the (100)PbS layer in oyonite, as seen perpendicular to (110).

140

Minerals 2018, 8, 192

On the border of the plane, mixed (Ag,Cu) and (Mn,Ag)-centered octahedra (i.e., M4 and M9 sites,
respectively) on one side, and Sb and (Sb,Mn)-centered sites alternate (M6 and M1 sites, respectively).
In the center of the plane, a pure Sb site (M5) alternates with a pure As site (M10) along c in one
column and mixed (Sb,As) and (As,Sb) sites in the other column (M7 and M2 sites, respectively).
The composition of the diagonal (100)PbS plane is (Cu0.20 Ag1.25 Mn0.86 Sb3.65 As2.04 S10 ), that can be
simpliﬁed as (Ag1.5 MnSb3.5 As2 S10 ). Thus, the simpliﬁed chemical composition of oyonite is given by
2 × Pb2 S2 + 2 × (Ag1.5 MnSb3.5 As2 S10 ) = Ag3 Mn2 Pb4 Sb7 As4 S24 .
The chemical formula, as obtained through the single-crystal X-ray diffraction study,
is Cu0.40 Ag2.48 Mn1.74 Pb3.82 Sb7.48 As4.08 S24 (Z = 2).
5.2. Relation to the Other Species
Oyonite belongs to the andorite sub-series of the Sb-rich homeotypic members within
the lillianite homologous series ([9]).
Following the calculation procedure for the order
N and the Ag+ + Me3+ = 2Pb2+ substitution proposed by [6], considering minor cations, the homologue
order from chemical analysis is N = 3.58, slightly smaller than the crystallographic value N = 4 indicatedby the crystal structure reﬁnement, whereas the substitution percentage is 83%. The idealized chemicalcomposition corresponds to N = 4 and a substitution percentage of 75%.
The simpliﬁed chemical formula of the members of the andorite homeotypic series is
Agx Pb3−2x Me3+ 2+x S6 , where Me = (Sb,Bi). Within this series, the classiﬁcation of its members canbe indicated by the percentage of the andorite component Andn , where n (%) = 100 x. In ideal
oyonite, n = 75. Following [6] and [10], mineral species belonging to the andorite series havecompositions close to integer values of m in the general formula Ag16−m Pb16+2m Sb48−m S96 , wherem = 16 × [1 − (n/100)]. The known species have m = 8 (uchucchacuaite [11] and menchettiite [4]),6 (ﬁzélyite [12]), 5 (ramdohrite [13]), 1 (quatrandorite [14]), 0 (senandorite [15]), −1 (andreadiniite [16]),−2 (arsenquatrandorite [17]), −3 (roshchinite [18]), and −5 (jasrouxite [19]). Oyonite, having an
andorite component And75 , corresponds to a new member in the sequence, having m = 4.
Oyonite is the third member of the andorite series where Mn plays a structural role. Basedon 12 S atoms per formula unit (Z = 2), these three minerals have formulae AgMnPb3 Sb5 S12(uchucchacuaite [1,11]), AgMn1.5 Pb2.5 Sb3 As2 S12 (menchettiite [4]), and Ag1.5 MnPb2 Sb3.5 As2 S12(oyonite). Oyonite is related to uchucchacuaite through the heterovalent substitutions 0.5M1 Sb3+
+ M5 Pb2+ + 0.5M9 Mn2+ = 0.5M1 Mn2+ + M5 Sb3+ + 0.5M9 Ag+ and by the substitution of Sb-by-As at theM2 and M10 sites. It differs from menchettiite for a higher And component, related to a higher (Ag+Sb)
content in the (311)PbS layers (e.g., Sb at M1 and Ag at M9). A comparison between site occupancies inthe three Mn-rich members of the andorite group is given in Table 7.
Table 7. Summary of site occupancies in Mn-bearing ramdohrite-type minerals.

M1
M2
M3
M4
M5
M6
M7
M8
M9
M10

Uchucchacuaite [11]Menchettiite [4]Oyonite (This Work)

Sb
Sb
Pb
Ag
Pb
Sb
Sb
Pb
Mn0.9 Ag0.05 Sb0.05
Sb

Mn0.60 Pb0.40
As0.57 Sb0.43
Pb0.65 Sb0.35
Ag
Sb0.77 Pb0.23
Sb
Sb0.68 As0.32
Pb
Mn
As

Sb0.69 Mn0.31
As0.96 Sb0.04
Pb0.91 Sb0.09
Ag0.80 Cu0.20
Sb
Sb
Sb0.92 As0.08
Pb
Mn0.55 Ag0.45
As

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/5/192/s1,
CIF: oyonite.

141

Minerals 2018, 8, 192

Author Contributions: F.N.K. acquired the sample; L.B. found and described the new mineral; L.B. and C.B.
conceived and designed the experiments; L.B. performed the experiments; L.B. and C.B. analyzed the data;
L.B. wrote the paper with input from C.B. and F.N.K.
Acknowledgments: The research was supported by “progetto d’Ateneo 2015, University of Firenze” to Luca Bindi.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

References
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Oudin, E.; Picot, P.; Pillard, F.; Moëlo, Y.; Burke, E.A.J.; Zakrzewski, M.A. La bénavidésite, Pb4 (Mn,Fe)Sb6 S14 ,un noveau minéral de la série de la jamesonite. Bull. Mineral. 1982, 105, 166–169.
Bonazzi, P.; Keutsch, F.N.; Bindi, L. Manganoquadratite, AgMnAsS3 , a new manganese-bearing sulfosalt
from the Uchucchacua polymetallic deposit, Lima Department, Peru: Description and crystal structure.
Am. Mineral. 2012, 97, 1199–1205. [CrossRef]
Bindi, L.; Keutsch, F.N.; Bonazzi, P. Menchettiite, AgPb2.40 Mn1.60 Sb3 As2 S12 , a new sulfosalt belonging to thelillianite series from the Uchucchacua polymetallic deposit, Lima Department, Peru. Am. Mineral. 2012, 97,
440–446. [CrossRef]
Oxford Diffraction. CrysAlis RED (Version 1.171.31.2) and ABSPACK in CrysAlis RED; Oxford Diffraction Ltd.:
Abingdon, UK, 2006.
Makovicky, E.; Karup-Møller, S. Chemistry and crystallography of the lillianite homologous series; Part I,
General properties and deﬁnitions. Neues Jahrb. Mineral. Abh. 1977, 130, 264–287.
Sheldrick, G.M. A short history of SHELX. Acta Crystallogr. 2008, 64, 112–122. [CrossRef] [PubMed]
Wilson, A.J.C. (Ed.) International Tables for Crystallography Volume C, Mathematical, Physical and Chemical Tables;
Kluwer Academic: Dordrecth, NL, USA, 1992.
Moëlo, Y.; Makovicky, E.; Mozgova, N.N.; Jambor, J.L.; Cook, N.; Pring, A.; Paar, W.H.; Nickel, E.H.;
Graeser, S.; Karup-Møller, S.; et al. Sulfosalt systematics: A review. Report of the sulfosalt sub-committee of
the IMA Commission on Ore Mineralogy. Eur. J. Mineral. 2008, 20, 7–46. [CrossRef]Moëlo, Y.; Makovicky, E.; Karup-Møller, S. Sulfures complexes plombo-argentifères: Minéralogie et cristallochimie
de la série andorite-fizelyite, (Pb,Mn,Fe,Cd,Sn)3−2x (Ag,Cu)x (Sb,Bi,As)2+x (S,Se)6 . Doc. BRGM 1989, 167, 107.
Yang, H.; Downs, R.T.; Evans, S.H.; Feinglos, M.N.; Tait, K.T. Crystal structure of uchucchacuaite,
AgMnPb3 Sb5 S12 , and its relationship with ramdohrite and ﬁzélyite. Am. Mineral. 2011, 96, 1186–1189.
[CrossRef]
Yang, H.; Downs, R.T.; Burt, J.B.; Costin, G. Structure reﬁnement of an untwined single crystal of Ag-excess
ﬁzélyite, Ag5.94 Pb13.74 Sb20.84 S48 . Can. Mineral. 2009, 47, 1257–1264. [CrossRef]
Makovicky, E.;
Mumme, W.G.;
Gable, R.W. The crystal structure of ramdohrite,
Pb5.9 Fe0.1 Mn0.1 In0.1 Cd0.2 Ag2.8 Sb10.8 S24 : A new reﬁnement. Am. Mineral. 2013, 98, 773–779. [CrossRef]
Nespolo, M.; Ozawa, T.; Kawasaki, Y.; Sugiyama, K. Structural relations and pseudosymmetries in the
andorite homologous series. J. Mineral. Petrol. Sci. 2012, 107, 226–243. [CrossRef]
Sawada, H.; Kawada, I.; Hellner, E.; Tokonami, M. The crystal structure of senandorite (andorite VI):
PbAgSb3 S6 . Z. Kristallogr. 1987, 180, 141–150. [CrossRef]
Biagioni, C.; Moëlo, Y.; Orlandi, P.; Paar, W.H. Andreadiniite, IMA 2014-049. CNMNC Newsletter No. 22,
October 2014, page 1244. Mineral. Mag. 2014, 78, 1241–1248.
Topa, D.; Makovicky, E.; Putz, H.; Zagler, G.; Tajjedin, H. Arsenquatrandorite, IMA 2012-087. CNMNC
Newsletter No. 16, August 2013, page 2696. Mineral. Mag. 2013, 77, 2695–2709.
Makovicky, E.; Stöger, B.; Topa, D. The incommensurately modulated crystal structure of roshchinite,
Cu0.09 Ag1.04 Pb0.65 Sb2.82 As0.37 S6.08 . Z. Kristallogr.-Cryst. Mater. 2018, 233, 255–267. [CrossRef]
Makovicky, E.; Topa, D. The crystal structure of jasrouxite, a Pb-Ag-As-Sb member of the lillianite
homologous series. Eur. J. Mineral. 2014, 26, 145–155. [CrossRef]© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleCerromojonite, CuPbBiSe3, from El Dragón (Bolivia):
A New Member of the Bournonite Group
Hans-Jürgen Förster 1, *, Luca Bindi 2 , Günter Grundmann 3 and Chris J. Stanley 4
1
2
3
4

*

Helmholtz Centre Potsdam German Research Centre for Geosciences GFZ, DE-14473 Potsdam, Germany
Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy;luca.bindi@uniﬁ.it
Chair of Engineering Geology, Technical University Munich, Arcisstr. 23, DE-80333 Munich, Germany;
grundmann.g@gmx.de
Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK;c.stanley@nhm.ac.uk
Correspondence: forhj@gfz-potsdam.de; Tel.: +49-0331-288-28843

Received: 25 July 2018; Accepted: 30 August 2018; Published: 21 September 2018

Abstract: Cerromojonite, ideally CuPbBiSe3 , represents a new selenide from the El Dragón mine,
Department of Potosí, Bolivia. It either occurs as minute grains (up to 30 μm in size) in intersticesof hansblockite/quijarroite intergrowths, forming an angular network-like intersertal texture, or as
elongated, thin-tabular crystals (up to 200 μm long and 40 μm wide) within lath-shaped or acicularmineral aggregates (interpreted as pseudomorphs) up to 2 mm in length and 200 μm in width.
It is non-ﬂuorescent, black, and opaque, with a metallic luster and black streak. It is brittle,
with an irregular fracture, and no obvious cleavage and parting. In plane-polarized incident
light, cerromojonite is grey to cream-white, and weakly pleochroic, showing no internal reﬂections.
Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades ofbrown and grey. Lamellar twinning on {110} is common. The reﬂectance values in air for the
COM standard wavelengths (R1 and R2 ) are: 48.8 and 50.3 (470 nm), 48.2 and 51.8 (546 nm),
47.8 and 52.0 (589 nm), and 47.2 and 52.0 (650 nm). Electron-microprobe analyses yielded a mean
composition of: Cu 7.91, Ag 2.35, Hg 7.42, Pb 16.39, Fe 0.04, Ni 0.02, Bi 32.61, Se 33.37, total
100.14 wt %. The empirical formula (based on 6 atoms pfu) is (Cu0.89 Hg0.11 )Σ = 1.00 (Pb0.56 Ag0.16 Hg0.15
Bi0.11 Fe0.01 )Σ = 0.99 Bi1.00 Se3.01 . The ideal formula is CuPbBiSe3 . Cerromojonite is orthorhombic(space group Pn21 m), with a = 8.202(1) Å, b = 8.741(1) Å, c = 8.029(1) Å, V = 575.7(1) Å3 , Z = 4.
Calculated density is 7.035 g·cm−3 . The ﬁve strongest measured X-ray powder diffraction lines (d inÅ (I/I0 ) (hkl)) are: 3.86 (25) (120), 2.783 (100) (122), 2.727 (55) (212), 2.608 (40) (310), and 1.999 (25) (004).
Cerromojonite is a new member of the bournonite group, representing the Se-analogue of součekite,
CuPbBi(S,Se)3 . It is deposited from strongly oxidizing low-T hydrothermal ﬂuids at a f Se2 /f S2 ratio>1, both as primary and secondary phase. The new species has been approved by the IMA-CNMNC(2018-040) and is named for Cerro Mojon, the highest mountain peak closest to the El Dragón mine.
Keywords: cerromojonite; selenium; copper; lead; mercury; bismuth; součekite; bournonite group;
El Dragón; Bolivia

1. Introduction
The Bolivian Andes host two mineralogically important selenide occurrences: Pacajake, districtof Hiaco de Charcas; and El Dragón, Province of Antonio Quijarro; both in the Departmentof Potosí. The El Dragón mineralization represents a multi-phase assemblage of primary andsecondary minerals, among which Se-bearing phases are the most prominent. It is the type
locality for eldragónite, Cu6 BiSe4 (Se2 ) [1]; favreauite, PbBiCu6 O4 (SeO3 )4 (OH)·H2 O [2]; grundmannite,
Minerals 2018, 8, 420; doi:10.3390/min8100420

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Minerals 2018, 8, 420

CuBiSe2 [3]; hansblockite, (Cu,Hg)(Bi,Pb)Se2 [4]; alfredopetrovite, Al2 (Se4+ O3 )3 ·6H2 O [5]; quijarroite,
Cu6 HgPb2 Bi4 Se12 [6]; and also contains the lately discovered rare orthorhombic dimorph of CuSe2 ,petříčekite [7]. A comprehensive survey of the geology and origin of the El Dragón Se-mineralization
was published by Grundmann and Förster (2017) [8], who also provided a full list of minerals recorded
as from this locality.
This paper provides a description of a new species in the Cu–Hg–Pb–Bi–Se system, cerromojonite,
ideally CuPbBiSe3 , from El Dragón. This new species and its name have been approved by theCommission on New Minerals, Nomenclature and Classiﬁcation (CNMNC) of the IMA, proposal
2018-040. The X-rayed crystal is preserved by one of the authors (L.B.) at the Dipartimento di Scienzedella Terra, Università degli Studi di Firenze. The polished section, from which the crystal was extracted(holotype), is housed in the collections of the Natural History Museum, London, with the catalogue
number BM 2018, 11. Cotype material, consisting of another cerromojonite-bearing polished section,
is deposited within the Mineralogical State Collection Munich (Mineralogische Staatssammlung
München, Museum “Reich der Kristalle”), with the inventory number MSM 73583.
The species is named for Cerro Mojon, the highest mountain peak nearest to El Dragón (4292 m
above sea level), located about 800 m northeast of the mine.
Unnamed phase “C”, described at El Dragón already in 2016 [3], for which no structural data wereobtained, compositionally resembles cerromojonite. A species similar to cerromojonite was speculated
to occur in carbonate veins in the Schlema–Alberoda U–Se–polymetallic deposit (Erzgebirge, Germany),
forming tiny inclusions which are intimately intergrown with berzelianite [9,10]. However,neither compositional data of pure material nor structural data were provided for this material.
2. Geology
The abandoned El Dragón mine is situated in the Cordillera Oriental (southwestern Bolivia),
about 30 km southwest of Cerro Rico de Potosí. It is located at 19◦ 49 23.90” S (latitude), 65◦ 55 00.60” W(longitude), at an altitude of 4160 m above sea level. The adit of the mine is on the orographic
left side of the Rio Jaya Mayu, cutting through a series of thinly-stratiﬁed, pyrite-rich black shales,
and reddish-grey, hematite-bearing siltstones, dipping 40◦ to the north. The almost vertical ore vein is
located in the center of a 1.5-m-wide shear zone (average trend 135 degrees). In 1988, the selenium
mineralization consisted of a single vein of small longitudinal extension (maximum 15-m-long gallery),ranging mostly from 0.5 to 2 cm in thickness.
3. Physical and Optical Properties
Cerromojonite occurs within two different mineral assemblages, which are interpreted as
representing genetically distinct types.
Type-I cerromojonite (maximum grain size ~30 μm) crystallized in the interstices ofquijarroite/hansblockite intersertal intergrowths, partly together with penroseite (NiSe2 ), klockmannite(CuSe), watkinsonite (Cu2 PbBi4 Se8 ), clausthalite (PbSe), and, more rarely, petrovicite (Cu3 HgPbBiSe5 )(Figure 1). These aggregates are cemented by umangite (Cu3 Se2 ) and klockmannite. They were
deposited at the surfaces of krut’aite–penroseite (CuSe2 –NiSe2 ) solid solutions.
Cerromojonite type-II occurs within lath-shaped or acicular mineral aggregates (up to 2 mm
in length and 200 μm in width), that are interpreted as pseudomorphs after the above describedintersertal aggregates (Figure 2). In this case, it forms elongated thin-tabular crystals (up to 200 μm
long and 40 μm wide), intimately (subparallel) intergrown with watkinsonite, and less frequently,
quijarroite (Figure 3), clausthalite, unnamed CuNi2 Se4 [11], and (according to energy-dispersive
electron-microprobe analysis) two new Cu–(Ag)–Hg–Pb–Bi selenides (Figure 4), which were all
cemented by klockmannite. These pseudomorphs occasionally show parallel intergrowth of grains,which is implied by the serrated prismatic grain surfaces. They are usually deposited in interstices
in brecciated krut’aite–penroseite (CuSe2 –NiSe2 ) grains. The appearance of the cerromojonite grains
resembles a spinifex texture, indicating fast crystallization. Type-II cerromojonite and associated144

Minerals 2018, 8, 420

minerals are themselves altered by late klockmannite, fracture-ﬁlling chalcopyrite, covellite, goethite,endmember petříčekite and krut’aite (CuSe2 ), and native selenium (Figure 5).
Cerromojonite is black in color and possesses a black streak. The mineral is opaque in transmitted
light, exhibits a metallic luster, and is non-ﬂuorescent. No cleavage and parting is observed, and thefracture is irregular. Density and Mohs hardness could not be measured owing to the small crystal
size. The calculated density is 7.035 g/cm3 (for Z = 4), based on the empirical formula (see below) andthe unit-cell parameters derived from X-ray single-crystal reﬁnement.

