Mineralogical
Crystallography
Edited by

Vladislav V. GurzhiyPrinted Edition of the Special Issue Published in Crystals

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Mineralogical Crystallography

Mineralogical Crystallography

EditorVladislav V. Gurzhiy

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EditorVladislav V. GurzhiySaint Petersburg State University
Russian Federation

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Contents
About the Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Vladislav V. Gurzhiy
Mineralogical Crystallography
Reprinted from: Crystals 2020, 10, 805, doi:10.3390/cryst10090805 . . . . . . . . . . . . . . . . . .

1

Vladislav V. Gurzhiy, Ivan V. Kuporev, Vadim M. Kovrugin, Mikhail N. Murashko,
Anatoly V. Kasatkin and Jakub Plá šil
Crystal Chemistry and Structural Complexity of Natural and Synthetic Uranyl Selenites
Reprinted from: Crystals 2019, 9, 639, doi:10.3390/cryst9120639 . . . . . . . . . . . . . . . . . . . .

5Olga S. Tyumentseva, Ilya V. Kornyakov, Sergey N. Britvin, Andrey A. Zolotarev andVladislav V. Gurzhiy
Crystallographic Insights into Uranyl Sulfate Minerals Formation: Synthesis and Crystal
Structures of Three Novel Cesium Uranyl Sulfates
Reprinted from: Crystals 2019, 9, 660, doi:10.3390/cryst9120660 . . . . . . . . . . . . . . . . . . . . 33
Igor V. Pekov, Natalia V. Zubkova, Ilya I. Chaikovskiy, Elena P. Chirkova,
Dmitry I. Belakovskiy, Vasiliy O. Yapaskurt, Yana V. Bychkova, Inna Lykova,
Sergey N. Britvin and Dmitry Yu. Pushcharovsky
Krasnoshteinite, Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O, a New Microporous Mineral with a Novel
Type of Borate Polyanion
Reprinted from: Crystals 2020, 10, 301, doi:10.3390/cryst10040301 . . . . . . . . . . . . . . . . . . 47
Sergey N. Britvin, Maria G. Krzhizhanovskaya, Vladimir N. Bocharov andEdita V. Obolonskaya
Crystal Chemistry of Stanﬁeldite, C a7 M2 Mg9 (PO4 )12 ( M = C a, M g, F e2+ ), a S tructural B ase of
Ca3 Mg3 (PO4 )4 Phosphors
Reprinted from: Crystals 2020, 10, 464, doi:10.3390/cryst10060464 . . . . . . . . . . . . . . . . . . 61
Paola Comodi, Azzurra Zucchini, Tonci Balić-Žunić, Michael Hanﬂand and Ines Collings
The High Pressure Behavior of Galenobismutite, PbBi2 S4 : A Synchrotron Single Crystal X-ray
Diffraction Study
Reprinted from: Crystals 2019, 9, 210, doi:10.3390/cryst9040210 . . . . . . . . . . . . . . . . . . . . 75
Yunfan Miao, Youwei Pang, Yu Ye, Joseph R. Smyth, Junfeng Zhang, Dan Liu, Xiang Wangand Xi Zhu
Crystal Structures and High-Temperature Vibrational Spectra for Synthetic Boron andAluminum Doped Hydrous Coesite
Reprinted from: Crystals 2019, 9, 642, doi:10.3390/cryst9120642 . . . . . . . . . . . . . . . . . . . . 93
Alina R. Izatulina, Anton M. Nikolaev, Mariya A. Kuz’mina, Olga V. Frank-Kamenetskaya
and Vladimir V. Malyshev
Bacterial Effect on the Crystallization of Mineral Phases in a Solution Simulating Human Urine
Reprinted from: Crystals 2019, 9, 259, doi:10.3390/cryst9050259 . . . . . . . . . . . . . . . . . . . . 111
Aleksei V. Rusakov, Mariya A. Kuzmina, Alina R. Izatulina and Olga V. Frank-Kamenetskaya
Synthesis and Characterization of (Ca,Sr)[C2 O4 ]·nH2 O Solid Solutions: Variations of Phase
Composition, Crystal Morphologies and in Ionic Substitutions
Reprinted from: Crystals 2019, 9, 654, doi:10.3390/cryst9120654 . . . . . . . . . . . . . . . . . . . . 123v

Alejandro De la Rosa-Tilapa, Agustı́n Maceda and Teresa Terrazas
Characterization of Biominerals in Cacteae Species by FTIR
Reprinted from: Crystals 2020, 10, 432, doi:10.3390/cryst10060432 . . . . . . . . . . . . . . . . . . 135
Donata Konopacka-Łyskawa
Synthesis Methods and Favorable Conditions for Spherical Vaterite Precipitation: A Review
Reprinted from: Crystals 2019, 9, 223, doi:10.3390/cryst9040223 . . . . . . . . . . . . . . . . . . . . 147
Min Tang and Yi-Liang Li
A Complex Assemblage of Crystal Habits of Pyrite in the Volcanic Hot Springs from Kamchatka,
Russia: Implications for the Mineral Signature of Life on Mars
Reprinted from: Crystals 2020, 10, 535, doi:10.3390/cryst10060535 . . . . . . . . . . . . . . . . . . 163

viAbout the Editor
Vladislav V. Gurzhiy is an associate professor at the Crystallography Department of the Instituteof Earth Sciences, Saint Petersburg State University, and holds the position of Chairman of theInstitute of Earth Sciences’ Scientiﬁc Committee. Dr. Gurzhiy graduated from St. Petersburg State
University in 2007 and completed his PhD in 2009, with a dissertation entitled ”Crystal chemistryof uranyl selenates with organic and inorganic cations”. His main research interests are related
to the crystal chemistry of minerals and their synthetic analogs, in particular, compounds bearing
uranium and transuranium elements, and biominerals. Dr. Gurzhiy is a full member of the Russian
Mineralogical Society and the Saint-Petersburg Society of Naturalists. He has published over 150
peer-reviewed journal articles and several book chapters. Dr. Gurzhiy has been honored with several
awards, including the Yu.T. Struchkov Prize and Academia Europaea Prize, for research works in theﬁelds of X-ray crystallography and Earth sciences.

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crystals
Editorial

Mineralogical Crystallography
Vladislav V. GurzhiyDepartment of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9,
St. Petersburg 199034, Russia; vladislav.gurzhiy@spbu.ru or vladgeo17@mail.ru
Received: 4 September 2020; Accepted: 9 September 2020; Published: 11 September 2020

Keywords: mineral; crystallography; crystal chemistry; X-ray diﬀraction; crystal structure; crystal
growth; mineral evolution

Crystallography remains, for mineralogy, one of the main sources of information on natural
crystalline substances. A description of mineral species shape is carried out according to the principles
of geometric crystallography; the crystal structure of minerals is determined using X-ray crystallography
techniques, and physical crystallography approaches allow one to evaluate various properties ofminerals, etc. However, the reverse comparison should not be forgotten as well: the crystallography
science, in its current form, was born in the course of mineralogical research, long before preparative
chemistry received such extensive development. It is worth noting that, even today, investigations of
the crystallographic characteristics of minerals regularly open up new horizons in materials science,
because the possibilities of nature (fascinating chemical diversity; great variation of thermodynamic
parameters; and, of course, almost endless processing time) are still not available for reproduction
in any of the world’s laboratories. This Special Issue is devoted to mineralogical crystallography,
the oldest branch of crystallographic science, and combines important surveys covering such topics as:
discovery of new mineral species; crystal chemistry of minerals and their synthetic analogs; behaviorof minerals at non-ambient conditions; biomineralogy; and crystal growth techniques.
We hope that the current set of reviews and articles will arouse genuine interest among readers
and, perhaps, push them to their own successful research in the ﬁeld of mineralogical crystallography.
1. Crystal Chemistry of Minerals and Their Synthetic Analogs
Gurzhiy et al. [1] reviewed the crystal chemistry of the family of natural and synthetic
uranyl selenite compounds, paying special attention to the pathways of synthesis and topologicalanalysis of the known crystal structures. Crystal structures of two minerals were reﬁned. The H
atoms positions belonging to the interstitial H2 O molecules in the structure of demesmaekerite,
Pb2 Cu5 [(UO2 )2 (SeO3 )6 (OH)6 ](H2 O)2 , were assigned. The reﬁnement of the guilleminite crystal
structure allowed the determination of an additional site arranged within the void of the interlayer
space and occupied by an H2 O molecule, which suggests the new formula of guilleminite to be written
as Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 . This paper could be regarded as the ﬁrst review on the mineralogy
and crystal chemistry of the named group of compounds.
Tyumentseva et al. [2] studied the alteration of the uranyl oxide hydroxy-hydrate mineral
schoepite [(UO2 )8 O2 (OH)12 ](H2 O)12 at mild hydrothermal conditions in the presence of sulfate
oxyanions, which resulted in the crystallization of three novel compounds.
Comparison of the isotypic natural and synthetic uranyl-bearing compounds [1,2] suggeststhat formation of all uranyl selenite and of the majority of uranyl sulfate minerals requires heating,which most likely can be attributed to the radioactive decay. The temperature range could be assumed
from the manner of the interpolyhedral linkage.

Crystals 2020, 10, 805; doi:10.3390/cryst10090805

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Crystals 2020, 10, 805

2. Discovery of New Mineral Species
Pekov et al. [3] discovered the new hydrous aluminum chloroborate mineral krasnoshteinite(Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O), with a zeolite-like microporous structure and a three-dimensional
system of wide channels containing Cl- anions and weakly bonded H2 O molecules. The crystal
structure of krasnoshteinite is also remarkable due to the presence of a novel insular borate polyanion[B2 O4 (OH)2 ]4− .
Britvin et al. [4] reported on the crystal structure of natural Ca-Mg-phosphate stanﬁeldite,
Ca7 M2 Mg7 (PO4 )12 (M = Ca, Mg, Fe2+ ), derived from the pallasite meteorite Brahin for the ﬁrst time.
The authors reviewed the existing analytical data and showed that there is no evidence that thephosphor base with the formula Ca3 Mg3 (PO4 )4 exists.
3. Behavior of Minerals at Non-Ambient Conditions
Comodi et al. [5] studied the transformation of the crystal structure of galenobismutite, PbBi2 S4 ,under pressure up to 20.9 GPa. The structure undergoes reversible and completely elastic transitions.
The size and the shape of Bi- and Pb-centered polyhedra suggest that the high-pressure structure ofgalenobismutite can host Na and Al in the lower mantle, which are incompatible with the periclase or
perovskite crystal structures.
Hydrous coesite crystals, a high-pressure SiO2 polymorph, were synthesized with various B3+ and3+
Al contents and in situ high-temperature Raman and FTIR spectra were collected at ambient pressure
by Miao et al. [6]. Crystals were observed to be stable up to 1500 K. Al substitution signiﬁcantly
reduces the H+ concentration in coesite, so the mechanism is controlled by oxygen vacancies, while the
B incorporation may prefer the electrostatically coupled substitution (Si4+ = B3+ + H+ ).
4. Biomineralogy
Izatulina et al. [7] studied the eﬀect of bacteria that are present in human urine on the crystallizationof oxalate and phosphate mineral phases, the most common constituents of renal stones. It was shownthat the inﬂammatory process will contribute to the decrease in oxalate supersaturation in urine due tocalcium oxalate crystallization, while the change in urine pH and the products of bacterial metabolism
will be of major importance in the case of phosphate mineralization.
Rusakov et al. [8] reported on the mechanisms of Sr-to-Ca substitution in the structures of calcium
oxalate minerals that were found in lichen thalli on Sr-bearing apatite rock. It was shown that theincorporation of Sr ions is less preferable than Ca into the structures of whewellite and weddellite,
and substitution rates are slightly higher for weddellite than for whewellite, which is most likely
caused by the denser manner of the interpolyhedral linkage in the latter structure.
Five Cacteae species were studied using various experimental techniques to characterize thebiomineral composition within their diﬀerent tissues by De la Rosa-Tilapa et al. [9]. Calcium carbonatesand silicate phases were detected along with common calcium oxalates.
5. Crystal Growth Techniques
Konopacka-Łyskawa [10] reviewed the state of the art of the vaterite crystallization techniques.
Vaterite is known to be the least thermodynamically stable anhydrous calcium carbonate polymorph,very rarely found in nature. However, synthetic vaterite has large potential in pharmacology andmanufacturing. Well-known classical and new methods used for vaterite precipitation were discussedwith particular attention to the parameters aﬀecting the formation of spherical particles.
Tang and Yi-Liang [11] revealed that speciﬁc geochemical microenvironments and the bacterial
activities in the long-lived volcanic hot springs from Kamchatka result in the development andpreservation of the complex pyrite crystal habits. Application of similar techniques to other systems
may help in the identiﬁcation of biogenic iron sulﬁdes in sediments on Earth and other planets.
Funding: This research received no external funding.