Figure 1. Back-scattered electron (BSE) image of type-I cerromojonite intergrown with hansblockite
and quijarroite, forming an angular network-like intersertal texture. Abbreviations: ce = cerromojonite,hb = hansblockite, qu = quijarroite.

Figure 2. BSE image showing a pseudomorph composed of bright type-II cerromojonite, watkinsonite,and quijarroite, cemented by medium-grey klockmannite. Abbreviations: k–p = krut’aite–penroseitesolid solutions, kl = klockmannite, g = goethite.

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Minerals 2018, 8, 420

Figure 3. BSE image of type-II cerromojonite (ce) intergrown with quijarroite (qu) and watkinsonite (w).

Figure 4. BSE image of parallel-intergrown type-II cerromojonite (ce) grains with darker domains of an
unknown Cu–(Ag)–Hg–Pb–Bi selenide. The biggest grain of this potentially new species is marked bya question mark.

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Minerals 2018, 8, 420

Figure 5. Type-II cerromojonite-bearing pseudomorph progressively altered by goethite and sulﬁdes(reﬂected light, horizontal ﬁeld of view is 500 μm). Abbreviations: ce = cerromojonite, ch = chalcopyrite,
cl = clausthalite, co = covellite, g = goethite, kl = klockmannite, k–p = krut’aite–penroseite solid solution,p = penroseite, w = watkinsonite, u = unnamed CuNi2 Se4 .

In plane-polarized incident light, cerromojonite is grey to cream-white. In the assemblage withklockmannite and watkinsonite, it is weakly pleochroic or bireﬂectant. The mineral does not show any
internal reﬂections. Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation
tints in shades of brown and grey (Figure 6). Twinning of cerromojonite is expressed either by distinct
sharp polysynthetic lamellae (Figure 7) or by ﬁnely divided twinning in fan-shaped aggregates.

Figure 6. Type-II cerromojonite and associated minerals in plane polarized light (a), and at partiallycrossed polarizers (b) (horizontal ﬁeld of view is 200 μm). Lamellar twinning on {110} of cerromojonite
is well displayed in (b). Abbreviations: ce = cerromojonite, g = goethite, kl = klockmannite, u =unnamed CuNi2 Se4 .

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Minerals 2018, 8, 420

Figure 7. Type-II cerromojonite associated with klockmannite, unnamed CuNi2 Se4 , clausthalite,
and later-formed goethite in reﬂected light: (a) 1 polarizer, (b) partially crossed polarizers.
Horizontal ﬁeld of view is 200 μm. Abbreviations: ce = cerromojonite, kl = klockmannite,w = watkinsonite, cl = clausthalite, u = unnamed CuNi2 Se4 , g = goethite.

Quantitative reﬂectance measurements were performed in air relative to a WTiC standard(Zeiss number 314) by means of a J & M TIDAS diode array spectrometer (J & M Analytik AG,
Essingen, Germany), running ONYX software (Version 1.1, Cavendish Instruments Ltd., Shefﬁeld,
UK) on a Zeiss Axioplan ore microscope (Carl Zeiss AG, Oberkochen, Germany) (Table 1, Figure 8).
Measurements were made on unoriented grains at extinction positions leading to the designation of R1(minimum) and R2 (maximum).

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Minerals 2018, 8, 420

Table 1. Reﬂectance data.
λ (nm)

R1 (%)

R2 (%)λ (nm)

R1 (%)

R2 (%)

400
420
440
460
480
500
520
540

47.0
47.2
47.5
47.8
48.1
48.3
48.3
48.3

48.0
48.6
49.3
50.0
50.6
51.1
51.5
51.7

560
580
600
620
640
660
680
700

48.1
47.9
47.7
47.5
47.3
47.1
46.9
46.8

51.9
52.0
52.1
52.1
52.0
51.9
51.7
51.6

Reﬂectance percentages (R1 and R2 ) for the four Commission on Minerals (COM) wavelengths
are: 48.8, 50.3 (470 nm); 48.2, 51.8 (546 nm); 47.8, 52.0 (589 nm); 47.2, 52.0 (650 nm).

Figure 8. Reﬂectance spectra of type-II cerromojonite and its most frequently associated Cu–Bi
selenides: quijarroite [6] and watkinsonite [12].

4. Chemical Data
Cerromojonite was checked for concentrations of Cu, Ag, Pb, Hg, Fe, Co, Ni, As, Sb, Bi, S, and Se.
Twenty-four spot analyses of type-II cerromojonite from the holotype section, were performed using
a JEOL JXA-8230 electron microprobe (WDS mode, 20 kV, 20 nA, 1–2 μm beam size) (JEOL Ltd.,
Akishima, Japan). The composition of the grain used for the structural study corresponds chemically tothe other grains analyzed by microprobe, which were proved to be homogeneous within 2σ standard
deviations of the analyzed elements. The counting time on the peak was 20 s, with half that time on
background on both sites of the peak. The following standards, emission lines, and analyzing crystals(in parentheses) were used: Cu–eskebornite, Kα (LIFL); Ag–bohdanowiczite, Lα (PETJ); Pb–galena, Mα
(PETH); Hg–cinnabar, Lα (LIFL); Fe–chalcopyrite, Kα (LIFL); Co–cobaltite, Kα (LIFL); Ni–pentlandite,Kα (LIFL); As–skutterudite, Lα (TAP); Sb–skutterudite, Lα (PETJ); Bi–synthetic Bi2 Se3 , Mα (PETH);
S–chalcopyrite, Kα (PETJ); Se–synthetic Bi2 Se3 , Kα (LIFL). The software-implemented PRZ (XPP metal)
data-correction routine (JEOL EPMA version 10) (JEOL Ltd., Akishima, Japan), which is based on theφ(ρZ) method [13], was used for data processing. Table 2 compiles the analytical data for cerromojonite

149

Minerals 2018, 8, 420(means of 24 spot analyses, ranges, and standard deviations). Table 3 provides a selection of resultsfrom microprobe spot analyses of cerromojonite, together with the elemental detection limits (d.l.).
Table 2. Chemical data for cerromojonite.
Element

Mean

Range

e.s.d.

Cu (wt %)
Ag
Hg
Pb
Fe
Ni
Bi
Se
Total

7.91
2.35
7.42
16.39
0.04
0.02
32.61
33.37
100.11

7.40–8.16
2.16–2.54
7.19–7.60
16.15–16.77
0.00–0.18
0.00–0.18
32.19–32.91
32.93–33.81
99.24–100.79

0.18
0.11
0.10
0.13
0.04
0.04
0.20
0.24
0.42

(Cu0.89 Hg0.11 )Σ = 1.00 (Pb0.56 Ag0.16 Hg0.15 Bi0.11 Fe0.01 )Σ = 0.99 Bi1.00 Se3.01 is the empirical formula ofcerromojonite (based on 6 atoms pfu). The ideal formula of the mineral is CuPbBiSe3, corresponding to
the ideal contents of the elements (in wt %) Cu 8.87, Pb 28.92, Bi 29.15, Se 33.06, sum 100.00.
Table 3. Representative results of electron-microprobe spot analyses of cerromojonite.
Element

d.l. (ppm)

1

2

3

4

5

6

Cu (wt %)
Ag
Hg
Pb
Fe
Ni
Bi
Se
Total

250
200
1100
400
150
200
300
800

7.40
2.45
7.48
16.29
b.d.l.
b.d.l.
32.74
33.48
99.86

7.75
2.54
7.46
16.31
0.14
0.04
32.76
33.27
100.28

8.09
2.30
7.37
16.50
b.d.l.
b.d.l.
32.78
33.58
100.64

8.03
2.33
7.38
16.46
b.d.l.
b.d.l.
32.80
33.31
100.22

7.91
2.54
7.32
16.34
b.d.l.
0.11
32.42
33.07
99.74

7.91
2.27
7.33
16.36
b.d.l.
0.03
32.51
33.56
99.98

0.84
0.16
0.27
0.56
1.12
3.04

0.87
0.17
0.26
0.56
0.02
1.12
3.00

0.90
0.15
0.26
0.56
1.11
3.01

0.90
0.15
0.26
0.57
1.12
3.01

0.89
0.17
0.26
0.56
0.01
1.11
2.99

0.89
0.15
0.26
0.56
1.11
3.03

Cu (a.p.f.u.)
Ag
Hg
Pb
Fe
Ni
Bi
Se

a.p.f.u. = atoms per formula unit; b.d.l. = below limit of detection.

5. X-ray Crystallography and Description of the Crystal Structure
X-ray powder diffraction data (Table 4) were obtained from the same fragment used for the
single-crystal study (see below), with an Oxford Diffraction Excalibur PX Ultra diffractometer(Oxford Diffraction, Oxford, UK), ﬁtted with a 165 mm diagonal Onyx CCD detector, and using
copper radiation (CuKα, λ = 1.54138 Å). The working conditions were 40 kV and 40 mA, with 1 hourof exposure; the detector-to-sample distance was 7 cm.

150

Minerals 2018, 8, 420

Table 4. Measured and calculated X-ray powder diffraction data (d in Å) for cerromojonite.
The strongest measured diffraction lines are given in bold.
hkl

dmeas

Imeas

dcalc

Icalc

020
002
120
210
112
220
022
202
122
130
212
310
222
320
132
312
040
004
330
042
142
412
332
124
242
422
224
134
314
252
044
522
530
334
260
600
126
452
216
534
264
604
416
722
182
456

4.08
4.00
3.86
3.70
3.32
2.991
2.783
2.747
2.727
2.608
2.186
1.999
1.992
1.867
1.788
1.592
1.494
1.415
-

10
20
25
10
10
10
100
10
55
40
10
25
20
10
20
20
10
10
-

4.37054.10104.01453.8571
3.71273.33332.99062.95652.86882.78142.74562.72572.60932.39832.31782.2663
2.18782.18532.0073
1.99371.9193
1.86881.78751.78561.7806
1.73841.68491.66661.6204
1.59101.49281.47831.43441.4294
1.41451.37281.36701.2642
1.26281.25891.16441.13311.12991.11151.08931.0457
0.9434

4
9
15
20
9
9
10
6
7
100
12
50
37
5
3
9
5
14
21
21
4
12
22
6
4
6
3
3
4
14
10
7
4
4
12
4
4
6
8
3
3
4
4
3
5
3
3

Note: calculated diffraction pattern obtained with the atom coordinates reported in Table 6 (only reﬂections with
Irel ≥ 3 are listed).

The program Crysalis RED [14] was used to convert the observed diffraction rings to a conventional
powder diffraction pattern. Least squares reﬁnement gave the following orthorhombic unit-cell values:
a = 8.2004(6) Å, b = 8.7461(5) Å, c = 8.0159(5) Å, V = 574.91(5) Å3 , and Z = 4.
A small crystal (0.040 × 0.055 × 0.060 mm3 ) of type-II cerromojonite was handpicked from theholotype specimen (it is registered under the number #123 in the mineralogical collection of one of the
Authors, G.G.). The crystal was preliminarily examined with a Bruker-Enraf MACH3 single-crystal
151

Minerals 2018, 8, 420

diffractometer (Bruker, Karlsruhe, Germany), using graphite-monochromatized MoKα radiation.
Single-crystal X-ray diffraction intensity data were collected using an Oxford Diffraction Xcalibur
diffractometer equipped with an Oxford Diffraction CCD detector, with graphite-monochromatized
MoKα radiation (λ = 0.71073 Å). The data were integrated, and corrected for standard Lorentz andpolarization factors, with the CrysAlis RED package [12]. The program ABSPACK in CrysAlis RED [12]was used for the absorption correction. Table 5 reports details of the selected crystal, data collection,and reﬁnement.
Table 5. Data and experimental details for the selected cerromojonite crystal.
Crystal Data
Ideal formula
Crystal size (mm3 )
Form
Color
Crystal system
Space group
a (Å)
b (Å)
c (Å)
V (Å3 )Z

CuPbBiSe3
0.040 × 0.055 × 0.060
Block
Black
Orthorhombic
Pn21 m8.202(1)8.741(1)8.029(1)575.7(1)4
Data Collection

Instrument
Radiation type
Temperature (K)Detector to sample distance (cm)Number of frames
Measuring time (s)Maximum covered 2θ (◦ )Absorption correction
Collected reﬂections
Unique reﬂections
Reﬂections with Fo > 4σ(Fo )Rint
RσRange of h, k, lReﬁnement
Final R1 [Fo > 4σ(Fo )]Final R1 (all data)
S
Number reﬁned parametersΔρmax (e Å−3 )Δρmin (e Å−3 )Oxford Diffraction Xcalibur 3
MoKα (λ = 0.71073 Å)
293(3)
6
889
50
59.30
multi-scan [12]
3504
1359
701
0.0356
0.04120 ≤ h ≤ 11, −12 ≤ k ≤ 8, 0 ≤ l ≤ 10
Full-matrix least squares on F2
0.0256
0.0315
1.09
68
1.81
−2.06

Statistical tests (|E2 − 1| = 0.821) and systematic absences agreed with the acentric space group
Pn21 m. The crystal structure was reﬁned starting from the atomic coordinates of bournonite [15].
Given the observed larger unit-cell volume of cerromojonite (i.e., 575.7 Å3 ) compared to bournonite(i.e., 552.3 Å3 ; [15]), the site occupancy factor (s.o.f.) at the crystallographic sites was allowed tovary (Pb vs. Ag and Bi vs. Ag for the Pb and Bi sites; Cu vs. Hg for the Cu site; Se vs. S for
the anionic site), using scattering curves for neutral atoms taken from the International Tables
for Crystallography [16]. After several cycles of anisotropic reﬁnement, a ﬁnal R1 = 0.0256 for
701 reﬂections with Fo > 4 σ(Fo ) and 68 reﬁned parameters was achieved (0.0315 for all 1359 reﬂections).
Atomic coordinates, site occupancies, and equivalent isotropic displacement parameters are listed inTable 6, whereas anisotropic displacement parameters are given in Table 7. Selected bond distances and152

Minerals 2018, 8, 420

bond-valence sums are provided in Table 8. The Crystallographic Information File (CIF) is available as
Supplementary Material.
Table 6. Atoms, site occupancy factors (s.o.f.), fractional atomic coordinates (x, y, z), and equivalentisotropic displacement parameters (Ueq , Å2 ) for the selected cerromojonite crystal.
Atoms.o.f.

x

y

z

Ueq

Pb1
Pb2
Bi1
Bi2
Cu
Se1
Se2
Se3
Se4

Pb0.80(2) Ag0.20
Pb0.74(2) Ag0.26
Bi1.00
Bi1.00Cu0.870(8) Hg0.130
Se1.00Se1.00Se1.00Se1.000.07291(13)0.56972(12)0.07446(12)0.55647(12)0.27526(17)0.2450(3)0.2316(3)0.08564(19)0.57919(18)0.9709(3)0.1758(3)0.9807(2)0.1819(2)0.42151(15)0.2494(4)0.2595(4)0.65945(19)0.4826(2)00.0108(5)0.0115(5)0.0118(4)0.0134(4)0.0120(6)0.0103(6)0.0105(6)0.0102(4)0.0096(4)

1
2
1
2

0
0.2439(3)0120.2374(4)0.2675(4)Table 7. Anisotropic displacement parameters (U) of the atoms for the selected cerromojonite crystal.
Atom

U 11

U 22

U 33

U 12

U 13

U 23

Pb1
Pb2
Bi1
Bi2
Cu
Se1
Se2
Se3
Se4

0.0116(7)0.0102(8)0.0123(6)0.0149(6)0.0131(8)0.0108(11)0.0114(10)0.0101(7)0.0111(7)0.0118(11)0.0135(10)0.0122(10)0.0136(8)0.0125(7)0.0119(16)0.0111(15)0.0108(7)0.0108(7)0.0089(7)0.0107(8)0.0110(7)0.0118(6)0.0104(9)0.0081(12)0.0089(12)0.0096(11)0.0070(12)0.0005(5)−0.0008(7)−0.0001(4)−0.0001(6)0.0007(4)−0.0007(10)0.0001(10)−0.0004(6)−0.0002(7)

0.000
0.000
0.000
0.000
0.0007(7)
0.000
0.000
0.0000(7)−0.0009(6)

0.000
0.000
0.000
0.000
−0.0003(10)
0.000
0.000
0.0036(13)−0.0004(12)Table 8. Bond distances (in Å) and bond valence sums (BVS in valence units) in the structureof cerromojonite.
Pb1-Se1
Pb1-Se3 (×2)
Pb1-Se2
Pb1-Se3 (×2)
Pb1-Se4 (×2)
BVS

2.814(4)2.975(3)3.107(3)3.334(3)3.408(3)
1.93

Pb2-Se2Pb2-Se4 (×2)
Pb2-Se4 (×2)
Pb2-Se3 (×2)
BVS

2.868(3)2.993(3)3.268(3)3.412(3)1.62

Bi1-Se2
Bi1-Se3 (×2)
Bi1-Se1
Bi1-Se3 (×2)
Bi1-Se4 (×2)
BVS

2.757(4)2.793(3)3.314(3)3.519(3)3.563(3)2.96

Bi2-Se1
Bi2-Se4 (×2)
Bi2-Se4 (×2)
Bi2-Se3 (×2)
BVS

2.622(3)2.785(3)3.395(3)3.620(3)3.22

Cu-Se1
Cu-Se2
Cu-Se4
Cu-Se3BVS

2.482(3)2.522(3)2.557(2)2.598(2)1.29

v.u. = valence units.

The crystal structure of cerromojonite (Figure 9) is identical to those of the three members of thebournonite group: bournonite (PbCuSbS3 ), seligmannite (PbCuAsS3 ), and součekite (PbCuBi(S,Se)3 ).
It consists of [7,9] Pb-polyhedra, [3+2,3+3] Bi-polyhedra, and CuSe4 tetrahedra, which share corners and153

Minerals 2018, 8, 420

edges to form a 3-dimensional framework; CuSe4 tetrahedra share corners to form chains parallelto [001] (Figure 10). The two Pb sites were found to exhibit a mean electron number of 75.0 and
72.9 electrons, respectively. According to their structural environments, and taking into accountthe bond-valence sums calculated using the parameters of Breese and O’Keeffe [17], the following
site-populations were determined: Pb0.52 Ag0.20 Bi0.16 Hg0.12 and Pb0.60 Hg0.16 Ag0.12 Bi0.05 0.07 . Bi-sites
were thought to be ﬁlled by Bi only (according to the site-occupancy reﬁnement), whereas the Cu site
(35.6 electrons) was determined to be Cu0.88 Hg0.12 . Such a cation distribution is in agreement with theobserved bond distances.

Figure 9. The crystal structure of cerromojonite projected down [001] (six unit-cells). The unit-cell andorientation of the ﬁgure are outlined. Symbols: Bi = green dots, Pb = blue dots, Cu = light blue dots,
Se = orange dots.

Figure 10. The crystal structure of cerromojonite projected down [100] (six unit-cells). Symbols as in
Figure 9. CuSe4 tetrahedra are depicted as light blue polyhedra. The unit-cell and orientation of theﬁgure are outlined.

154

Minerals 2018, 8, 420

The overall crystallochemical formula, as obtained through the single-crystal X-ray diffraction
study, is [Cu0.880 Hg0.120 ]Bi[Pb0.560 Ag0.155 Hg0.142 Bi0.107 0.036 ]Se3 (Z = 4), and is in excellent agreementwith that obtained from electron microprobe data.
6. Discussion
Cerromojonite is a new member of the bournonite group and represents the Se-analogue
of součekite, CuPbBi(S,Se)3 [12,18]. Interestingly, whereas previously analyzed součekite always
contained appreciable amounts of Se (together with minor Te) substituting for S, cerromojonite fromEl Dragón is practically devoid of S, containing S at concentrations below its detection limit of ~200 ppm.
Moreover, součekite is characterized by an almost ideal occupancy of the Cu-, Pb-, and Bi-sites,in contrast to cerromojonite, where signiﬁcant amounts of other cations, in particular Ag and Hg,entered the structure.
Conclusions on the physico-chemical environment of cerromojonite formation could be drawn
from the associated Cu selenides. It is apparent that type-I cerromojonite crystallized together withumangite and klockmannite, implying that selenium fugacities (f Se2 ) ﬂuctuated around values deﬁned
by the umangite–klockmannite univariant reaction. Type-II cerromojonite precipitated in equilibriumwith klockmannite, outside the stability ﬁelds of umangite and krut’aite. At T = 100 ◦ C, a temperature
typical for the formation of vein-type selenide deposits, and an elevated oxygen fugacity deﬁned by
the magnetite–hematite buffer, these paragenetic relations are consistent with the range of log f Se2
between −14.6 and −11.6 [19]. The absence of krut’aite and sulﬁdes (chalcopyrite, pyrite) deﬁne themaximum log of sulfur fugacity (f S2 ) to be roughly −19.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/10/420/
s1, CIF: Cerromojonite.
Author Contributions: G.G. collected the samples and manufactured the polished sections; H.-J.F. and G.G.
found the new mineral; H.-J.F. conducted the electron-microprobe analyses; L.B. performed the X-ray structuralinvestigations; C.J.S. and G.G. determined the optical and physical properties; H.-J.F. wrote the paper.
Funding: This research received no external funding.
Acknowledgments: The research was supported by “progetto d’Ateneo 2015, University of Firenze” to L.B. C.J.S.
acknowledges Natural Environment Research Council grant NE/M010848/1 Tellurium and Selenium Cycling
and Supply. Oona Appelt (GFZ) provided assistance with the electron-microprobe work. Constructive commentsof two anonymous reviewers helped to improve the paper.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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crystal structure. Can. Mineral. 2012, 50, 281–294. [CrossRef]
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Förster, H.-J.; Bindi, L.; Stanley, C.J. Grundmannite, CuBiSe2 , the Se-analogue of emplectite: A new mineralfrom the El Dragón mine, Potosí, Bolivia. Eur. J. Mineral. 2016, 28, 467–477. [CrossRef]
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Mgriite, bismuth petrovicite and associated selenides from carbonate veins of the Erzgebirge. Novye Dannyeo Mineralakh 1991, 37, 81–101. (In Russian)Jambor, J.L.; Pertsev, N.N.; Roberts, A.C. New mineral names. Amer. Mineral. 1995, 80, 845–850.
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156

minerals
ArticleFiemmeite Cu2(C2O4)(OH)2 ·2H2O, a New Mineralfrom Val di Fiemme, Trentino, Italy
Francesco Demartin 1, *, Italo Campostrini 1 , Paolo Ferretti 2 and Ivano Rocchetti 2
1
2*

Dipartimento di Chimica, Università degli Studi di Milano, Via C. Golgi 19, I-20133 Milano, Italy;
italo.campostrini@unimi.it
MUSE, Museo delle Scienze di Trento, Corso del Lavoro e della Scienza 3, I-38122 Trento, Italy;
paolo.ferretti@muse.it (P.F.); ivanorocchetti@tiscali.it (I.R.)
Correspondence: francesco.demartin@unimi.it; Tel.: +39-02-503-14457

Received: 28 May 2018; Accepted: 11 June 2018; Published: 12 June 2018

Abstract: The new mineral species ﬁemmeite, Cu2 (C2 O4 )(OH)2 ·2H2 O, was found NE of the Passo di
San Lugano, Val di Fiemme, Carano, Trento, Italy (latitude 46.312◦ N, longitude 11.406◦ E). It occurs
in coaliﬁed woods at the base of the Val Gardena Sandstone (upper Permian) which were permeatedby mineralizing solutions containing Cu, U, As, Pb and Zn. The oxalate anions have originatedfrom diagenesis of the plant remains included in sandstones. The mineral forms aggregate up to
1 mm across of sky blue platelets with single crystals reaching maximum dimensions of about 50 μm.
Associated minerals are: baryte, olivenite, middlebackite, moolooite, brochantite, cuprite, devilline,
malachite, azurite, zeunerite/metazeunerite, tennantite, chalcocite, galena. Fiemmeite is monoclinic,
space group: P21 /c with a = 3.4245(6), b = 10.141(2), c = 19.397(3) Å, β = 90.71(1)◦ , V = 673.6(2) Å3 ,Z = 4. The calculated density is 2.802 g/cm3 while the observed density is 2.78(1) g/cm3 . The six
strongest reﬂections in the X-ray powder diffraction pattern are: [dobs in Å (I)(hkl)] 5.079(100)(020),
3.072(58)(112), 9.71(55)(002), 4.501(50)(022), 7.02(28)(012), 2.686(25)(114). The crystal structure wasreﬁned from single-crystal data to a ﬁnal R1 = 0.0386 for 1942 observed reﬂections [I > 2σ(I)] withall the hydrogen atoms located from a Difference–Fourier map. The asymmetric unit contains two
independent Cu2+ cations that display a distorted square-bipyramidal (4+2) coordination, one oxalateanion, two hydroxyl anions and two water molecules. The coordination polyhedra of the twocopper atoms share common edges to form polymeric rows running along [100] with composition[Cu2 (C2 O4 )(OH)2 ·2H2 O]n . These rows are held together by a well-established pattern of hydrogenbonds between the oxalate oxygens not involved in the coordination to copper, the hydrogen atomsof the water molecules and the hydroxyl anions.
Keywords: ﬁemmeite; new oxalate mineral; Val di Fiemme; Trentino; Italy

1. Introduction
The presence of small Cu ore deposits, in the area close to the Passo di San Lugano, Val di Fiemme,
Carano, Trento, Italy is well known since the XV and XVI century, as documented by the remainsof the old mining sites. From a stratigraphic point of view, the deposits are located within the Val
Gardena Sandstone (upper Permian) a few meters above the limit with the ignimbrites of the AthesianVolcanic Group (lower Permian). The sedimentary sequence of the upper Permian consists of thecontinental deposits of alluvial plain of the Val Gardena Sandstone. The unconformity at the base of
the Val Gardena Sandstone indicates a prolonged sub-aerial exposure with erosion of the volcanic
substrate and consequent articulated topography. In this context, the deposition of the Val Gardena
Sandstone began, in an environment of alluvial plain. The basal portion of the Val Gardena Sandstonecorresponds to the ﬁrst of the ﬁve third-order depositional sequences identiﬁed by Massari et al. [1],and is represented mainly by deposits of alternating alluvial conoids and reddened mudstones with
Minerals 2018, 8, 248; doi:10.3390/min8060248

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pedogenized horizons and evaporites. The mineralization is set at the height of a Cu and U rich level,
located at the base of the Val Gardena Sandstone (upper Permian). The major ore concentrations
are found in deposits of carbon frustules and especially inside coaliﬁed trunks of up to metric sizes,
impregnated with framboidal pyrite, covellite, tennantite and uraninite and surrounded by evident
colored halos of supergenic minerals. This mineralization, referable to “sandstone-uranium type”
deposits, can be explained by a genetic model given by a continental source made up of graniteor acid volcanites that are eroded in an arid continental climate and then transported with U andother heavy metals, such as Cu, Pb, Zn, in the form of ions dissolved in clastic aquifers, in ourcase the alluvial conoid deposits of the Val Gardena Sandstone. The deposition of these ions is dueto the strong decrease in solubility resulting from the reaction between mineralized groundwaterand the strongly reducing environment given by the accumulation of trunks and organic matter inthe channels or deposits of overbank. The peculiar greyish color of the sandstones for a range ofa few meters around the levels with coaliﬁed trunks is a typical example of a reduction front (rollfront) [2]. In this environment (latitude 46.312◦ N, longitude 11.406◦ E) we have recently identiﬁed
the second world occurrence of middlebackite Cu2 C2 O4 (OH)2 [3], a new copper oxalate discovered at
the Iron Monarch quarry, Middleback Range, Australia and approved by the IMA CNMNC in 2016
(IMA 2015-115) [4]. A systematic investigation by micro-Raman spectroscopy of the mineralogicalphases deposited (see Appendix A), allowed us to recognize that, besides middlebackite, other copper
oxalates were present such as moolooite CuC2 O4 ·H2 O and another new mineral with chemical formula
Cu2 (C2 O4 )(OH)2 ·2H2 O, that was approved as a new species by the IMA Commission on New Minerals,
Nomenclature and Classiﬁcation (No. 2017-115) with the name ﬁemmeite, after the type locality where
it was found. This paper deals with the description of the new mineral ﬁemmeite, together with
its crystal structure determination. Holotype material of ﬁemmeite is deposited in the ReferenceCollection of MUSE, Museo delle Scienze di Trento, sample No. 5249.
2. Experimental Data
2.1. Mineral Description and Physical Properties
Fiemmeite occurs in coaliﬁed woods as aggregates up to 1 mm across (Figure 1) made of sky blue
elongated platelets with maximum dimensions about 50 μm. Associated minerals are: baryte, olivenite,
middlebackite, moolooite, brochantite, cuprite, devilline, malachite, azurite, zeunerite/metazeunerite,
tennantite, chalcocite and galena. The streak is pale blue and the lustre is from vitreous to waxy. It isbrittle with cleavage almost perfect parallel to {010} or {001}, according to the weakest bonds observedin the crystal structure, and fracture uneven in the other directions. Its hardness was not determined
due to the minute size of the crystals. Twinning was not observed. The mineral is non-ﬂuorescent bothunder long- and short-wave ultraviolet radiation. A measurement of the density, obtained by ﬂotation
in a diiodomethane-benzene mixture gives the value of 2.78(1) g/cm3 . The density calculated using
the empirical formula and single-crystal unit-cell data is 2.802 g/cm3 .
Crystals appear as highly birefringent platelets but we could not perform a precise opticalcharacterization. The maximum and minimum refractive index we could measure are 1.90 and 1.54.
A Gladstone-Dale calculation using the data of Mandarino [5,6] gives a mean refractive index of 1.64.

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Minerals 2018, 8, 248

Figure 1. Fiemmeite aggregates with olivenite on coaliﬁed wood (base width 4 mm).

2.2. Chemical Data
Insufﬁcient material is available for a direct determination of H2 O or C2 O3 with a CHN analyzer.
The presence of H2 O and C2 O3 was conﬁrmed by crystal structure analysis and Raman spectroscopy.
Crystals rapidly decompose under a microprobe electron beam even using a low voltage current anda wide electron beam, therefore quantitative determination of Cu by microprobe analysis was notpossible. However ﬁve chemical analyses of the copper and zinc content could be carried out, before
damage of the sample, by means of a JEOL JSM 5500LV electron microscope equipped with an IXRF
EDS 2000 microprobe (Table 1) with the following conditions: 20 kV, 10−11 A, 2 μm beam diameter. No
other signiﬁcant element quantities, besides Cu, Zn, O and C, were detected.
Table 1. Chemical data for ﬁemmeite.
Constituent

wt %

Range

Stand. Dev.

Probe Standard

wt % **

Cu
Zn
O
C
H
Total

44.00
0.09
44.40 *
8.34 *
2.10 *
98.93

43.79–44.24
0.06–0.12

0.19
0.02

Synth. CuO
Synth. ZnO

44.57
0
44.89
8.42
2.12
100.00* theoretical for the empirical formula (based on 8 anions) Cu1.996 Zn0.004 (C2 O4 )(OH)2 ·2H2 O; ** theoretical for theempirical formula Cu2 (C2 O4 )(OH)2 ·2H2 O.

2.3. Micro Raman Spectroscopy
The Raman spectrum (Figure 2a,b) was obtained with an ANDOR 303 spectrometer equippedwith a CCD camera iDus DV420A-OE, and with a Nikon CF plan 50×/0.55 objective. The 532 nm lineof an OXXIUS solid state laser was used for excitation. The laser power was set to 10 mW in orderto prevent damage of the crystal, with an aperture of 75 μm and 8.2 mm working distance. The twobands at 1683 and 1705 cm−1 can be assigned to νa (C=O), that at 1457 cm−1 to νs (C-O) + νs (C-C),
that at 903 cm−1 to νs (C-O) + δ(O-C=O), that at 853 cm-1 to νs (C-O)/δ(O-C=O). Those at 466, 517
and 543 cm−1 are attributable to ν(Cu-O) + ν(C-C). The remaining below 298 cm−1 are assigned toout-of-plane bends and to lattice modes [7]. The observed bands at 3438 and 3471 cm−1 (Figure 2b) of
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Minerals 2018, 8, 248

the OH/H2 O region are consistent with the range of hydrogen bond lengths found (2.655–2.903 Å),
according to the Libowitzky correlation [8].

(a)

(b)
Figure 2. Micro-Raman spectrum of ﬁemmeite. (a) Oxalate and lattice mode bands; (b) hydrogenbond bands.

2.4. Crystallography
X-ray powder diffraction data of ﬁemmeite were obtained using a Bruker D8 diffractometer withgraphite monochromatized CuKα radiation (Table 2). The unit-cell parameters, reﬁned from powder
data using the UNITCELL software [9], are: a = 3.4345(5), b = 10.159(2), c = 19.412(3) Å, β = 90.83(1)◦ ,V = 677.5(1) Å3 .
Details of the single-crystal X-ray diffraction data collection and reﬁnement are given in Table 3.
7512 intensities were collected at room temperature on a Bruker Apex II diffractometer with MoKα
radiation (λ = 0.71073 Å) up to 2θ = 63.16◦ , of which 2118 unique. SADABS [10] absorption correction
was applied. The structure was solved with direct methods [11] and reﬁned with SHELXL-2017 [12]to a ﬁnal R1 = 0.0386 for 1942 observed reﬂections [I > 2σ(I)]. All the non-hydrogen atoms werereﬁned anisotropically. All the hydrogen atoms were located in a Difference-Fourier map and reﬁned.
The coordinates and displacement parameters of the atoms are reported in Table 4; selected interatomic
distances and angles and bond-valence values are listed in Table 5. The CIF ﬁle of ﬁemmeite is available
as Supplementary Material.