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Acknowledgments: As the Guest Editor, I would like to acknowledge all the authors for their valuable contribution
to this Special Issue, which is expressed in fascinating and inspiring papers.
Conﬂicts of Interest: The author declares no conﬂict of interest.

References
1.
2.

3.

4.

5.

6.
7.
8.

9.
10.
11.

Gurzhiy, V.; Kuporev, I.; Kovrugin, V.; Murashko, M.; Kasatkin, A.; Plášil, J. Crystal chemistry and structuralcomplexity of natural and synthetic uranyl selenites. Crystals 2019, 9, 639. [CrossRef]
Tyumentseva, O.; Kornyakov, I.; Britvin, S.; Zolotarev, A.; Gurzhiy, V. Crystallographic insights into uranyl
sulfate minerals formation: Synthesis and crystal structures of three novel cesium uranyl sulfates. Crystals
2019, 9, 660. [CrossRef]
Pekov, I.; Zubkova, N.; Chaikovskiy, I.; Chirkova, E.; Belakovskiy, D.; Yapaskurt, V.; Bychkova, Y.; Lykova, I.;
Britvin, S.; Pushcharovsky, D. Krasnoshteinite, Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O, a new microporous mineralwith a novel type of Borate Polyanion. Crystals 2020, 10, 301. [CrossRef]
Britvin, S.; Krzhizhanovskaya, M.; Bocharov, V.; Obolonskaya, E. Crystal chemistry of Stanﬁeldite,
Ca7 M2 Mg9 (PO4 )12 (M = Ca, Mg, Fe2+ ), a Structural Base of Ca3 Mg3 (PO4 )4 Phosphors. Crystals 2020,
10, 464. [CrossRef]
Comodi, P.; Zucchini, A.; Balić-Žunić, T.; Hanﬂand, M.; Collings, I. The high pressure behavior of
galenobismutite, PbBi2 S4 : A synchrotron single crystal X-ray diﬀraction study. Crystals 2019, 9, 210.
[CrossRef]
Miao, Y.; Pang, Y.; Ye, Y.; Smyth, J.; Zhang, J.; Liu, D.; Wang, X.; Zhu, X. Crystal structures and high-temperature
vibrational spectra for synthetic boron and aluminum doped hydrous coesite. Crystals 2019, 9, 642. [CrossRef]
Izatulina, A.; Nikolaev, A.; Kuz’mina, M.; Frank-Kamenetskaya, O.; Malyshev, V. Bacterial eﬀect on the
crystallization of mineral phases in a solution simulating human urine. Crystals 2019, 9, 259. [CrossRef]
Rusakov, A.; Kuzmina, M.; Izatulina, A.; Frank-Kamenetskaya, O. Synthesis and characterization of(Ca,Sr)[C2 O4 ]·nH2 O solid solutions: Variations of phase composition, crystal morphologies and in ionicsubstitutions. Crystals 2019, 9, 654. [CrossRef]
De la Rosa-Tilapa, A.; Maceda, A.; Terrazas, T. Characterization of biominerals in Cacteae species by FTIR.
Crystals 2020, 10, 432. [CrossRef]
Konopacka-Łyskawa, D. Synthesis methods and favorable conditions for spherical vaterite precipitation:
A review. Crystals 2019, 9, 223. [CrossRef]
Tang, M.; Li, Y. A complex assemblage of crystal habits of pyrite in the volcanic hot springs from Kamchatka,
Russia: Implications for the mineral signature of life on Mars. Crystals 2020, 10, 535. [CrossRef]© 2020 by the author. 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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Review

Crystal Chemistry and Structural Complexity of
Natural and Synthetic Uranyl Selenites
Vladislav V. Gurzhiy 1, *, Ivan V. Kuporev 1 , Vadim M. Kovrugin 1 , Mikhail N. Murashko 1 ,Anatoly V. Kasatkin 2 and Jakub Plášil 3
1