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Table 2. X-ray powder diffraction data for ﬁemmeite.
dobs (Å)

Iobs

dcalc (Å) $Icalc $h, k, ldobs (Å)

Iobs

dcalc (Å) $Icalc $h, k, l

9.71
7.02

55
28

2.151

13

3.237

5

1.998

3

3.198
3.072
3.001
2.891

5
58
2
20

1.943

8

1.873

5

2.730

15

2.686

25

002
012
013
020
021
004
022
023
031
−1 0 2006−1 −1 1
032
112
033
113
−1 0 4−1 −2 1
−1 2 2026−1 −1 4114
−1 −2 3123035
041
−1 −1 5
−1 −2 4
042
124
027
008
−1 −3 1043
−1 −3 2036
106132
−1 1 6−1 −3 3
133

5
12

100
12
2
50
4

68
34
3
100
10
5
40
6
2
4
7
1
7
65
6
18
2
5
2
9
13
32
2
1
6
1
6
7
2
5
3
1
1
2
2
3
2
3
9
2
4

2.251
2.190

5.079
4.914
4.855
4.501
3.996

9.698
7.009
5.452
5.071
4.906
4.849
4.493
3.990
3.330
3.241
3.233
3.193
3.192
3.087
2.996
2.886
2.813
2.811
2.731
2.726
2.711
2.682
2.608
2.589
2.549
2.514
2.503
2.474
2.453
2.438
2.431
2.424
2.390
2.360
2.340
2.336
2.336
2.330
2.303
2.261
2.248

1.755

7

1.695

9

1.657

3

2.247
2.187
2.162
2.147
2.122
2.052
2.036
2.028
2.017
1.997
1.995
1.972
1.953
1.938
1.935
1.905
1.883
1.873
1.870
1.824
1.809
1.797
1.752
1.745
1.719
1.716
1.697
1.689
1.688
1.687
1.666
1.656
1.639
1.642
1.636
1.609
1.603
1.598
1.596
1.578
1.544

3
16
6
17
3
1
2
6
2
1
7
1
1
13
4
3
2
2
6
6
2
4
10
1
1
3
2
7
6
3
2
6
5
4
3
3
4
4
2
2
3

044
028
−1 −3 4134
126
−1 −3 5135
−1 −4 1051
−1 −4 2046
127
−1 1 8−1 3 6053
0 1 10−1 −4 4144047
−1 3 7−1 −4 5055
048
150
−1 −5 2152
−1 0 10−2 0 2210−1 −5 3−2−1 21 1 10−1 −5 4049
147
−1 2 10−2 −2 2139064
−2 −2 3−2 −2 4

2.552

3

2.511
2.468

2
2

2.442

4

2.310

6

Note: $ Pattern calculated on the basis of the single crystal data and structure reﬁnement.

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Minerals 2018, 8, 248

Table 3. Single-crystal diffraction data and reﬁnement parameters for ﬁemmeite.
Crystal System

Monoclinic

Space Group
a (Å)
b (Å)
c (Å)
β (◦ )
V (Å3 )Z
Radiation
μ (mm−1 )Dcalc (g·cm−3 )Measured reﬂections
Rint
Independent reﬂections
Range of h, k, lObserved reﬂections [I > 2σ(I)]Parameters reﬁned
Final R1 [I > 2σ(I)] and wR2 (all data)
GooF
Max/min residuals (e/Å3 )P21 /c (No. 14)
3.4245(6)10.141(2)19.397(3)90.71(1)673.6(2)4MoKα (λ = 0.71073 Å)
6.322
2.802
7512
0.02942118−5 ≤ h ≤ 4, −14 ≤ k ≤ 14, −28 ≤ l ≤ 28
1942
133
0.0386, 0.0905
1.176
1.37/−0.73

Notes: R1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = {Σ[w(Fo2 − Fc2 )2 ]/Σ[w(Fo2 )2 ]}1/2 ; w = 1/[σ2 (Fo2 ) + (0.0268q)2 +3.8682q] where q = [max(0, Fo2 ) + 2Fc2 ]/3; GooF = {Σ[w(Fo2 − Fc2 )]/(n − p)}1/2 where n is the number of reﬂections
and p is the number of reﬁned parameters.

Table 4. Atom coordinates and displacement parameters [Ueq /Uij , Å2 ].
Atom

x/a

y/b

z/c

U eq

Cu1
Cu2
C1
C2
O1
O2
O3
O4
OH5
OH6
Ow7
Ow8
H5
H6
H71
H72
H81
H82

0.19372(13)0.61198(13)0.0211(11)−0.1380(10)0.2069(8)−0.1081(8)−0.0369(9)−0.2941(9)0.6196(7)0.1952(8)0.9519(9)0.5910(10)0.666(16)0.182(14)0.861(16)0.932(19)0.706(17)0.504(15)0.24133(4)0.29545(4)0.1394(3)0.2827(4)0.1067(3)0.3439(3)0.0695(3)0.3269(3)0.1665(2)0.3715(3)0.1899(3)0.4305(3)0.0765(16)0.4626(13)0.193(6)0.0986(14)0.423(7)0.517(2)0.50020(2)0.37303(2)0.6268(2)0.6259(2)0.5731(1)0.5690(1)0.6773(1)0.6784(1)0.4475(1)0.4273(1)0.3121(1)0.3023(1)0.441(3)0.434(3)0.2664(11)0.318(3)0.2589(15)0.309(3)0.0126(1)0.0131(1)0.0130(6)0.0132(6)0.0160(5)0.0169(5)0.0228(6)0.0221(6)0.0130(5)0.0139(5)0.0193(5)0.0263(7)0.032(15)0.026(14)0.037(16)0.046(18)0.047(19)0.028(14)

Atom

U 11

U 22

Cu1
Cu2
C1
C2
O1
O2
O3
O4
OH5
OH6
Ow7
Ow8

0.0151(2)0.0156(2)0.0148(15)0.0096(14)0.0211(13)0.0236(14)0.0360(17)0.0328(16)0.0149(12)0.0159(12)0.0301(15)0.0406(18)0.0118(2)0.0120(2)0.0107(14)0.0151(15)0.0124(12)0.0124(12)0.0152(13)0.0182(13)0.0097(11)0.0116(11)0.0139(12)0.0192(14)

U 33

U 23

0.01106(19) 0.00217(14)0.01179(19) 0.00236(14)0.0137(14) −0.0003(11)0.0149(15) −0.0014(12)0.0147(11)0.0011(9)0.0147(11)0.0019(9)0.0175(13) 0.0055(10)0.0155(12) −0.0012(10)0.0145(11)0.0019(9)0.0144(11)0.0026(9)0.0141(12) −0.0007(9)0.0194(13) 0.0079(11)

U 13

U 12

0.00295(14)0.00336(14)0.0008(11)−0.0002(11)0.0044(9)0.0056(9)0.0091(11)0.0075(11)0.0029(9)0.0034(9)0.0045(10)0.0132(12)0.00301(15)0.00312(15)0.0013(12)0.0009(12)0.0056(10)0.0053(10)0.0069(12)0.0090(12)0.0042(9)0.0033(9)0.0041(11)0.0133(13)The anisotropic displacement factor exponent takes the form: −2π2 (U11 h2 (a*)2 + ... + 2U12 hka*b* + ...); Ueq according
to Fischer and Tillmans [13].

162Minerals 2018, 8, 248

Table 5. Selected interatomic distances (Å), angles (◦ ) and bond-valence sums (vu).
Atom1-Atom2

Distancevu

Atom1-Atom2

Distancevu

Cu1-OH6
Cu1-OH5
Cu1-O1
Cu1-O2
Cu1-OH5#1
Cu1-O2#2

0.50
0.46
0.43
0.40
0.16
0.03
1.98

Cu2-Ow8
Cu2-OH5
Cu2-OH6
Cu2-Ow7
Cu2-OH6#2
Cu2-Ow7#1

1.939(3)1.948(3)1.943(3)1.983(3)2.375(3)2.754(3)Total
1.552(5)1.240(4)1.270(4)angle
114.5(3)125.2(3)119.3(3)

0.46
0.45
0.46
0.41
0.14
0.05
1.97

Cu1-Cu2
C1-O3
C1-O1
Atom1-Atom2-Atom3
O1-C1-O3
C1-C2-O2
C1-C2-O4

1.935(3)1.946(2)1.966(3)1.992(3)2.332(3)2.916(3)Total
2.9198(7)1.228(4)1.271(4)angle
126.2(3)115.8(3)118.9(3)Atom1 . . . Atom2
OH5 . . . O1#3
OH6 . . . O2#4
Ow7 . . . O3#3
Ow7 . . . O4#5
Ow8 . . . O4#4
Ow8 . . . O3#5

distance
2.862(4)2.903(4)2.655(4)2.722(4)2.691(4)2.753(4)

C1-C2
C2-O4
C2-O2
Atom1-Atom2-Atom3
O1-C1-C2
O2-C2-O4
C2-C1-O3
Hydrogen bond interactions
Atom1-Atom2 . . . Atom3
OH5-H5 . . . O1#3
OH6-H6 . . . O2#4
Ow7-H72 . . . O3#3
Ow7-H71 . . . O4#5
Ow8-H82 . . . O4#4
Ow8-H81 . . . O3#5

angle
177(5)
169(5)
163(6)
171(6)
175(5)
172(6)

Symmetry codes: #1 = x − 1,y,z; #2 = x + 1,y,z; #3 = 1 − x, −y, 1 − z; #4 = −x, 1 − y, 1 − z; #5 = x + 1, 1/2 − y,z − 1/2. Bond-valence parameters after [14].

3. Crystal Structure Description and Discussion
The asymmetric unit of ﬁemmeite contains two independent Cu2+ cations, one oxalate anion,
two hydroxyl anions and two water molecules (Figure 3a). Both copper cations display a distorted
square-bipyramidal (4+2) coordination with four equatorial distances in the range 1.935(3)–1.992(3) Å,
one apical distance intermediate [2.332(3)–2.375(3) Å] and one apical distance longer 2.754(3)–2.916(3) Å.
Cu1 is coordinated by three oxygens of the oxalate anion and by three OH- anions. Cu2 is coordinatedby three water molecules and by three OH− anions.
The coordination polyhedra of the two copper atoms shares common edges to form polymericrows, with composition [Cu2 (C2 O4 )(OH)2 ·2H2 O]n , running along [100]. These rows are held together
by a well-established pattern of hydrogen bonds (see Table 5) between the oxalate oxygens O3 andO4, not involved in the coordination to copper and the hydrogen atoms of the water molecules andbetween the other oxalate oxygens O1 and O2 and the hydroxyl anions.
Fiemmeite is the third example of oxalate containing only Cu2+ as cation, the others being
moolooite CuC2 O4 ·H2 O [15], and middlebackite Cu2 C2 O4 (OH)2 [4]. Other copper(II) oxalates withadditional cations are wheatleyite Na2 Cu(C2 O4 )2 ·2H2 O [16] and antipinite KNa3 Cu2 (C2 O4 )4 [17].
A comparison of the crystal structure of ﬁemmeite with that of the other oxalates containing only
copper can be done with middlebackite only, because the structure of moolooite is unknown.

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Minerals 2018, 8, 248

(a)

(b)
Figure 3. Projection along [100] with atoms labelling (a) and perspective view (b) of the crystal structureof ﬁemmeite with hydrogen bonds represented as dashed lines.

A portion of the polymeric rows with composition [Cu2 (C2 O4 )(OH)2 ·2H2 O]n is reported in
Figure 4a and can be compared with those observed in the structure of middlebackite (Figure 4b).
In ﬁemmeite, the oxalate anion acts as a chelating bidentate ligand with only one of the two
independent copper ions, whereas in middlebackite the same anion is tetradentate, acting as a bridge
between dimeric octahedral copper units, to form rows extending along [101]. In middlebackite, theserows are interconnected to form channels where the hydroxyl hydrogens are located.

164

Minerals 2018, 8, 248

(a)

(b)

Figure 4. Portions of the polymeric rows observed in ﬁemmeite (a) and middlebackite (b).

No structural relationships to the other copper-containing oxalates or synthetic compound has
been established. In fact, the crystal structure of the synthetic analogue of wheatleyite consists ofcolumns of Cu-centered edge-sharing square bipyramids running along [100] (a axis 3.6 Å) andcorrugated layers of seven-coordinate Na-centered polyhedra, held together by the oxalate anions andH bonds of water molecules in the vertices of Na-centered polyhedra. In antipinite, columns of edge
sharing Cu-centered bipyramids running along [100] are instead combined into a layer by pairs ofother Cu bipyramids.
Fiemmeite belongs to class 50.01.06 in the New Dana classiﬁcation (Salts of Organic Acids,
Oxalates) and to 10.AB (Oxalates) in the Nickel–Strunz classiﬁcation [18].
Supplementary
Materials:
The
www.mdpi.com/2075-163X/8/6/248/s1.

CIF

ﬁle

of

ﬁemmeiteisavailable

onlineat

Author Contributions: I.R. made the Raman measurements that allowed to discover the new mineral. F.D.
performed the diffraction experiments and structure solution. P.F. and I.C. performed the chemical analyses andthe determination of the mineral properties. P.F. and I.R. collected most of the material studied.
Funding: This research was funded by Euregio Science Fund (call 2014, IPN16) of the Europaregion Euregio;
project “The end-Permian mass extinction in the Southern and Eastern Alps: extinction rates vs. taphonomic
biases in different depositional environments”.
Acknowledgments: The authors thank the Department of Innovation, Research and University of theAutonomous Province of Bozen/Bolzano for covering the Open Access publication costs. Stefano Dallabona is
gratefully thanked for having donated some specimens for study. We also thank Alessandro Guastoni for having
provided us with the Raman Spectrum of moolooite from Monte Cervandone and three anonymous referees for
useful suggestions.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

Appendix A

Figure A1. A comparison of the Raman spectra of ﬁemmeite, middlebackite and moolooite from Passo
di San Lugano and moolooite from Monte Cervandone, Val d’Ossola, Italy.

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Minerals 2018, 8, 248

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2000.
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Polidori, G.; Spagna, R. SIR97, a new tool for crystal structure determination and reﬁnement.
J. Appl. Crystallogr. 1999, 32, 115–119. [CrossRef]
Sheldrick, G.M. Crystal structure reﬁnement with SHELXL. Acta Crystallogr. C Struct. Chem. 2015, 71, 3–8.
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a natural sodium copper salt of oxalic acid. Am. Mineral. 1986, 71, 1240–1242.
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2007, 45, 983–1055. [CrossRef]© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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minerals
ArticleTiberiobardiite,
Cu9Al(SiO3OH)2(OH)12(H2O)6(SO4)1.5 ·10H2O, a New
Mineral Related to Chalcophyllite from the Cretaio
Cu Prospect, Massa Marittima, Grosseto (Tuscany,
Italy): Occurrence and Crystal Structure
Cristian Biagioni 1, *, Marco Pasero 1 and Federica Zaccarini 2
1
2*

Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria 53, I-56126 Pisa, Italy;
marco.pasero@unipi.it
Department of Applied Geological Sciences and Geophysics, University of Leoben, Peter Tunner Str. 5,
A-8700 Leoben, Austria; Federica.Zaccarini@unileoben.ac.at
Correspondence: cristian.biagioni@unipi.it; Tel.: +39-050-221-5789

Received: 23 March 2018; Accepted: 9 April 2018; Published: 11 April 2018

Abstract: The new mineral species tiberiobardiite, ideally Cu9 Al(SiO3 OH)2 (OH)12 (H2 O)6 (SO4 )1.5 ·10H2 O,
has been discovered in the Cretaio Cu prospect, Massa Marittima, Grosseto, Tuscany, Italy, as very
rare, light green, vitreous, tabular {0001}, pseudo-hexagonal crystals, up to 200 μm in size and 5 μm
in thickness, associated with brochantite. Electron microprobe analysis gave (in wt %, average of
5 spot analyses): SO3 10.37, P2 O5 3.41, As2 O5 0.05, SiO2 8.13, Al2 O3 5.54, Fe2 O3 0.74, CuO 62.05,
and ZnO 0.03, for a total of 90.32. Based on an idealized O content of 42 atoms per formula unit,
assuming the presence of 16 H2 O groups and 13.5 cations (without H), the empirical formula oftiberiobardiite is (Cu8.69 Al0.21 Fe0.10 )Σ9.00 Al1.00 (Si1.51 P0.54 )Σ2.05 S1.44 O12.53 (OH)13.47 ·16H2 O. The main
diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 9.4 (s),4.67 (s), 2.576 (m), 2.330 (m), and 2.041 (mw). The crystal structure study revealed tiberiobardiite to betrigonal, space group R 3, with unit-cell parameters a = 10.6860(4), c = 28.3239(10) Å, V = 2801.0(2) Å3 ,and Z = 3. The crystal structure was reﬁned to a ﬁnal R1 = 0.060 for 1747 reﬂections with Fo > 4σ (Fo )and 99 reﬁned parameters. Tiberiobardiite is the Si-analogue of chalcophyllite, with Si4+ replacing
As5+ through the coupled substitution As5+ + O2− = Si4+ + (OH)− . The name tiberiobardiite honors
Tiberio Bardi (b. 1960) for his contribution to the study of the mineralogy of Tuscany.
Keywords: tiberiobardiite; chalcophyllite group; copper; silicate; sulfate; Cretaio; Tuscany; Italy