2
3

*

Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg 199034,
Russian; st054910@student.spbu.ru (I.V.K.); kovrugin_vm@hotmail.com (V.M.K.);
mzmurashko@gmail.com (M.N.M.)
Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninskiy pr. 18, 2, Moscow 119071,
Russian; kasatkin@inbox.ru
Institute of Physics, The Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 2, 18221 Praha 8,
Czech Republic; plasil@fzu.cz
Correspondence: vladislav.gurzhiy@spbu.ru or vladgeo17@mail.ru

Received: 10 November 2019; Accepted: 28 November 2019; Published: 30 November 2019

Abstract: Comparison of the natural and synthetic phases allows an overview to be made andeven an understanding of the crystal growth processes and mechanisms of the particular crystal
structure formation. Thus, in this work, we review the crystal chemistry of the family of uranyl
selenite compounds, paying special attention to the pathways of synthesis and topological analysisof the known crystal structures. Comparison of the isotypic natural and synthetic uranyl-bearing
compounds suggests that uranyl selenite mineral formation requires heating, which most likely canbe attributed to the radioactive decay. Structural complexity studies revealed that the majority ofsynthetic compounds have the topological symmetry of uranyl selenite building blocks equal to
the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved
regardless of the interstitial ﬁlling of the structures. Whereas the real symmetry of U-Se complexes
in the structures of minerals is lower than their topological symmetry, which means that interstitialcations and H2 O molecules signiﬁcantly aﬀect the structural architecture of natural compounds. At thesame time, structural complexity parameters for the whole structure are usually higher for the minerals
than those for the synthetic compounds of a similar or close organization, which probably indicatesthe preferred existence of such natural-born architectures. In addition, the reexamination of the crystal
structures of two uranyl selenite minerals guilleminite and demesmaekerite is reported. As a result of
the single crystal X-ray diﬀraction analysis of demesmaekerite, Pb2 Cu5 [(UO2 )2 (SeO3 )6 (OH)6 ](H2 O)2 ,the H atoms positions belonging to the interstitial H2 O molecules were assigned. The reﬁnement of
the guilleminite crystal structure allowed the determination of an additional site arranged within
the void of the interlayer space and occupied by an H2 O molecule, which suggests the formula ofguilleminite to be written as Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 instead of Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)3 .
Keywords: uranyl; selenite; selenate; crystal structure; topology; structural complexity; demesmaekerite;
guillemenite; haynesite

1. Introduction
All natural compounds of U(VI) and selenium are selenites. Uranyl selenites can be justifiably
attributed to rare mineral species. Nowadays, there are only seven uranyl selenite mineral species
approved by the International Mineralogical Association as of 20 October 2019 (for comparison,
there are >40 uranyl sulfates and ~50 uranyl phosphates): Guilleminite, Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)3 [1],demesmaekerite, Pb2 Cu5 [(UO2 )2 (SeO3 )6 (OH)6 ](H2 O)2 [2], marthozite, Cu[(UO2 )3 (SeO3 )2 O2 ](H2 O)8 [3],Crystals 2019, 9, 639; doi:10.3390/cryst9120639

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Crystals 2019, 9, 639

derriksite, Cu4 [(UO2 )(SeO3 )2 ](OH)6 [4], haynesite, [(UO2 )3 (SeO3 )2 (OH)2 ](H2 O)5 [5], piretite,
Ca(UO2 )3 (SeO3 )2 (OH)4 ·4H2 O [6], and larisaite, Na(H3 O)[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 [7]. Their occurrenceis limited to just a few localities. First, these are Musonoi and Shinkolobwe mines in DR Congo [6],two of the minerals were only found in the Repete mine (San Juan County, Utah, USA) [5], and a
few more occurrences in Europe could be mentioned (small uranium deposit Zálesí in the Czech
Republic, Liauzun in France, and La Creusaz U prospect in Switzerland) [8]. Nevertheless, apart frommineralogy, uranyl selenites are of great interest from the geochemical and radiochemical points of view.
It is known that fission products contain 53 g per ton [9] of long-lived 79 Se isotope with a half-life of
1.1 × 106 years [10] after three years of nuclear fuel irradiation in the reactor. Thus, an understandingof the processes of mineral formation in nature and their synthetic analogs in laboratories can help
in the processing of nuclear wastes. Crystal chemical and structural investigations are key points insuch a material’s scientific studies due to the essential knowledge of how the variation in the chemicalcomposition and growth conditions affects the crystal structure formation.
Herein, we review the topological diversity and growth conditions of natural and synthetic
uranyl selenites. Crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite
were reﬁned. The structural complexity approach was implemented to determine the preference of aparticular topological type, taking into account existing geometrical isomers.
2. Materials and Methods
2.1. Occurrence
The samples of minerals studied in this work were taken from the Fersman MineralogicalMuseum, Museum of Natural History in Luxembourg and private collections of authors of thecurrent paper (V.V.G., A.V.K.). Guilleminite: from the Museum (69465 and 82312), from V.V.G. (6111).
Demesmaekerite: from J.P. Haynesite: from the Fersman Museum (88922 and 94267), from V.V.G. (5767),
from A.V.K. (247X). The samples of guilleminite and demesmaekerite originate from the Musonoi, DR
Congo. The samples of haynesite originate from the Repete mine, Utah, UT, USA.
2.2. Single-Crystal X-Ray Diﬀraction Study
A single crystal of guilleminite (0.08 × 0.04 × 0.01 mm3 ) was selected under binoculars, encased inviscous cryoprotectant, and mounted on cryo-loop. Diﬀraction data were collected using a Bruker Kappa
Duo diﬀractometer (Bruker AXS, Madison, WI, USA) equipped with a CCD (charge-coupled device)
Apex II detector operated with monochromated microfocused MoKα radiation (λ[MoKα] = 0.71073 Å)
at 45 kV and 0.6 mA. Diﬀraction data were collected at 100 K with frame widths of 0.5◦ in ω and ϕ,
and an exposure of 70 s per frame. Diﬀraction data were integrated, and background, Lorentz, andpolarization correction were applied. An empirical absorption correction based on spherical harmonics
implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program [11].
The unit-cell parameters were reﬁned using the least-squares techniques. The crystal structure of
guilleminite was solved by a dual-space algorithm and reﬁned using the SHELX programs [12,13]
incorporated in the OLEX2 program package [14]. The ﬁnal model includes coordinates and anisotropic
displacement parameters for all non-H atoms. The H atoms of H2 O molecules were localized from
diﬀerence Fourier maps and were included in the reﬁnement, with Uiso (H) set to 1.5Ueq (O) and O–H
restrained to 0.95 Å.
A dark-olive green prismatic crystal of demesmaekerite (0.034 × 0.032 × 0.022 mm3 ) was mounted
on a glass ﬁber, and diﬀraction intensities were measured at room temperature with a Rigaku
SuperNova (Oxford, UK) single-crystal diﬀractometer. The diﬀraction experiment was done using
MoKα radiation from a micro-focus X-ray source collimated and monochromatized by mirror-optics
and the detection of the reﬂected X-rays was done by an Atlas S2 CCD detector. X-ray diﬀraction
data were collected at room-temperature with frame widths of 1.0◦ in ω and an exposure of 80 s perframe. Diﬀraction data were integrated, and background, Lorentz, and polarization correction were6