1. Introduction
The occurrences of small Cu ore deposits in the ophiolitic sequences of the Northern Apennines(Italy) have been known for a long time, and they represent one of the few Cu sources in Italy.
Notwithstanding their past economic importance, little mineralogical and geological information
has been provided about these deposits (e.g., [1–4]). These studies mainly focused on the primaryore mineralogy, whereas secondary assemblages were usually not taken into account. On the other
hand, the potential occurrence of interesting mineral species was suggested by the identiﬁcation ofphosphates belonging to the mixite group (e.g., calciopetersite from Monte Beni, Firenzuola [5]).
The small Cu prospect of Cretaio (latitude 43◦ 03 10” N, longitude 10◦ 58 40” E), near the small
village of Prata, Massa Marittima, Grosseto, Tuscany, Italy, is characterized by several alteration phasesof the primary Cu-Fe ore minerals [6]. During the study of this mineral assemblage, a phase related
to chalcophyllite was identiﬁed. Its unit-cell volume was signiﬁcantly smaller than chalcophyllite

Minerals 2018, 8, 152; doi:10.3390/min8040152

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Minerals 2018, 8, 152(ΔV = −2.5%), thus indicating the likely occurrence of some chemical substitutions. Indeed, qualitative
chemical analyses, as well as Raman spectroscopy, suggested the absence of As in the studiedmaterial, making a structural investigation mandatory to fully characterize the mineral. Quantitative
chemical analysis and crystal structure reﬁnement proved that the studied phase is the Si-analogue ofchalcophyllite. This new mineral was named tiberiobardiite, in honor of the mineral collector Tiberio
Bardi (b. 1960), for his contribution to the study of Tuscan mineralogy. Tiberio Bardi provided thetype material of volaschioite [7] and promoted the mineralogical investigation of the ophiolite-hosted
mineralization occurring in the Monte Beni area, north of Florence [8], where he collected the specimenof calciopetersite used for the crystal structure study [5]. Recently, together with other mineral collectors(Cristiano Bazzoni, Mauro Bernocchi, Cesare Betti, and Riccardo Marini), Tiberio Bardi contributed
to the mineralogical rediscovery of the small Cu ore deposits of Cretaio, where he ﬁrst collected thetype material of tiberiobardiite. The mineral and its name have been approved by the IMA CNMNC,
under the number 2016-096. The holotype specimen is deposited in the mineralogical collection of
the Museo di Storia Naturale of the University of Pisa, Via Roma 79, Calci (Pisa), under the catalogue
number 19900.
The occurrence, physical properties, and crystal structure of tiberiobardiite are described inthis paper.
2. Geological Setting and Occurrence of Tiberiobardiite
No detailed geological studies have been performed on the small Cu ore deposit of Cretaio.
The available data are limited to some mining reports dating to the early decades of the 20th century [6].
The ore deposit is represented by a small concentration of Cu sulﬁdes (bornite, chalcocite, and covellite)
and hematite, scattered as stockwork veins within highly deformed gabbro belonging to the Liguride
units. Minor chalcopyrite has been observed only in quartz + calcite veins.
The primary sulﬁdes are strongly altered into a series of secondary minerals: antlerite, brochantite,chalcanthite, chalcoalumite, connellite, langite, libethenite, malachite, posnjakite, serpierite/devilline,
and spangolite. Tiberiobardiite was found associated with abundant brochantite. Its origin is likely
related to the supergene alteration of the Cu ore minerals in an oxidizing and hydrous low-T
environment. The individual elements were derived from Cu ores (Cu and S) and from rock-forming
minerals occurring in the gabbro (Al, Si, and minor P).
3. Experimental Data
3.1. Mineral Description and Physical Properties
Tiberiobardiite (Figure 1) occurs as thin, tabular {0001} crystals, having a pseudo-hexagonal
outline, up to 200 μm in diameter and 5 μm in thickness. The mineral is green, with a pale green streak;it is transparent, and its luster is vitreous. Tiberiobardiite is brittle, with a perfect {0001} cleavage.
Its fracture is irregular. Its hardness and density, as well as optical properties, were not measured,owing to the small amount of available material. The calculated density, based on the ideal formula,is 2.528 g/cm3 . The mean refractive index, obtained from the Gladstone-Dale relationship [9,10],
using the ideal formula and the calculated density, is 1.568.

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Minerals 2018, 8, 152

Figure 1. Tiberiobardiite, thin, pseudo-hexagonal crystals, up to 40 μm in size, with iron oxides. Cretaio,
Prata, Massa Marittima, Grosseto, Tuscany. Holotype specimen, #19900, collection of the Museo diStoria Naturale of the University of Pisa.

3.2. Chemical Data
Preliminary energy dispersive spectrometry (EDS) analysis showed the presence of Cu, Al, Fe, P,
Si, and S as the only elements with Z > 8.
Quantitative chemical analyses were carried out using a Superprobe JEOL JXA 8200 electron
microprobe operating in WDS mode at the Eugen F. Stumpﬂ laboratory, Leoben University, Austria.
The experimental conditions were as follows: accelerating voltage 20 kV, beam current 10 nA, and beam
size 3 μm. The standards were (element and emission line) as follows: chromite (Al Kα), chalcopyrite(S Kα, Cu Kα), zircon (Si Kα), pyrite (Fe Kα), skutterudite (As Lα), apatite (P Kα), and sphalerite(Zn Kα). The following diffracting crystals were selected: PETJ for S, Si, and P; TAP for Al and As;
and LIFH for Fe, Cu, and Zn. Direct H2 O determination was not performed, owing to the scarcityof the material. The occurrence of H2 O, suggested by the crystal structure reﬁnement, was further
conﬁrmed by micro-Raman spectroscopy (see below).
The collection of reliable chemical data for tiberiobardiite was a very difﬁcult task, owing to thesmall size of the available crystals, which made the preparation of a good-quality polished surface
difﬁcult. Moreover, the mineral is unstable under the electron beam, showing evidence of strong
dehydration. Indeed, the structural determination (see below) indicated the occurrence of 16 H2 O, 13.5
cations (without H atoms), and ~14 OH groups, implying the occurrence of ~36.6 wt % H2 O. It should
be noted that by adding such an H2 O content to the total given in Table 1, an unrealistically high totalof ~126.9 wt % is obtained. Notwithstanding such an important limitation, the atomic ratios among
cations are in good agreement with the results of the structural study.
Table 1. Chemical data of tiberiobardiite.
Oxide

wt % (n = 5)

Rangee.s.d.

SO3
P2 O5
As2 O5SiO2Al2 O3Fe2 O3
CuO
ZnO
Total

10.37
3.41
0.05
8.13
5.54
0.74
62.05
0.03
90.32

9.67–10.94
3.02–3.80
0.00–0.17
7.29–9.03
4.93–6.47
0.61–0.83
57.44–65.20
0.00–0.10
84.86–93.70

0.51
0.28
0.07
0.77
0.57
0.09
3.06
0.04
3.35

On the basis of an idealized O content of 42 atoms per formula unit (apfu), to becompared with 42.2 found in the structural study (see below), assuming the occurrenceof 16 H2 O groups and 13.5 cations (without H), the empirical formula of tiberiobardiite
169

Minerals 2018, 8, 152

is (Cu8.69 Al0.21 Fe0.10 )Σ9.00 Al1.00 (Si1.51 P0.54 )Σ2.05 S1.44 O12.53 (OH)13.47 ·16H2 O, with rounding errors.
The ideal formula is Cu9 Al[SiO3 (OH)]2 (SO4 )1.5 (OH)12 ·16H2 O, which corresponds to the following(in wt %): SO3 8.45, SiO2 8.45, Al2 O3 3.59, CuO 50.36, and H2 O 29.15, for a sum of 100.00.
3.3. Micro-Raman Spectroscopy
Unpolarized micro-Raman spectra were collected on an unpolished sample of tiberiobardiite
in nearly backscattered geometry using a Jobin-Yvon Horiba XploRA Plus apparatus equipped witha motorized x-y stage and an Olympus BX41 microscope with a 50× objective. The 532 nm lineof a solid-state laser was used. The minimum lateral and depth resolution was set to a few μm.
The system was calibrated using the 520.6 cm−1 Raman band of silicon before each experimental
session. The spectra were collected through multiple acquisitions with single counting times of 120 s.
The backscattered radiation was analyzed with a 600 mm−1 grating monochromator.
Figure 2 shows the observed Raman spectrum of tiberiobardiite. The region between 100 and
1200 cm−1 (Figure 2a) can be divided into three ranges, as follows:
Range 100–300 cm−1 : bands at 124, 203, and 261 cm−1 , possibly related to lattice modes;
Range 300–700 cm−1 : bands at 394, 440, 487, 544, and 589 cm−1 , related to the bending modes ofthe [SiO3 (OH)] and (SO4 ) groups;(iii) Range 700–1200 cm−1 : two bands at 965 and 1097 cm−1 occur. The former could be due to thecontribution of the [SiO3 (OH)] and (SO4 ) groups, whereas the weak band at 1097 cm−1 is likely
related to the (SO4 ) antisymmetric stretching, in agreement with the band position in the Raman
spectrum of chalcophyllite (1100 cm−1 ; [11]). A similar interpretation was proposed for the bands
observed between 900 and 1150 cm−1 in barrotite [12]. It is worth noting the absence of the strong
band at ~840 cm−1 occurring in the Raman spectrum of chalcophyllite, which was interpreted as
being due to the (AsO4 ) symmetric stretching mode [11]. The same feature, at a slightly lower
wavenumber (825 cm−1 ), was reported in barrotite [12].
(i)
(ii)

Figure 2. Micro-Raman spectrum of tiberiobardiite in the spectral range 100–1200 cm−1 (a) and
3000–4000 cm−1 (b). In (b), the components contributing to the broad band are shown.

In the region between 3000 and 4000 cm−1 , a strong and broad band was observed (Figure 2b).
Such a band is related to O–H stretching vibrations. The band deconvolution, performed using thesoftware Fityk [13], revealed the presence of three bands, centered at 3218, 3418, and 3555 cm−1 .
By using the relationships between O–H stretching frequencies and O· · · O distances [14], the three
Raman bands could correspond to three different kinds of hydrogen bonds (i.e., short (=strong) bonds(O· · · O ~2.70 Å), intermediate (O· · · O ~2.80 Å), and long (=weak) bonds (O· · · O ~3.00 Å)).
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Minerals 2018, 8, 152

3.4. Crystallography
Powder X-ray diffraction data of tiberiobardiite were collected using a 114.6 mm Gandolﬁ camera
and Ni-ﬁltered Cu Kα radiation (Table 2). Unit-cell parameters were not reﬁned from the X-ray powder
diffraction pattern owing to the multiple indexing for most of the observed reﬂections.
Table 2. Observed and calculated X-ray powder diffraction data (d in Å) for tiberiobardiite.
Iobs

dobs

Icalc

dcalc

hkl

Iobs

dobs

Icalc

dcalc

hkl

s

9.4

100

9.36

003

-

-

2

2.646

1 2 −7

-

-

4
2

5.62
5.34

104
110

m

2.576

2
13

2.571
2.571

223
2 2 −3

-

-

m

2.330

10

2.325

2 2 −6

5

2.325

226

7
1

2.037
2.037

2 2 −9
229

2

4.83

1 0 −5

27

4.72

006

4.67

4
5

4.65
4.65

113
1 1 −3

mw

2.041

-

-

3

3.873

2 0 −4

vw

1.859

2

1.854

2 0 14

-

-

6
5

3.584
3.147

205
009

vw

1.776

2
3

1.769
1.769

2 2 122 2 −12

-

-

1
1

3.136
3.085

214
300

w

1.548

4
1

1.542
1.542

600
2 2 −15

-

-

2
2

2.976
2.932

125
3 0 −3

w

1.528

2
2

1.522
1.522

603
6 0 −3

vw
-

2.73
-

3
2

2.712
2.708

1 1 −91 0 10

w

1.469

2
2

1.466
1.466

606
6 0 −6

w

2.68

13

2.672

220

s

Note: intensity and dhkl were calculated using the software PowderCell 2.3 [15] based on the structural model given
in Table 4. Only reﬂections with Icalc >1 are listed, if not observed. The ﬁve strongest reﬂections are given in
bold. Observed intensities were visually estimated (s = strong; m = medium; mw = medium-weak; w = weak;
vw = very weak).

A single-crystal X-ray diffraction study was performed using a Bruker Smart Breeze diffractometerequipped with an air-cooled CCD detector and graphite-monochromatized Mo Kα radiation.
The detector-to-crystal working distance was 50 mm. Data were corrected for the Lorentz-polarization
factor, absorption, and background effects, using the package of software Apex2 [16]. Unit-cellparameters of tiberiobardiite are a = 10.6860(4), c = 28.3239(10) Å, V = 2801.0(2) Å3 , and space group
R 3, suggesting isotypic relationships between tiberiobardiite and chalcophyllite [17]. Consequently,
the crystal structure of the former was reﬁned using Shelxl-2014 [18] starting from the atomic
coordinates of the latter. Scattering curves for neutral atoms were taken from the International Tables
for Crystallography [19]. After several cycles of isotropic reﬁnement, the R factor converged to 0.22,
lowered to 0.16 after taking into account the twinning according to the {10-10} plane. In agreementwith the chemical data, the site hosting As in chalcophyllite was found to be occupied by lighter atoms(i.e., Si and P). Its site occupancy was ﬁxed based on electron microprobe data, owing to the similarity
between the scattering factors of Si and P. In agreement with [17], the displacement parameters of the
S atom and the O atoms belonging to the SO4 group (i.e., O6 and O7) were large. The reﬁnement of
the site occupancy at the S site pointed to S0.82(1) , then ﬁxed to 0.75 to achieve the electrostatic balance.
The Ow8 site was found to be split into two sub-positions Ow8a and Ow8b. The latter was too close toOw9, and consequently, the site occupancy at Ow9 was constrained to be the same as that at Ow8a inorder to avoid the too short Ow9–Ow8b distance. Hydrogen atoms were not located. After several
cycles of anisotropic reﬁnement for all the atoms, the R1 converged to 0.0602 for the 1747 reﬂections
with Fo > 4σ (Fo ). Details of the data collection and reﬁnement are given in Table 3. CIF is available as
Supplementary Materials.

171

Minerals 2018, 8, 152

Table 3. Crystal and experimental details for tiberiobardiite.
Crystal data
0.180 × 0.050 × 0.005
Trigonal, R 3
10.6860(4), 28.3239(10)2801.0(2)3Crystal size (mm)Cell setting, space groupa, c (Å)
V (Å3 )Z
Data collection and reﬁnement

Mo Kα, λ = 0.71073
293
58.35
18,9122809
1747
0.06330.0799−10 ≤ h ≤ 14, −14 ≤ k ≤ 13, −38 ≤ l ≤ 37
0.06020.11670.13541.044
991.26 (at 0.98 from Si), −1.10 (at 0.65 from Cu2)

Radiation, wavelength (Å)
Temperature (K)2θmax (◦ )Measured reﬂections
Unique reﬂections
Reﬂections with Fo > 4σ (Fo )Rint
RσRange of h, k, l
R [Fo > 4σ (Fo )]R (all data)wR (on F2 )GooF
Number of least-square parameters
Maximun and minimum residuals (e/Å3 )The crystal structure reﬁnement of tiberiobardiite points to the crystal chemical formula
Cu9 (Al0.96 Fe0.04 )[(Si0.75 P0.25 )O3 (OH0.75 O0.25 )]2 (OH)12 (H2 O)6 (SO4 )1.5 ·10.2H2 O (Z = 3).
4. Crystal Structure Description
4.1. General Features and Cation Coordinations
The atomic coordinates, site occupancies, and equivalent isotropic displacement parameters oftiberiobardiite are given in Table 4. Table 5 reports selected bond distances, whereas Table 6 shows thebond-valence balance.
The crystal structure of tiberiobardiite (Figure 3) is composed by ﬁve independent cation positions(Cu1, Cu2, Al, Si, and S) and nine anion sites in the asymmetric unit. It can be described as formed by{0001} heteropolyhedral layers, composed by CuΦ6 polyhedra (Φ = O, OH, H2 O), Al(OH)6 octahedra,and (Si,P)O3 (OH,O) tetrahedra, alternating with interlayers hosting SO4 and H2 O groups. The {0001}
heteropolyhedral layers are formed through the edge-sharing of Cu polyhedra, with three unoccupied
octahedral positions out of 12 available ones per unit cell. Such unoccupied positions actually host Al,
lying on the −3-point symmetry, and the Si site, on the 3-point symmetry, a little off the {0001} sheet.
The Si-centered tetrahedra are alternatively disposed above and below the sheet.
Table 4. Site labels, Wyckoff sites, site occupancy factor (s.o.f.), atom coordinates, and equivalentisotropic displacement parameters (Å2 ) for tiberiobardiite.
Site

Wyckoff Sites.o.f.

x/a

y/b

z/c

U eq

Cu1
Cu2
Al
Si
S
OH1
O2
Ow3
OH4
OH5
O6
O7
Ow8a
Ow8b
Ow9

9d
18f
3b
6c
6c
18f
18f
18f
18f
6c
6c
18f
18f
18f
18f

Cu1.00Cu1.00Al0.96 Fe0.04
Si0.75 P0.25
S0.75
O1.00
O1.00
O1.00
O1.00
O1.00
O0.75
O0.75
O0.70(1)O0.30(1)O0.70(2)1/2
0.33920(8)
0
2/3
1/3
0.3223(4)0.5279(4)0.3482(6)0.1312(4)2/3
1/3
0.2174(9)0.3180(10)0.249(3)0.4200(12)1/20.18074(7)
0
1/3
2/3
0.3489(4)0.3353(4)0.4550(5)0.1635(4)1/3
2/3
0.5274(9)0.3109(11)0.381(3)0.2991(14)1/2
0.50669(3)1/2
0.46619(10)0.32465(14)0.52847(15)0.48438(15)0.42895(18)0.46337(14)0.4107(3)0.3764(3)0.3075(3)0.6244(4)0.6179(8)0.3709(4)0.0196(3)0.0172(2)0.0094(12)0.0145(6)0.0348(10)0.0153(9)0.0213(10)0.0507(15)0.0152(10)0.037(2)0.030(2)0.059(2)0.080(3)0.080(3)0.103(4)172

Minerals 2018, 8, 152

Figure 3. Projection of the crystal structure of tiberiobardiite viewed down b (a) and c (b). Polyhedra:
green = Cu1 and Cu2 sites; light blue = Al site; blue = Si site; and yellow = S site. Circles: light blue =H2 O groups; blue = OH; and red = O.
Table 5. Selected bond distances (in Å) in tiberiobardiite.
Cu1

Al

–OH1

1.948(4) × 2

–O2
–Ow3

1.977(4) × 2
2.476(5) × 2–OH4

1.909(4) × 6

Cu2

–OH4

1.944(4)

–O2
–OH1
–OH1
–OH4
–Ow3

1.965(4)1.972(4)1.991(4)2.464(4)2.541(6)

Si

S

–OH5

1.571(9)–O2

1.580(4) × 3

–O7
–O6

1.463(8) × 3
1.466(10)The two independent Cu sites (Cu1 and Cu2) display a six-fold coordination, showing the typicaldistorted (4 + 2) octahedral coordination related to the Jahn–Teller effect of Cu2+ (e.g., [20]). Considering
the nature of the ligands Φ, two different kinds of polyhedra occur (i.e., Cu1O2 (OH)2 (H2 O)2 andCu2O(OH)4 (H2 O) (Figure 4)), in agreement with the structure of chalcophyllite [17]. The average ofthe four shorter Cu–Φ distances, corresponding to the equatorial bonds, is 1.962 and 1.968 Å for the
Cu1 and Cu2 sites, respectively, in agreement with previous studies [21]. The axial bonds are deﬁnitely
longer, ranging between 2.48 and 2.54 Å. The bond-valence sums (BVS) at the Cu1 and Cu2 sites are
2.10 and 2.05 valence units (v.u.), respectively (Table 6).