Crystals 2019, 9, 639

applied. An empirical absorption correction based on spherical harmonics implemented in the SCALE3
ABSPACK algorithm was applied in the CrysAlisPro program [11]. The structure was solved by the
charge-ﬂipping algorithm [12] and reﬁned using the Jana2006 program [15]. The ﬁnal reﬁnement
cycles were undertaken considering all atoms (except of hydrogen) reﬁned with anisotropic atomic
displacement parameters. The H atoms of H2 O molecules were localized from the diﬀerence Fourier
maps and were subsequently reﬁned with Uiso (H) set to 1.2*Ueq of the donor O atom and O–H softly
restrained to 0.95 Å.
Supplementary crystallographic data were deposited in the Inorganic Crystal Structure
Database (ICSD) and can be obtained by quoting the depository numbers CSD 1963864 and1964420 for guilleminite and demesmaekerite, respectively, at https://www.ccdc.cam.ac.uk/structures/(see Supplementary Materials).
2.3. Coordination of U and Se
The crystal structures of all the natural and synthetic compounds described herein are based on
the chained or layered substructural units built by the linkage of U- and Se-centered coordination
polyhedra. U(VI) atoms form approximately linear UO2 2+ uranyl ions (Ur) with two short U6+ ≡O2−
bonds. Ur cation is coordinated in the equatorial plane by other four to six oxygen atoms, to forma tetra-, penta-, or hexagonal bipyramid, respectively, as a coordination polyhedron of U6+ atoms.
The selenite group has a conﬁguration of a trigonal pyramid with its apical vertex occupied by the Se4+
cation possessing a stereochemically active lone-electron pair. In the crystal structures of number ofsynthetic uranyl selenium compounds, there are also Se(VI) species that form [SeO4 ]2− tetrahedra.
2.4. Graphical Representation and Anion Topologies
For topological analysis, the theory of graphical (nodal) representation of crystal structures [16]and the anion topology method [17] were used along with the classiﬁcation suggested in [18].
Anion topologies were used to describe the layered complexes having edge-sharing polymerization of
uranyl coordination polyhedra. For the rest of the structures, graphical representation was used. Eachgraph has a special index ccD–U:Se–#, where cc means “cation-centered”, D indicates dimensionality(1—chains; and 2—sheets), U:Se ratio, # is the registration number of the unit. Each anion topology isindicated by a, so called, ring symbol, p1 r1 p2 r2 . . . , where p is the sum of vertices in a topological cycle,
and r is the number of the respective cycles in the reduced section of the layer.
Three-connected selenate tetrahedra, sharing three of its corners with adjacent uranyl bipyramids,
and 2- or 3-connected selenite pyramids, can possess the fourth non-shared corner or lone electron
pair, respectively, oriented either up, down, or disordered relative to the plane of the chain, layer,or, in particular, to the equatorial plane of the uranyl bipyramid. Such ambiguity gives rise togeometric isomerism with various orientations of the Se-centered polyhedra. To distinguish theisomers, their orientation matrices were assigned using symbols u (up), d (down), m (orientation
up-down topologically equivalent), or  (white vertex, Se-centered polyhedron, is missing in the graph).
2.5. Complexity Calculations
In order to characterize and quantify the impact of each substructural units on the formationof a particular architecture, the structural complexity approach recently developed by S.V.
Krivovichev [19–23], which allows comparison of the structures in terms of their information content,
was used.
The complexity of the crystal structure was estimated as a Shannon information content per atom(IG ) and per unit cell (IG,total ) using the following equations:
IG = −k
i=1(bits/atom),pi log2 pi

7(1)

Crystals 2019, 9, 639

IG,total = −v IG = −v

k

i=1

pi log2 pi (bits/cell),(2)where k is the number of diﬀerent crystallographic orbits (independent sites) in the structure and pi isthe random choice probability for an atom from the i-th crystallographic orbit, that is:pi = mi /v,(3)where mi is a multiplicity of a crystallographic orbit (i.e., the number of atoms of a speciﬁc Wyckoﬀ site
in the reduced unit cell), and v is the total number of atoms in the reduced unit cell.
The reliable correlation of structural complexity parameters is possible only for compounds withthe same or very close chemical composition (e.g., polymorphs), whereas changes in the hydration
state, nature of interstitial complexes, and size and shape of organic molecules could signiﬁcantly
aﬀect the overall complexity behavior. In this light, within the current crystal chemical review,structural complexity parameters of various building blocks (uranyl selenite units, interstitial structure,
H-bonding system) were calculated to analyze their contributions to the complexity of the whole
structure. This approach suggested by S.V. Krivovichev [24] and recently successfully implemented
in [25,26] allows the factors that inﬂuence the symmetry preservation or reduction of uranyl selenite
units to be revealed, and it shows which of the multiple blocks plays the most important role in aparticular structure formation.
3. Results
3.1. Uranyl Selenite Minerals
Guilleminite, Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)3 [1,27], and demesmaekerite, Pb2 Cu5 [(UO2 )2 (SeO3 )6(OH)6 ](H2 O)2 [2,28], were the ﬁrst uranyl selenites found in nature (Table 1). These minerals occur inthe lower part of the oxidized zone of the copper-cobalt deposit of Musonoi (Katanga, DR Congo).
The ﬁrst mineral was named after the general director of the Union Minière du Haut-Katanga(UMHK), co-founder of the International Mineralogical Association, French chemist and mineralogist,
Jean-Claude Guillemin. Guilleminite crystallizes in the orthorhombic Pmn21 space group and forms
small tabular crystals and canary yellow crusts. It occurs in association with malachite, uranophane-α,
wulfenite, etc. The second mineral was named in honor of the director of the geological departmentof the UMHK, Belgian geologist Gaston Demesmaeker. Demesmaekerite crystallizes in the triclinic
P-1 space group in the form of lamellar and elongated crystals of bottle-green to dark olive-green
color in association with malachite, uranophane–α, chalcomenite, and other uranyl-selenites: Namely
marthozite and derriksite as well as guilleminite.
Marthozite, Cu[(UO2 )3 (SeO3 )2 O2 ](H2 O)8 [3,29], was also found in the Musonoi mine within a few
years after, and named to honor Aimé Marthoz, former director of the UMHK. Marthozite crystallizes
in the orthorhombic Pbn21 space group, in the form of well-faceted green crystals, in association withthe other selenites, including guilleminite and demesmaekerite, as well as kasolite, cuprosklodowskite,malachite, chalcomenite, and sengierite. Mineral is isotypic with guilleminite.
A few years later, derriksite, Cu4 [(UO2 )(SeO3 )2 ](OH)6 [4,30], was found at the same deposit inCongo, and named after Jean-Marie François Joseph Derriks, a Belgian geologist and administratorof the UMHK. Derriksite crystallizes in the orthorhombic Pn21 m space group, as sub-green up tobottle-green-colored crystals, elongated at [001] or incrustations and ﬁne-crystalline crusts on digenite
and the mineral is associated with marthozite, demesmaekerite, kasolite, malachite, etc.

8

7

6

5

4

3

Cu[(UO2 )3 (SeO3 )2 O2 ](H2 O)8marthozite
Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4guilleminite
Na(H3 O)[(UO2 )3 (SeO3 )2 O2 ](H2 O)4larisaite[(UO2 )3 (SeO3 )2 (OH)2 ](H2 O)5haynesite
Ca[(UO2 )3 (SeO3 )2 (OH)4 ](H2 O)4
piretite
61 52 42 32

cc1–1:3–2

Pb2 Cu5 [(UO2 )2 (SeO3 )6 (OH)6 ](H2 O)2demesmaekerite2

Layers with edge-linkage

cc1–1:2–1

Topology

Cu4 [(UO2 )(SeO3 )2 ](OH)6
derriksite

ChainsFormula/Mineral Name

1

No.

19.088(8)/90b, Å/β, ◦6.935/90
7.010(3)/90

Pmn21 or
Pmnm

6.9806(9)/90

16.762(1)/90

6.9879(4)/90

17.135(7)/90

8.025/90

7.646(1)/90

7.2522(5)/90

16.454(1)/90

11.9663(9)/89.891(8) 10.0615(14)/100.341(11)5.570(2)/90a, Å/α, ◦Pnc2 or Pncm

P11m

Pmn21

Pbn21

P-1

Pn21 m

Sp. Gr.

Table 1. Crystallographic characteristics of natural uranyl selenites.

17.606(4)/90

17.430/90

17.249(2)/90.039(4)7.0629(4)/90

17.223(1)/90

5.6318(8)/91.339(9)5.965(2)/90c, Å/γ, ◦

[22]

[21]

[19]