Figure 4. Bonding environment of Cu sites in tiberiobardiite.

173

Minerals 2018, 8, 152

Table 6. Bond-valence sums (in valence units (v.u.)) in tiberiobardiite.
Site

Cu1

Cu2

OH1

0.48↓× 2

O2
Ow3

0.45↓× 2
0.12↓× 2

0.45
0.43
0.46
0.10
0.49
0.12

OH4

Al

S

2.02
0.22

0.51↓× 6

1.12
1.14
1.55
1.53↓× 3

2.10

2.05

Σanions
1.36

1.11↓× 3

OH5
O6
O7
Ow8a
Ow8b
Ow9
Σcations

Si

3.06

4.47

1.14
1.55
1.53
-

6.14 *

Note: bond parameters after [22]. In mixed or partially occupied sites, the bond-valence sum has been calculated
taking into account the site occupancy. Right superscripts indicate the number of bonds for each cation. * Assuming
full-occupancy at the S site.

Aluminum is hosted in a regular octahedron, connected with the Cu polyhedra throughedge-sharing, and is coordinated by OH groups only. In agreement with the chemical data, aluminumis partially replaced by minor Fe3+ . The bond-valence sum is in good accord with the theoretical value(i.e., 3.06 v.u.).
The Si-centered tetrahedra are connected through corner-sharing to the (Cu–Al)-polyhedra {0001}
sheets. The fourth oxygen atom (OH5) points to the interlayer and is involved in the hydrogen bondsystem characterizing tiberiobardiite. The average bond distance at the Si site, 1.578 Å, agrees witha mixed (Si and P) occupancy, as suggested by the chemical data. Owing to the similar scatteringfactors of Si and P, the site occupancy was ﬁxed to (Si0.75 P0.25 ), in accord with electron microprobe
data. The corresponding BVS is 4.47 v.u., slightly higher than the calculated value of 4.25 v.u. based onsite occupancy.
The {0001} sheet has the simpliﬁed chemical composition {Cu9 Al[(Si0.75 P0.25 )O3 (OH0.75 O0.25 )]2(OH)12 (H2 O)6 }3+ .
Successive sheets are separated by interlayers hosting SO4 and H2 O groups. The S site is
statistically occupied by S atoms, with a site occupancy ﬁxed, based on the chemical data, to 0.75.
Similarly, the site occupancy at the O6 and O7 sites, bonded to S, were ﬁxed to the same value.
The average <S–O> distance is 1.464 Å, in agreement with the average <S–O> distance in sulfate
minerals (i.e., 1.473 Å) [23]. In addition to the SO4 group, the interlayer is completed by two H2 O
groups, hosted at the Ow9 and the split Ow8a/b positions. In order to avoid too short O· · · O distances,
Ow9 and Ow8b cannot be fully occupied simultaneously. These sites are only partially occupied,with Ow8a and Ow9 having the same site occupancy (0.70), whereas Ow8b has a lower occupancy(0.30). The chemical formula of the interlayer is [(SO4 )1.5 ·10H2 O]3− .
4.2. The Hydrogen Bond System in Tiberiobardiite
The nine anion positions can be divided, based on their BVS (Table 6), in O2− , OH− , and H2 Ohosting sites. Table 7 reports the observed O· · · O distances shorter than 3 Å; their values agreewith those calculated from the O–H stretching frequencies measured through Raman spectroscopy.
Coupling this information with the occurrence of several underbonded anion positions, the importantrole played by the hydrogen bonds in tiberiobardiite can be hypothesized.

174

Minerals 2018, 8, 152

Table 7. O· · · O distances (in Å) and corresponding bond strengths (in v.u.) in tiberiobardiite, calculated
according to [24].
O· · · O

d (Å)

v.u.

OH1 · · · Ow8a
OH1 · · · Ow8b
Ow3 · · · O6
Ow3 · · · Ow9
OH4 · · · O7
OH5 · · · Ow8b
OH5 · · · Ow9
Ow8a · · · O7
Ow8a · · · O7
Ow8b · · · O7
Ow9 · · · O7
Ow8a · · · Ow9
Ow8a · · · Ow9
Ow8b · · · Ow9

2.745(11)2.72(2)2.778(7)2.711(13)2.960(8)2.83(3)2.718(11)2.727(13)2.958(13)2.95(3)2.835(13)2.632(14)2.876(16)2.69(2)

0.21
0.22
0.19
0.22
0.14
0.17
0.22
0.21
0.14
0.14
0.17
0.27
0.16
0.23

The hydrogen bond scheme is well-deﬁned as regards the anion forming the {0001}
heteropolyhedral layers, whereas it is more difﬁcult to be described for interlayer H2 O groups.
The heteropolyhedral layer is composed by anion sites OH1 to OH5. The valence sum at O2(2.02 v.u.) agrees with its occupancy by O2− , whereas Ow3 (0.22 v.u.) is consistent with a H2 O group.
OH1 is a donor of an H-bond to Ow8a/Ow8b (at 2.74/2.72 Å), and OH4 is a donor to O7 (at 2.96 Å);
the BVS at OH1 and OH4, after correction for the H-bond in agreement with [24], are 1.15/1.14 and0.98 v.u., respectively. This bond scheme is analogous to that observed in chalcophyllite [17]. On thecontrary, whereas O5 is an acceptor of three H-bonds from Ow9 in chalcophyllite [17], the mixed (OH,O) nature of OH5 in tiberiobardiite, related to the Si4+ + OH− = P5+ + O2− substitution, requires adifferent H-bond pattern. Different conﬁgurations are possible, considering the possible occurrencesof Ow8b and Ow9. Taking into account the site occupancy of these two sites (i.e., 0.70 Ow9 and0.30 Ow8b, close to 2/3 Ow9 and 1/3 Ow8b), a model with OH5 bonded to two Ow9 and one Ow8b
could be proposed. On the basis of the Si–P atomic ratio, an (OH)0.75 O0.25 occupancy at OH5 should
occur, corresponding to an expected weighted BVS of 1.25 v.u. It is likely that Ow9 and Ow8b could
be donors of the H-bond, whereas the second Ow9 could accept an H-bond from OH5. Accordingly,
the BVS at OH5 would be 1.31 v.u., consistent with the proposed site occupancy.
In the interlayer, the BVS values are in accord with the H2 O groups at the Ow8a/Ow8b and Ow9sites (0 v.u.), consistent with their role as interlayer H2 O groups. The oxygen atoms belonging to
the SO4 group are signiﬁcantly underbonded (1.55 and 1.53 v.u. at the O6 and O7 sites, respectively)
and they are acceptors of H-bonds. O6 is an acceptor of H-bonds from three symmetry-related H2 O
groups hosted at the Ow3 site, achieving a BVS of 2.12 v.u. Oxygen at the O7 site is involved in amore complicated H-bond system than that shown in chalcophyllite, where this site is H-bonded to
Ow8 (at 2.66 Å), Ow9 (at 2.88 Å), and OH4 (at a longer distance of 3.03 Å) [17]. Indeed, the split
nature of Ow8 and the partial occupancy of the H2 O-hosting sites allowed different conﬁgurations.
Taking into account O· · · O distances as well as O· · · H2 O· · · O angles, a hypothetical model aroundO7 in tiberiobardiite can be proposed. This site could be H-bonded with Ow8a (at 2.73 Å) and Ow9(at 2.84 Å), as in chalcophyllite, with an additional bond with OH4 (at 2.96 Å). The O7· · · Ow8a
distance of 2.96 Å could not likely represent a hydrogen bond, because the O7· · · Ow8a· · · Ow9 wouldbe ~133◦ , to be compared with two possible O7· · · Ow8a· · · Ow9 conﬁgurations (bifurcated H-bond,as in chalcophyllite [17]) showing angles of ~109◦ and 102◦ , respectively, closer to the average value of
108.3◦ observed in crystalline hydrates [25]. In such a way, the BVS at O7 would be 2.05 v.u.
In agreement with [17], when the SO4 group is absent, additional H2 O groups could occur at
the O sites. Consequently, it could be difﬁcult to propose a reliable H-bond system involving the
Ow8a/Ow8b and Ow9 sites, that could show variable H-bond patterns. As a matter of fact, hydrogen
175Minerals 2018, 8, 152

bonds are fundamental in bonding successive {0001} heteropolyhedral layers and in “trapping” theSO4 groups in the interlayers.
5. Discussion
5.1. Relationships with Chalcophyllite
Tiberiobardiite is the Si-analogue of chalcophyllite, with Si4+ replacing As5+ through the coupled
substitution As5+ + O2− = Si4+ + OH− . These two minerals form the chalcophyllite group (Table 8),
in agreement with [26]. The occurrence of P5+ partially replacing Si4+ allows us to also hypothesize theexistence of the P-analogue, ideally [Cu9 Al(PO4 )2 (OH)12 (H2 O)6 ](SO4 )1.5 ·10–12H2 O.
These species are related to barrotite, {Cu9 Al[SiO3 (OH)]2 (OH)12 (H2 O)6 }{(SO4 )[AsO3 (OH)]0.5 }·2H2 O.
In fact, the crystal structure of barrotite was not solved, and consequently, the actual relationships
with the chalcophyllite group minerals are unknown. Indeed, As could be hosted in an independent
tetrahedral site, or it could partially replace (SO4 ) groups in the interlayer, through the substitution(SO4 )2− = [AsO3 (OH)]2− . In this latter case, the ideal formula of barrotite could be the same as that oftiberiobardiite, from which it differs in the lower hydration state and the shorter c axis (Table 8).
Table 8. Members of the chalcophyllite group and related minerals.
MineralChemical Formula

a (Å)

c (Å)

V (Å3 )

Ref.

10.76
10.69

28.68
28.32

2873.32801.0[17]
this work

10.65

21.95

2156.5[12]

Chalcophyllite group
Chalcophyllite
Tiberiobardiite[Cu9 Al(AsO4 )2 (OH)12 (H2 O)6 ](SO4 )1.5 ·12H2 O{Cu9 Al[SiO3 (OH)]2 (OH)12 (H2 O)6 }(SO4 )1.5 ·10H2 O
Unclassiﬁed

Barrotite{Cu9 Al[SiO3 (OH)]2 (OH)12 (H2 O)6 }{(SO4 )[AsO3 (OH)]0.5 }·2H2 O

5.2. Secondary Cu2+ Oxysalt Minerals in Ophiolite-Hosted Ore Deposits from Tuscany (Italy)
The secondary mineral assemblages associated with ophiolite-hosted ore deposits from Tuscany(Italy) have received little attention by mineralogists, and very few data are currently available (Table 9).
In addition to the widespread occurrence of the copper carbonates azurite and malachite, known sincethe 19th century (e.g., [27]), several common sulfates have been identiﬁed in the last thirty years (i.e.,brochantite, chalcanthite, langite, posnjakite, and serpierite) (e.g., [28,29]). The study of the mineral
association occurring in the small Cretaio prospect allowed to increase the knowledge of these phases,
with further description of antlerite, some Cu–Al sulfates (chalcoalumite, cyanotrichite, and spangolite),and a mineral belonging to the devilline group [6]. Spangolite is characterized by the occurrence of
the Cl anion, as well as the two Cu halides clinoatacamite and connellite; the latter was identiﬁed
from the same occurrence of spangolite at Cretaio and in close association with clinoatacamite, cuprite,
and native copper also from the Montecastelli Cu deposit, along the Pavone River valley (e.g., [30]).
It is worth noting the occurrence of some copper phosphates in the ophiolite-hosted deposits(i.e., calciopetersite and libethenite from Monte Beni [5] and Cretaio [6]). Moreover, a still unidentiﬁed
member of the mixite group has been recently identiﬁed in the mineral assemblages from Cretaio.
Likely, phosphorus could be derived from the interaction between the acid solutions derived from
sulﬁde weathering and apatite, occurring as accessory minerals in the host rocks. Phosphorus isenriched also in tiberiobardiite, and the potential occurrence of the P-analogue of tiberiobardiite andchalcophyllite could be likely found in the phosphate-rich assemblages from ophiolite-hosted deposits.

176

Minerals 2018, 8, 152

Table 9. Secondary Cu2+ oxysalts in ophiolite-hosted ore deposits from Tuscany.
MineralChemical Formula

Occurrences

Cu2 (OH)3 Cl
Cu19 (SO4 )(OH)32 Cl4 ·3H2 O

Montecastelli
Cretaio, Montecastelli

Cu3 (CO3 )2 (OH)2Cu2 (CO3 )(OH)2

Widespread
Widespread

Cu3 (SO4 )(OH)4Cu4 (SO4 )(OH)6Cu(SO4 )·5H2 O
CuAl4 (SO4 )(OH)12 ·3H2 O
Cu4 Al2 (SO4 )(OH)12 ·2H2 O
CaCu4 (SO4 )2 (OH)6 ·3H2 O
Cu4 (SO4 )(OH)6 ·2H2 O
CuPb(SO4 )(OH)2
Cu4 (SO4 )(OH)6 ·H2 O
Cu6 Al(SO4 )(OH)12 Cl·3H2 O

Cretaio
Widespread
Cretaio, Impruneta
Cretaio
Cretaio
Cretaio, Impruneta
Cretaio, Impruneta, MVC
MVC
Cretaio, MVC, CdC
Cretaio

CaCu6 (PO4 )2 (PO3 OH)(OH)6 ·3H2 O
Cu2 (PO4 )(OH)

Monte Beni
Cretaio

Cu2−x Alx (H2-x Si2 O5 )(OH)4 ·nH2 O
Cu9 Al(SiO3 OH)2 (SO4 )1.5 (OH)2 ·16H2 O

Widespread
Cretaio

Halides
Clinoatacamite
Connellite
Carbonates
AzuriteMalachite
Sulfates
Antlerite
Brochantite
Chalchantite
Chalcoalumite
Cyanotrichite
Devilline/serpierite
LangiteLinaritePosnjakite
Spangolite
Phosphates
Calciopetersite
Libethenite
Silicates
Chrysocolla
Tiberiobardiite

Note: minerals occurring in more than three localities are indicated as “widespread”. CdC = Cetine di Camporbiano,
Pisa and MVC = Montecatini Val di Cecina, Pisa.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/4/152/s1,
CIF: tiberiobardiite.
Acknowledgments: Tiberio Bardi is acknowledged for providing us with the studied material. We wish to thankalso the mineral collectors Cristiano Bazzoni, Mauro Bernocchi, Cesare Betti, and Riccardo Marini for providing uswith studied material from the Cretaio prospect. The comments of six anonymous reviewers helped us improve
the paper.
Author Contributions: C.B. conceived of and designed the experiments; C.B. performed the diffraction
experiments; C.B. and M.P. analyzed the data; F.Z. performed the chemical analysis; and C.B. wrote the paper.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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178minerals
ArticleKurchatovite and Clinokurchatovite, Ideally
CaMgB2O5: An Example of Modular Polymorphism
Yulia A. Pankova 1 , Sergey V. Krivovichev 1,2, *, Igor V. Pekov 3 , Edward S. Grew 4
and Vasiliy O. Yapaskurt 3
1
2
3
4

*

Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University,
University Emb. 7/9, 199034 St. Petersburg, Russia; yulika1314@gmail.com
Nanomaterials Research Centre, Kola Science Centre, Russian Academy of Sciences, Fersmana 14,
184209 Apatity, Russia
Faculty of Geology, Moscow State University, Vorob’evy Gory, 119991 Moscow, Russia;
igorpekov@mail.ru (I.V.P.); yvo72@geol.msu.ru (V.O.Y.)
School of Earth and Climate Sciences, University of Maine, 5790 Bryand Global Science Center,
Orono, ME 04469, USA; esgrew@maine.edu
Correspondence: s.krivovichev@spbu.ru or krivovichev@admksc.apatity.ru; Tel.: +7-81555-7-53-50

Received: 4 July 2018; Accepted: 26 July 2018; Published: 2 August 2018

Abstract: Kurchatovite and clinokurchatovite, both of ideal composition CaMgB2 O5 , from thetype localities (Solongo, Buryatia, Russia, and Sayak-IV, Kazakhstan, respectively) have beenstudied using electron microprobe and single-crystal X-ray diffraction methods. The empiricalformulae of the samples are Ca1.01 Mg0.87 Mn0.11 Fe2+ 0.02 B1.99 O5 and Ca0.94 Mg0.91 Fe2+ 0.10 Mn0.04 B2.01 O5
for kurchatovite and clinokurchatovite, respectively. The crystal structures of the two minerals
are similar and based upon two-dimensional blocks arranged parallel to the c axis in kurchatovite
and parallel to the a axis in clinokurchatovite. The blocks are built up from diborate B2 O5 groups,
and Ca2+ and Mg2+ cations in seven- and six-fold coordination, respectively. Detailed analysis
of geometrical parameters of the adjacent blocks reveals that symmetrically different diborategroups have different degrees of conformation in terms of the δ angles between the planes of
two BO3 triangles sharing a common O atom, featuring two discrete sets of the δ values ofca. 55◦ (B’ blocks) and 34◦ (B” blocks). The stacking of the blocks in clinokurchatovite can be presented
as . . . (+B’)(+B”)(+B’)(+B”) . . . or [(+B’)(+B”)], whereas in kurchatovite it is more complex andcorresponds to the sequence . . . (+B’)(+B”)(+B’)(−B’)(−B”)(−B’)(+B’)(+B”)(+B’)(−B’)(−B”)(−B’). . . or [(+B’)(+B”)(+B’)(−B’)(−B”)(−B’)]. The B’:B” ratios for clinokurchatovite and kurchatovite
are 1:1 and 2:1, respectively. According to this description, the two minerals cannot be considered
as polytypes and their mutual relationship corresponds to the term modular polymorphs.
From the viewpoint of information-based measures of structural complexity, clinokurchatovite(IG = 4.170 bits/atom and IG,total = 300.235 bits/cell) is structurally simpler than kurchatovite(IG = 4.755 bits/atom and IG,total = 1027.056 bits/cell). The high structural complexity of kurchatovite
can be inferred from the modular character of its structure. The analysis of structural combinatorics
in terms of the modular approach allows to construct the whole family of theoretically possible“kurchatovite”-type structures that bear the same structural features common for kurchatovite andclinokurchatovite. However, the crystal structures of the latter minerals are the simplest and are theonly ones that have been observed in nature. The absence of other possible structures is remarkable
and can be explained by either the maximum-entropy of the least-action fundamental principles.
Keywords: kurchatovite; clinokurchatovite; crystal structure; borate; polymorphism; polytypism;
structural complexity; structural combinatorics; conﬁgurational entropy; least-action principle

Minerals 2018, 8, 332; doi:10.3390/min8080332

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Minerals 2018, 8, 332

1. Introduction
Kurchatovite and clinokurchatovite are rare anhydrous Ca-Mg borates reported in skarns fromseveral localities in Russia, Kazakhstan, Afghanistan and Japan [1–4].
Kurchatovite was discovered in a vesuvianite-garnet calc-silicate skarn in the Solongo boron
and iron deposit, Buryatia (Siberia, Russia), in close association with sphalerite, magnetite andturneaurite-johnbaumite series arsenates [1,5,6]. It was interpreted to have formed as an early mineralwhen boron was introduced with postmagmatic ﬂuids. A wet chemical analysis after deductionof impurities gave the following formula calculated based on ﬁve oxygen atoms per formula
unit (pfu): Ca0.955 (Mg0.816 Mn0.183 Fe0.040 )Σ1.039 B2.005 O5 . Single-crystal X-ray diffraction study gaveorthorhombic symmetry with a = 11.15(0.02), b = 36.4(0.1), and c = 5.55(0.01) Å in the originaldescription; whereas Yakubovich et al. [7] gave the space group as Pc21 b when the structure wasreﬁned. The possibility of a monoclinic analogue was ﬁrst suggested by syntheses [8] and presence oftwins [9–11], and subsequently conﬁrmed by single-crystal studies and crystal-structure reﬁnementsof synthetic and natural material, including a synthetic Ca-Mn analogue [7,12–15], eventually leading
to the formal description of clinokurchatovite as a new mineral [16]. The type locality is the Sayak-IV
gold and copper deposit, Balkhash Region, Kazakhstan, where clinokurchatovite occurs in skarnwith calcite, harkerite, grossular-andradite garnet, magnetite and, locally, ludwigite. Other reliably
reported localities of clinokurchatovite are the Novofrolovskoe copper deposit, Northern Urals, and the
Titovskoe boron deposit, Polar Yakutia, Siberia, both in Russia [1].
Kurchatovite from the Fuka mine, Okayama Prefecture, Japan, ranges in chemical composition(with negligible Mn) from the almost end member (2 mol % CaFeB2 O5 ) to the Fe-dominant analogue(53 mol % CaFeB2 O5 ); there is ﬁrst report of a kurchatovite-type mineral with Fe > Mg [4].
The crystal structures of kurchatovite and clinokurchatovite have been re-investigated by Callegari
et al. [17], who reported for kurchatovite the space group Pbca. According to Yakubovich et al. [7] andBelokoneva et al. [18], the crystal structures of kurchatovite and clinokurchatovite could be considered
as polytypes. Callegari et al. [17] mentioned this hypothesis as a possibility, leaving it aside “to theexperts in this manner”. However, if kurchatovite and clinokurchatovite are polytypes, then theyshould be considered as one mineral species existing in two polytypic varieties, which would warrant
the discreditation of one of them and re-deﬁnition of the other.
The aim of the present paper is to report on the results of crystal-structure studies of kurchatovite
and clinokurchatovite, to analyze their structural relations from the viewpoint of the concepts ofpolymorphism and polytypism, and to examine the validity of the two minerals as separate mineral
species in the frame of actual approach accepted in mineral nomenclature.
2. Materials and Methods
2.1. Description of Samples
The samples examined in the present work originate from the collection of one of the authors(I.V.P.), #425 (kurchatovite) and #428 (clinokurchatovite). Both samples are from the type localitiesof these minerals and were received from the senior author of their ﬁrst descriptions, Prof. Svetlana
Vyacheslavovna Malinko (1927–2002). The clinokurchatovite sample is a part of the holotype specimen
studied by Malinko and Pertsev [16].
Sample #425 is a fragment of drillcore of a borehole from the Solongo deposit. Kurchatovite
occurs as colorless to greyish equant grains up to 3 mm across. It is a major constituent of a massive
borate-rich rock that also contains sakhaite, calcite and magnetite and is crosscut by vimsite veinlets.
Sample #428 is a fragment of drill core from a borehole in the Sayak-IV deposit. Clinokurchatovite
forms colorless tabular grains up to 2 mm across associated with harkerite and minor magnetite ina massive skarn rock consisting mainly of calcite and andradite.

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2.2. Chemical Composition
The chemical compositions of kurchatovite and clinokurchatovite were studied using a JEOL
JSM-6480LV (JEOL, Japan) scanning electron microscope equipped with an INCA-Wave 500
wavelength-dispersive spectrometer (Laboratory of Analytical Techniques of High Spatial Resolution,
Deptartmentof Petrology, Moscow State University). The WDS mode was used, with an acceleration
voltage of 20 kV and a beam current of 10 nA; electron beam was rastered to 5 μm × 5 μm area.
The standards used were as follows: wollastonite (Ca), MgO (Mg), Mn (Mn), Fe (Fe), and BN (B).
Contents of other elements with atomic numbers higher than carbon were below detection limits.
Table 1 provides the summary of analytical results (the averages of eight- and six-point
analyses for kurchatovite and clinokurchatovite, respectively). The resulting empirical formulae
obtained in our study can be written as Ca1.01 Mg0.87 Mn0.11 Fe2+ 0.02 B1.99 O5 for kurchatovite andCa0.94 Mg0.91 Fe2+ 0.10 Mn0.04 B2.01 O5 for clinokurchatovite, and both are in good agreement with theprevious studies and the results of crystal-structure analysis.
Table 1. Chemical composition of kurchatovite from Solongo (#425) and clinokurchatovite from
Sayak-IV (#428).
Constituent

#425

CaO
MgO
MnO
FeO
B2 O3
Total

32.74
20.12
4.54
0.84
40.04
98.28

e.s.d. *#428

e.s.d.

30.82
21.58
1.73
4.05
40.89
99.07

0.98
0.81
0.09
0.54
2.01

wt %
1.12
0.76
0.12
0.06
2.44

calculated based on 5 O atoms per formula unit
Ca
Mg
Mn
Fe
B

1.01
0.87
0.11
0.02
1.99

0.94
0.91
0.04
0.10
2.01

* e.s.d. = estimated standard deviation.

2.3. Single-Crystal X-ray Diffraction
The crystals of kurchatovite and clinokurchatovite selected for data collection were mounted ona Bruker «SMART APEX» X-ray diffractometer operated at 50 kV and 40 mA and equipped with
the IμS microfocus source. More than a hemisphere of three-dimensional data was collected foreach crystal using monochromatic MoKα X-radiation, with frame widths of 0.5◦ , and with a 10 scount for each frame. The unit-cell parameters (Table 2) were refined using least-squares techniques.
The intensity data were integrated and corrected for Lorentz, polarization, and background effects using
the Bruker programs APEX and XPREP. An analytical multiscan absorption correction was performed
using SADABS. The observed systematic absences were consistent with the space groups Pbca andP21 /c for kurchatovite and clinokurchatovite, respectively, in agreement with the previous report byCallegari et al. [17]. The refinements converged to R1 = 0.0265 and 0.0287 based on 4240 and 2480 uniquereflections for kurchatovite and clinokurchatovite, respectively. The SHELX program package was used
for all structural calculations [19]. The final atomic coordinates and isotropic displacement parameters are
given in Tables 3 and 4 and selected interatomic distances are in Tables 5 and 6.

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Table 2. Crystal data and structure reﬁnement for kurchatovite and clinokurchatovite.
Kurchatovite

Clinokurchatovite

Crystallographic Data
Crystal system
Space group
a, Å
b, Å
Unit-cellc, Åparametersβ, deg.
V, Å3Z
Density (g/cm3 )Absorption coefﬁcient (mm−1 )Crystal size (mm3 )orthorhombic
Pbca
11.1543(5)5.5078(2)36.357(2)902233.6(2)
12
3.051
2.249
0.25 × 0.15 × 0.07

Monoclinic
P21 /c12.330(1)11.147(1)5.5154(5)101.604(2)742.56(12)
4
3.063
2.348
0.14 × 0.08 × 0.06

Data Collection Parameters
Temperature, K
Radiation type, wavelength2θ angles range, deg.
h, k, l values range
Reﬂections collected
Unique reﬂections (Rint )Observed reﬂections (>4σ Fσ )296(2)
296(2)
MoKα, 0.71073
1.120–36.270
1.686–36.194−18 → 18, −8 → 9, −54 → 60
−20 → 20, −18 → 18, −9 → 7
37,90813,2395260 (0.0311)
3425 (0.0291)
4240
2480
Structure-Reﬁnement Parameters
Full-matrix least-square analysis of F2
0.0328, 1.3584
0.0290, 0.2749
0.0265, 0.0656
0.0287, 0.0633
0.0392, 0.0706
0.0505, 0.0716
251
165
1.032
1.014
0.58, −0.43
0.49, −0.51

Reﬁnement method
Weight coefﬁcients a, b
R1 [F > 4σ(F)], wR2 [F > 4σ(F)]R1 [all data], wR2 [all data]
No. of reﬁned parametersS−3max , min , e·Å

Table 3. Atomic coordinates, site occupancies and isotropic displacement parameters (Å2 ) for kurchatovite.
Atom

Occupancy

x

y

z

U eq

Ca1
Ca2
Ca3

Ca
Ca
Ca

0.61922(2)0.62704(2)0.38191(2)0.87281(5)0.11587(4)0.68481(5)0.197511(7)0.029585(7)0.135926(7)0.00938(5)0.00855(5)0.00851(5)

Mg1
Mg2
Mg3

Mg0.83 Mn0.17
Mg0.85 Mn0.15
Mg0.84 Mn0.16

0.36899(3)0.37184(3)0.62831(3)0.90620(7)0.13959(6)0.63546(6)0.24556(1)0.07913(1)0.08798(1)0.0071(1)0.0070(1)0.0069(1)

B1
B2
B3
B4
B5
B6

B
B
B
B
B
B

0.6165(1)0.3801(1)0.3758(1)0.6232(1)0.3769(1)0.6158(1)0.9056(2)0.3481(2)0.6224(2)0.4041(2)0.1430(2)0.1422(2)0.27635(4)0.00382(4)0.05821(4)0.16340(4)0.16971(4)0.10852(4)0.0075(2)0.0075(2)0.0076(2)0.0074(2)0.0077(2)0.0076(2)

O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
O11
O12
O13
O14
O15

O
O
O
O
O
O
O
O
O
O
O
O
O
O
O

0.60741(8)0.39113(8)0.33020(8)0.69551(7)0.41787(8)0.62748(8)0.69533(7)0.54422(8)0.45005(7)0.66730(8)0.65926(8)0.45486(8)0.40938(8)0.30194(7)0.59502(8)0.1717(2)0.3799(2)0.1304(2)0.7625(2)0.1526(2)0.4165(2)0.2813(2)0.9772(2)0.4882(2)0.4040(2)0.6685(2)0.5679(2)0.9429(2)0.7933(2)0.6100(2)0.14686(2)0.18516(2)0.13512(2)0.25775(2)0.02313(2)-0.01965(3)0.08927(2)0.09097(2)0.08012(2)0.19815(2)0.03147(2)0.24184(2)0.18909(2)0.07285(2)0.14478(2)0.0099(2)0.0096(2)0.0097(2)0.0086(2)0.0086(2)0.0096(2)0.0086(2)0.0090(2)0.0086(2)0.0109(2)0.0100(2)0.0096(2)0.0084(2)0.0083(2)0.0090(2)182Minerals 2018, 8, 332

Table 4. Atomic coordinates, site occupancies and isotropic displacement parameters (Å2) for clinokurchatovite.
Atom

Occupancy

x

y

z

U eq

Ca1
Ca2

Ca
Ca

0.08981(2)0.40709(2)0.87295(2)0.11838(3)0.15613(5)0.86795(5)0.00945(6)0.00964(6)

Mg1
Mg2

Mg0.82 Fe0.18
Mg0.85 Fe0.15

0.23725(3)0.26425(3)0.12783(3)0.87208(4)0.24798(7)0.75442(7)0.0074(1)0.0075(1)

B1
B2
B3
B4

B
B
B
B

0.1737(1)0.3274(1)0.0110(1)0.4910(1)0.1244(1)0.8833(1)0.1199(1)0.8758(1)0.7008(3)0.2898(3)0.3523(3)0.6265(3)0.0085(3)0.0080(2)0.0081(3)0.0081(2)

O1
O2
O3
O4
O5
O6
O7
O8
O9
O10

O
O
O
O
O
O
O
O
O
O

0.21777(8)0.26893(8)0.23931(8)0.27471(8)0.05777(8)0.44314(8)0.06922(8)0.43363(8)0.09551(8)0.40463(8)0.19770(9)0.80443(9)0.04971(9)−0.04466(9)0.12793(9)0.89021(10)0.08205(9)−0.09361(9)0.84098(9)0.16960(9)0.8923(2)0.4030(2)0.5970(2)0.1016(2)0.6085(2)0.3708(2)0.1841(2)0.8032(2)0.7140(2)0.3182(2)0.0092(2)0.0090(2)0.0094(2)0.0095(2)0.0103(2)0.0107(2)0.0090(2)0.0091(2)0.0105(2)0.0107(2)Table 5. Selected bond lengths (Å) for the crystal structure of kurchatovite.
Ca1-O10
Ca1-O13
Ca1-O15
Ca1-O4
Ca1-O1Ca1-O12
Ca1-O10<Ca1-O>2.388(1)2.392(2)2.418(1)2.427(1)2.474(1)2.588(1)2.637(1)2.475

Ca2-O5
Ca2-O11
Ca2-O6
Ca2-O5
Ca2-O7
Ca2-O11
Ca2-O8<Ca2-O>2.354(1)2.402(1)2.438(1)2.472(1)2.474(1)2.491(1)2.533(1)2.452

Ca3-O3Ca3-O13
Ca3-O9Ca3-O15
Ca3-O2Ca3-O3Ca3-O14<Ca3-O>2.385(1)2.419(1)2.422(1)2.434(1)2.460(1)2.521(1)2.532(1)2.453

Mg1-O10
Mg1-O4
Mg1-O4
Mg1-O12
Mg1-O13
Mg1-O12<Mg1-O>2.0862(1)2.0936(1)2.0941(1)2.0994(1)2.1116(1)2.2054(1)2.115

Mg2-O14
Mg2-O3
Mg2-O5
Mg2-O9
Mg2-O14
Mg2-O8<Mg2-O>2.073(1)2.088(1)2.101(1)2.109(1)2.127(1)2.164(1)2.110

Mg3-O7
Mg3-O11
Mg3-O15
Mg3-O8
Mg3-O7
Mg3-O9<Mg3-O>2.089(1)2.091(1)2.103(1)2.106(1)2.126(1)2.166(1)
2.114