This work, [18][17]This work, [4][2]Reference

Crystals 2019, 9, 639

9

Crystals 2019, 9, 639

Next, natural uranyl selenite was discovered in 20 years across the Atlantic, in the Repete mine
(Utah, USA). Haynesite, [(UO2 )3 (SeO3 )2 (OH)2 ](H2 O)5 [5,31,32] is named after the American geologist
Patrick Eugene Haynes. Haynesite is orthorhombic, occurs as amber-yellow tablets, transparent totranslucent, elongated at [001], and as acicular prismatic rosettes up to 3 mm in diameter, and isassociated with andersonite, boltwoodite, gypsum, and calcite as crusts on mudstones and sandstones.
Piretite, Ca(UO2 )3 (SeO3 )2 (OH)4 ·4H2 O [6], calcium uranyl selenite from Shinkolobwe mine
(Katanga, DR Congo) is named after the Belgian crystallographer Paul Piret. Piretite is orthorhombic,
it crystallizes as lemon-yellow elongated tablets, irregular in outline and up to 3 mm, ﬂattened on (001),
or as needle-prismatic crystals up to 5 mm. It occurs in association with a masuyite-like uranyl-leadoxide as crusts on uraninite. It should be noted that crystal structures of haynesite and piretite havestill not been determined.
The last to date, uranyl selenite mineral, larisaite, Na(H3 O)[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 [7],was found in the Repete mine (Utah, UT, USA) and named in honor of Larisa Nikolaevna Belova,
a Russian mineralogist and crystallographer who made a signiﬁcant contribution to the knowledge
on uranium minerals. Larisaite occurs as canary-yellow lamellar crystals up to 1 mm long, and asradial aggregates up to 2 mm across; most crystals are ﬁssured and ribbed. The mineral is a supergene
product associated with calcite, quartz, gypsum, montmorillonite, wölsendorﬁte, andersonite, haynesite,
and uranophane–α in sedimentary rocks.
3.2. Synthetic Uranyl Compounds with Selenite Ions
The ﬁrst synthetic and the simplest uranyl selenite, [(UO2 )(SeO3 )], was obtained in 1978 [33](and its neptunyl analog has been recently reported [34] as well). Further, the research undertaken
by V. E. Mistryukov and Yu. N. Mikhailov from the Kurnakov Institute of General and Inorganic
Chemistry RAS (Russian Federation), and by V.N. Serezhkin and L.B. Serezhkina from the Samara State
University (Russian Federation) should be mentioned, who studied uranyl selenites with electroneutral
ligands and the ﬁrst Na-bearing synthetic uranyl selenite compounds. Nearly half of the synthetic
compounds described within this review were synthesized and characterized by T.E. Albrecht-Schmittand co-workers (Table 2). The signiﬁcant impact of their works on the development of uranyl selenites’
structural chemistry should be especially noted.
Synthetic compounds, whose structures are based on inorganic units with the linkage of Ur
to selenite oxyanions (Table 2), could be divided into two groups: Pure inorganic and organicallytemplated phases.

10Ca[(UO2 )(SeO3 )2 ]Sr[(UO2 )(SeO3 )2 ]Sr[(UO2 )(SeO3 )2 ](H2 O)2Na3 [H3 O][(UO2 )(SeO3 )2 ]2 (H2 O)

10
11

12
13

11

Ag2 [(UO2 )(SeO3 )2 ]24

Cs2 [(UO2 )4 (SeO3 )5 ](H2 O)2Cs2 [(UO2 )7 (SeO4 )2 (SeO3 )2 (OH)4 O2 ](H2 O)5UO2 Se2 O5

30

31

32

[(UO2 )(SeO3 )]Sr[(UO2 )3 (SeO3 )2 O2 ](H2 O)4
Li2 [(UO2 )3 (SeO3 )2 O2 ](H2 O)628
29

Ba[(UO2 )(SeO3 )2 ]26

27

Pb[(UO2 )(SeO3 )2 ]25

Layers with edge-linkage[NH4 ]2 [(UO2 )(SeO3 )2 ](H2 O)0.5[NH4 ][(UO2 )(SeO3 )(HSeO3 )]K[(UO2 )(HSeO3 )(SeO3 )]Rb[(UO2 )(HSeO3 )(SeO3 )]Cs[(UO2 )(HSeO3 )(SeO3 )]Cs[((U,Np)O2 )(HSeO3 )(SeO3 )]Tl[(UO2 )(HSeO3 )(SeO3 )]Cs[(UO2 )(SeO3 )(HSeO3 )](H2 O)3
Na[(UO2 )(SeO3 )(HSeO3 )](H2 O)4[H3 O][(UO2 )(SeO4 )(HSeO3 )]

14
15
16
17
18
19
20
21
22
23

Layers with corner-linkage[(UO2 )(HSeO3 )2 (H2 O)][(UO2 )(HSeO3 )2 ](H2 O)

ChainsFormula89No.

9.1381(3)/90

P−1

61 56 46 36
81 52 38

9.405(2)/93.01(3)10.913(3)/90

P21 /nP21 /m

61 53 46 35

61 52 42 32

5.408(2)/90

P21 /m6 1 3217.014(2)/90
7.5213(9)/90

7.3067(6)/90

C2/m
P21 /c11.9911(7)/90

P21 /c5.8555(6)/90

P21 /nPmc21

7.193(5)/90
8.348(2)/90
8.4164(4)/90
8.4167(5)/90
13.8529(7)/90
8.4966(2)/90
8.364(3)/90
8.673(2)/90
8.8032(5)/90
8.668(2)/90

7.0545(5)/106.995(1)9.543(6)/66.69(2)5.5502(6)/104.055(2)5.6722(4)/104.698(1)6.354(1)/90
9.924(5)/90a, Å/α, ◦P21 /cP21 /nP21 /nP21 /nP21 /cP21 /nP21 /nP21 /nP21 /nP21 /n

P−1
P−1

P−1
P−1

A2/a
C2/c

Sp. Gr.

cc2–1:2–21

cc2–1:2–19

cc2–1:2–5

cc2–1:2–4

cc1–1:2–15

cc1–1:2–14

cc1–1:2–1

Topology

11.574(2)/93.66(3)15.0098(5)/91.171(1)12.427(3)/90.393(3)7.0637(7)/100.544(2)7.0071(8)/98.834(2)9.278(1)/93.45(10)8.1239(7)/100.375(2)5.7814(3)/90