B1-O4
B1-O12
B1-O2<B1-O>

1.362(2)1.367(2)1.410(2)
1.380

B2-O5
B2-O11
B2-O6<B2-O>

1.353(2)1.359(2)1.421(2)1.378

B3-O14B3-O9
B3-O6<B3-O>

1.359(2)1.366(2)1.419(2)1.381

B4-O10
B4-O15
B4-O1<B4-O>

1.356(2)1.358(2)1.425(2)1.380

B5-O13B5-O3
B5-O2
<B5-O>

1.357(2)1.363(2)1.425(2)
1.382

B6-O7
B6-O8
B6-O1
<B6-O>

1.365(2)1.368(2)1.407(2)1.380

Table 6. Selected bond lengths (Å) for the crystal structure of kurchatovite.
Ca1-O7
Ca1-O9
Ca1-O5
Ca1-O2
Ca1-O7
Ca1-O9
Ca1-O4<Ca1-O>2.3531(1)2.4052(1)2.4384(1)2.4709(1)2.4774(1)2.4791(1)2.5311(1)2.451

Ca2-O10
Ca2-O8
Ca2-O3Ca2-O8
Ca2-O6
Ca2-O1Ca2-O10<Ca2-O>2.3789(1)2.4060(1)2.4198(1)2.4220(1)2.4759(1)2.5249(1)2.5541(1)2.455

Mg1-O10
Mg1-O1
Mg1-O7
Mg1-O3
Mg1-O1
Mg1-O4<Mg1-O>2.0746(1)2.0789(1)2.0936(1)2.1083(1)2.1334(1)2.1708(1)
2.110

B1-O1
B1-O3
B1-O5
<B1-O>
B2-O2
B2-O4
B2-O6
<B2-O>

1.3594(2)1.3645(2)1.4180(2)1.381
1.3649(2)1.3685(2)1.4097(2)1.381

Mg2-O9
Mg2-O8
Mg2-O2
Mg2-O4
Mg2-O2
Mg2-O3<Mg2-O>2.0764(1)2.0867(1)2.0911(1)2.1084(1)2.1279(1)2.1591(1)
2.108

B3-O7
B3-O9
B3-O5
<B3-O>
B4-O8
B4-O10
B4-O6<B4-O>

1.3499(2)1.3623(2)1.4175(2)1.377
1.3577(2)1.3598(2)1.4240(2)1.359

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3. Results
The crystal structures of kurchatovite and clinokurchatovite are shown in Figures 1a and 2a,
respectively. Their basic structural features are the same as reported by Yakubovich et al. [7,12,13],Simonov et al. [15], and Callegari et al. [17]. Mg2+ cations are coordinated octahedrally with theaverage <Mg-O> bond lengths in the range of 2.108–2.115 Å (individual bond lengths vary from
2.073 to 2.205 Å). Ca2+ cations are coordinated by seven O atoms each with the <Ca-O> bond lengths
in the range of 2.451–2.475 Å. B3+ cations are coordinated by three O atoms each to form BO3 triangles(<B-O> = 1.359–1.382 Å). Two BO3 triangles share common Obr atoms (Obr = bridging O atom) toform [B2 O5 ]4− diborate group. The B-Obr bonds are essentially longer (1.407–1.425 Å) than the B-Ot
bonds (1.353–1.369 Å; Ot = terminal O atoms), in agreement with the observations by Filatov andBubnova [20]. The B-Obr -B angles in the B2 O5 groups are in the range 118.6–122.4◦ .
According to the description ﬁrst developed by Yakubovich et al. [7], the crystal structures of
the two minerals are similar and based upon two-dimensional blocks of the kind shown in Figure 3a.
The blocks are about 6 Å thick and are arranged perpendicular to the c axis in kurchatovite andparallel to (100) in clinokurchatovite. Callegari et al. [17] proposed different approach to the structure
description by splitting the crystal structures into thinner blocks (‘monoclinic modules’) consisting of
B2 O5 groups, BO3 triangles, and Ca2+ and Mg2+ cations. In her detailed OD-analysis of kurchatovite
and clinokurchatovite, Belokoneva [18] subdivided in the two structures two layers, MgBO2.5 (L1 )and CaBO2.5 (L2 ), with local symmetries P(b)c21 and P(1)21 /c1, respectively. The L1 layer contains
B1O3 triangles and Mg2+ ions, whereas the L2 layer contains B2O3 triangles and Ca2+ ions. The layers
alternate to form L1 L2 pairs, which stack in the +1/4, +1/4, +1/4 . . . sequence in clinokurchatovite[a maximum degree of order 1 (MDO1) polytype] and in the +1/4, +1/4, +1/4, −1/4, −1/4, −1/4 . . .
sequence in kurchatovite, where +1/4 and −1/4 symbolize shifts and orientations of the layers along
the b axis in kurchatovite and the c axis in clinokurchatovite.
Despite their relevance and simplicity, the approaches developed by Callegari et al. [17] andBelokoneva [18] possess several disadvantages: (1) both approaches imply “splitting” of B2 O5 groups
into single BO3 triangles, which is theoretically feasible, but physically unrealistic; (2) Belokoneva’s
approach takes into account only the B1 and B2 sites, whereas, in clinokurchatovite, there are four B
sites; (3) both approaches are unable to explain the absence of the “hypothetical protostructure” derived
by Yakubovich et al. [7], i.e., the monoclinic structure consisting of one block only, i.e., with a ~6.2 Å if
one uses the monoclinic setting of clinokurchatovite.
To develop a transparent and reliable method for the analysis of relationships between
kurchatovite and clinokurchatovite, the two structures can ﬁrst be analyzed in terms of thearrangements of the B2 O5 groups (Figures 1b and 2b).

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Figure 1. The crystal structure of kurchatovite projected along the b axis (a) and the arrangement of B2 O5groups (b). The curved brackets in (a) denote the basic two-dimensional blocks, whereas the dashed linesin (b) indicate directions of division of the structure into single slices consisting of diborate groups.

In turn, the two arrangements can be split into two-dimensional slices indicated by dashed lines
in Figures 1b and 2b. The arrangement of the B2 O5 groups in the plane of the slices (Figure 4a,c) clearly
demonstrates the relations between the two structures in terms of the sequence of the groups along
the direction of the layer stacking. The Ca2+ and Mg2+ cations lie in the plane of the slices, possessing
ﬁve- and four-fold coordination, respectively (Figure 4b,d). The dot-and-dash lines in Figure 4a,c show
the separation of the slices into 6 Å-blocks corresponding to “protostructure” by Yakubovich et al. [7].
The description of the two structures in terms of these blocks is physically realistic, since it does not
imply hypothetical splitting of diborate groups into single BO3 triangles.

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Figure 2. The crystal structure of clinokurchatovite projected along the c axis (a) and the arrangementof B2 O5 groups (b). The dashed lines in (b) indicate directions of division of the structure into single
slices consisting of diborate groups.

Figure 3. The structure topology of the 6 Å-block in the crystal structures of kurchatovite andclinokurchatovite (a; the picture corresponds to one of the symmetrically independent blocks inkurchatovite) and the coordination ﬁgures of Ca (b) and Mg (c) atoms.

The proposed approach appears to support a polytypic relationship between kurchatovite
and clinokurchatovite, since both are based upon the layers of the same kind stacked in different
sequences. However, detailed analysis of geometrical parameters of the adjacent 6 Å-blocks reveals
a more complicated situation. As mentioned above, the geometries of the B2 O5 groups in thetwo structures are almost identical with the B–O bond lengths and valence angles varying oververy narrow ranges. However, a closer investigation of symmetrically different diborate groups
demonstrates that they have different degrees of conformation in terms of the δ angles between
the planes of two BO3 triangles sharing a common O atom. The diagrams in Figure 5 show thatthe B2 O5 groups belong to two discrete types with the δ values of ca. 55◦ and 34◦ , respectively.
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In both minerals, each 6 Å-block consists exclusively of diborate groups with the same δ valueof either 55.5 ± 0.5◦ or 33.7 ± 0.5◦ . To make the distinction clear, we denote the 55◦ block as B’,and the 34◦ block as B”. Thus, taking into account the directions of the blocks along the c axis inclinokurchatovite and the b axis in kurchatovite symbolized as + or − (related by the 180◦ rotation
around the vertical axis), the stacking of the 6 Å-blocks in clinokurchatovite can be described as . . .
(+B’)(+B”)(+B’)(+B”) . . . or [(+B’)(+B”)]. In contrast, the stacking in kurchatovite is more complex andcorresponds to the sequence . . . (+B’)(+B”)(+B’)(−B’)(−B”)(−B’)(+B’)(+B”)(+B’)(−B’)(−B”)(−B’). . . or [(+B’)(+B”)(+B’)(−B’)(−B”)(−B’)]. The B’:B” ratios for clinokurchatovite and kurchatovite are
1:1 and 2:1, respectively. It is worthy to note that the proposed description of the two structures interms of two different layers allows for the explanation of the absence of the “protostructure” by
Yakubovich et al. [7]. In fact, clinokurchatovite is the simplest structure possible in this mineral group(see also below).

Figure 4. The slices of the crystal structures of clinokurchatovite (a,b) and kurchatovite (c,d)
and consisting of diborate groups (divided along the dashed lines as shown in Figures 1 and 2).
The dot-and-dash lines in (a,c) indicate the relative positions of the B’ and B” of different orientations(distinguished by the “+” and “−” signs).

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Figure 5. The arrangements of diborate groups in the crystal structures of kurchatovite (a) and clinokurchatovite(b), featuring their angular characteristics (the B–O–B angles are shown in black; the values in blue boxes
correspond to the δ angles between the planes of the adjacent BO3 triangles).

4. Discussion
According to Guinier et al. [21], “ . . . an element or compound is polytypic if it occurs in several
different structural modiﬁcations, each of which may be regarded as built up by stacking layers of(nearly) identical structure and composition, and if the modiﬁcations differ only in their stacking
sequence”. This deﬁnition implies that the two structures belong to the same polytypic family if they:(1) contain nearly identical layers; (2) differ only in the stacking sequence. While the crystal structuresof kurchatovite and clinokurchatovite conform to the ﬁrst requirement, that is, they are built up fromlayers of the same kind, they do not conform to the second requirement, since the two structures cannot
be obtained from one another simply by changing the stacking sequence. It is possible to obtain the
crystal structure of kurchatovite from that of clinokurchatovite by the following sequence of operations:(a) splitting the structure of the latter into modules consisting of three 6 Å-blocks having the B’:B” ratioof 2:1; (b) stacking the modules in such a way that the adjacent modules are in different orientations,which can also be described as a chemical twinning [22,23]. According to the modular approach [22],this procedure for obtaining kurchatovite from clinokurchatovite means that the crystal structure ofclinokurchatovite may be considered a basic structure, whereas that of kurchatovite is a derivative
structure. Thus, the two minerals are not polytypes of one another; instead their relationship isbetter understood in terms of modular polymorphism. Re-deﬁnition of the two mineral species is
not warranted. It is noteworthy that the situation would be very different if the crystal structures ofkurchatovite and clinokurchatovite were to have the same B’:B” ratio. The veatchite polytypes [24] arean example of polytypism in which two types of layers are involved in borate minerals, but overallsuch polytypism is very rare.
The description in terms of the B’:B” blocks raises the important question about the potential
occurrence of other members of the family consisting of the same elements. Considering the sequencesof blocks in the two structures, two empirical rules can be derived:(i) the B” blocks are always surrounded by the B’ blocks of the same directionality (i.e., of thesame sign);

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Minerals 2018, 8, 332(ii) the B’ blocks may occur in two combinations: either surrounded by two B” blocks of the same signor by one B” block of the same sign and one B’ block of the different sign. Thus, the following eight triplets
are allowed that satisfy the (i) and (ii) conditions:
I (+B’)(+B”)(+B’) II (−B’)(−B”)(−B’) III (+B”)(+B’)(+B”) IV (−B”)(−B’)(−B”)
V (+B”)(+B’)(-B’) VI (−B’)(+B’)(+B”) VII (−B”)(−B’)(+B’) VIII (+B’)(−B’)(−B”)
The number of symbolic sequences that may contain these triplets is obviously inﬁnite, and thesequences may be both periodic and aperiodic. We note that, to produce a sequence that satisfy therules, two triplets should overlap in two symbols: two right symbols of the ﬁrst (preceding) one andtwo left symbols of the second (following) one. Thus, all the triplets can be split into the following ten
overlapping pairs (see an example in Figure 6a):
I-III II-IV I-V II-VII III-I IV-II V-VIII VI-I VII-VI VIII-II
The relations between the triplets are illustrated in Figure 6b, which represents an analogue ofde Brujin diagram for cellular automata [25–27] that was considered as a model of crystallization
processes [28,29]. The arrow lines in the diagram indicate the only possible sequence of triplets startingat the preceding triplet and ending at the following. Any realizable symbolic sequence is produced by
the loop of arrows starting and ending at the same triplet.
Let us consider the possible symbolic sequences and their structural realizations in more detail:1. The overlapping periodic sequences I↔III, III↔I, II↔IV and IV↔II (Figure 6c) produce thestructure of clinokurchatovite. This is the simplest structure possible in this group and contains two
blocks within its identity period (e.g., [(+B’)(+B”)]).
2. Starting from triplet I and going to triplet V and so forth results in the following 6-memberedsequence . . . →VI→[I→V→VIII→II→VII→VI]→I→ . . . (Figure 6d), which corresponds exactly to
the 6-membered sequence of blocks observed in the crystal structure of kurchatovite.
3. An inﬁnite number of periodic and aperiodic sequences can be obtained by inserting into the
6-membered sequence of triplets given above any even subsequence from the sequences I↔III andII↔IV. For instance, the sequence [I→V→VIII→II→(IV→II)→VII→VI] is produced by the insertingof the (IV→II) element into the sequence [I→V→VIII→II→VII→VI] after the triplet II. However,all such derivative sequences are at least two blocks longer than the sequence producing kurchatovite.

Figure 6. The illustration of the rule of overlapping triplets using the example of the I–V pair(a), the diagram of relations between the triplets (b), and the parts of diagram that result in thegeneration of clinokurchatovite (c) and kurchatovite (d) structures. See text for details.

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Therefore, the following important conclusion is in order: given the rules (i) and (ii), the sequencesof blocks observed in the crystal structures of clinokurchatovite and kurchatovite contain the minimal
number of blocks per identity period. All other sequences are either aperiodic or contain at least two
more blocks within their identity periods. It is remarkable that the natural structures are observed forthe minimal possible periodic sequences only.
The information-theoretic analysis of structural complexity of the two minerals [23,30] showsthat clinokurchatovite (IG = 4.170 bits/atom and IG,total = 300.235 bits/cell) is structurally simpler
than kurchatovite (IG = 4.755 bits/atom and IG,total = 1027.056 bits/cell). A greater complexity forthe derivative structure is to be expected given the proposed modular relationship between the two
structures and the fact that the sequence of blocks in kurchatovite is 6-membered, whereas thatin clinokurchatovite is 2-membered. According to the complexity-based classiﬁcation of minerals
proposed by Krivovichev [30], clinokurchatovite is intermediate, whereas kurchatovite is very complex.
As it has been shown above, all other periodic structures in the “kurchatovite family” would containat least 8-membered sequences of blocks and therefore would be even more structurally complex
than kurchatovite.
Grew et al. [31] examined structural complexity of boron minerals in detail and reported theaverage Shannon information content per unit cell for this class of minerals as 340 bits/cell (i.e., close to
the value found for clinokurchatovite). In fact, 46% of all boron minerals are intermediate in complexity(100 to 500 bits/cell), whereas only 3% are very complex (>1000 bits/cell). The reasons for the highstructural complexity of kurchatovite can be inferred from the modular character of its structure,
which is one of the most common mechanisms for the formation of complex crystal structures [30,31].
5. Conclusions
In conclusion, our study shows that kurchatovite and clinokurchatovite are not polytypes,
but polymorphs and therefore re-consideration of their status as of separate mineral species is notwarranted. However, the structures of the two minerals are closely related: the crystal structure ofkurchatovite may be considered as a derivative of clinokurchatovite through the modular approach.
In fact, we have demonstrated that, considering common structural features of kurchatovite andclinokurchatovite, the whole family of periodic and aperiodic structures can be generated. However,
the two minerals under consideration possess the simplest periodic structures possible for the family,
whereas all other structures would be structurally more complex. This may explain the absence ofother “kurchatovite”-like polymorphs of CaMgB2 O5 in nature: only simplest structures from theinﬁnite array of possible conﬁgurations are realized. This might be viewed as a consequence ofthe maximum-entropy principle (since information-based structural complexity provides a negative
contribution to conﬁgurational entropy [32]) or as a result of the least-action principle, which was
expressed by Pierre Loius Maupertouis as “nature is thrifty in all its actions”
This study also demonstrates that diversity and complexity of mineral structures involve different
and sometimes complicated structural combinatorics, from which combinatorics of polytypes is justa single and relatively simple branch.
Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/8/332/s1,
Crystallographic Information Files (CIFs) for kurchatovite and clinokurchatovite.
Author Contributions: Conceptualization, S.V.K. and E.S.G.; Methodology, S.V.K. and I.V.P.; Investigation, Y.A.P.,
S.V.K., I.V.P. and V.O.Y.; Writing—Original Draft Preparation, Y.A.P., S.V.K., I.V.P., and E.S.G.; Writing—Review
and Editing, S.V.K. and E.S.G.; Visualization, S.V.K.
Funding: This research was funded by the Russian Foundation for Basic Research (in part of X-ray studies;
grant 17-05-01027 to Y.A.P. and S.V.K.) and Presidium of the Russian Academy of Sciences (in part of complexity
analysis).
Acknowledgments: The X-ray diffraction studies were performed in the X-ray Diffraction Resource Centre of
St. Petersburg State University.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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