6.5051(7)/96.796(2)10.368(5)/91.470(5)10.326(2)/97.06(2)10.1435(5)/97.556(1)10.2581(6)/96.825(1)10.6153(6)/101.094(1)10.3910(3)/93.693(1)10.346(4)/97.269(8)10.452(3)/105.147(4)10.4610(7)/105.054(2)10.655(2)/97.88(2)7.4656(5)/108.028(1)9.602(7)/84.10(2)6.6415(7)/93.342(2)6.7627(5)/93.708(1)12.578(2)/82.35(1)12.546(5)/98.090(5)b, Å/β, ◦9.972(2)/90
6.324(5)/90c, Å/γ, ◦6.698(2)/109.69(1)15.1732(5)/90

18.448(4)/90

7.1084(7)/90
17.328(2)/90

4.254(1)/90

13.651(1)/90

11.2525(6)/90

21.164(2)/90

13.823(5)/90
9.929(2)/90
9.6913(5)/90
9.8542(5)/90
12.5921(7)/90
10.2087(3)/90
9.834(4)/90
13.235(4)/90
13.1312(7)/90
9.846(2)/90

10.0484(6)/98.875(1)11.742(8)/63.69(1)11.013(1)/110.589(2)11.2622(8)/109.489(1)Table 2. Crystallographic characteristics of synthetic uranyl selenites and selenite-selenates.

[47]

[46]

[46]

[38]
[46]

[33]

[37]

[41]

[41]

[36]
[40]
[41]
[41]
[41,42]
[42]
[41]
[43]
[44]
[45]

[37]
[39]

[37]
[38]

[35]
[36]

Reference

Crystals 2019, 9, 639

cc2–2:3–4[C4 H15 N3 ][H3 O]0.5 [(UO2 )2(SeO4 )2.93 (SeO3 )0.07 (H2 O)](NO3 )0.5

[C5 H14 N]4 [(UO2 )3 (SeO4 )4(HSeO3 )(H2 O)](H2 SeO3 )(HSeO4 )[C2 H8 N]3 (C2 H7 N)[(UO2 )3(SeO4 )4 (HSeO3 )(H2 O)][C2 H8 N]2 [H3 O][(UO2 )3(SeO4 )4 (HSeO3 )(H2 O)](H2 SeO3 )0.2

[C8 H15 N2 ]2 [(UO2 )4 (SeO3 )5 ]

42

43

44

12

46

45
61 53 46 35

cc2–3:5–3

P21 /ncc2–1:2–14[C2 H8 N][(H5 O2 )(H2 O)][(UO2 )2 (SeO4 )3 (H2 SeO3 )](H2 O)

41

18.860(2)/90

8.3116(4)/90

P21 /mPnma

11.659(1)/90

11.707(1)/73.90(1)11.1679(4)/90

Pnma

P−1

P21 /c14.798(1)/90

11.055(2)/90

18.010(2)/90

18.636(1)/97.582(1)14.956(2)/90

14.817(1)/76.22(1)10.9040(4)/98.019(1)10.024(1)/111.628(1)10.645(2)/108.05(2)12.264(2)/95.23(3)13.399(5)/108.230(4)13.885(1)/103.212(2)13.170(3)/90
11.553(2)/90
8.475(3)/90
8.993(3)/90
10.748(1)/109.960(1)Pbca
P21 /nP21 /nP21 /nP−1

cc2–1:2–4

36

37
38
39
40

8.6903(4)/90.3040(8)13.086(1)/90
10.9378(5)/90

17.555(1)/90

6.950(1)/97.618(3)b, Å/β, ◦P21 /c21.888(3)/90a, Å/α, ◦

Pnma

34

35

cc1–1:2–12
C2/m

Sp. Gr.

[C4 H12 N][(UO2 )(SeO3 )(NO3 )]Topology[C6 H14 N2 ]0.5 [(UO2 )(HSeO3 )(SeO3 )](H2 O)0.5 (CH3 CO2 H)0.5[C4 H12 N2 ]0.5 [(UO2 )(HSeO3 )(SeO3 )][(C2 H8 N2 )H2 ][(UO2 )(SeO3 )(HSeO3 )](NO3 )(H2 O)0.5[C5 H14 N][(UO2 )(SeO4 )(HSeO3 )][C2 H8 N][(UO2 )(SeO4 )(HSeO3 )][C5 H6 N][(UO2 )(SeO4 )(HSeO3 )][C9 H24 N2 ][(UO2 )(SeO4 )(HSeO3 )](NO3 )Organically templated

Formula33

No.

Table 2. Cont.

11.140(1)/90

11.5623(5)/90

22.194(2)/90

16.977(2)/89.36(1)17.991(1)/90

16.418(1)/90

12.138(2)/90
10.404(3)/90
10.640(4)/90
14.636(1)/90.409(1)18.009(4)/90

9.9913(5)/90

10.5984(7)/90

8.350(1)/90c, Å/γ, ◦

[58]

[56]

[56]

[57]

[56]

[55]

[51]
[52]
[53]
[54]

[50]

[49]

[49]

[48]

Reference

Crystals 2019, 9, 639

Crystals 2019, 9, 639

Most of the inorganic uranyl selenites were obtained during low or medium temperaturehydrothermal experiments in the temperature range of 100 to 220 ◦ C using Teﬂon-lined steel autoclaves.
Various reagents were used as the source of uranium (U(VI) oxide, uranyl hydroxide, uranyl nitratehexahydrate, uranyl acetate dihydrate), whereas selenous acid (H2 SeO3 ) was the only source of Se(IV).
To be precise, either acid itself or SeO2 were used in the reactions, but Se(IV) dioxide reacts with water
to form selenous acid. H2 SeO3 is a very weak acid and it hardly dissociates at room temperature, which
explains the required heating for the reaction. Several compounds obtained in diﬀerent ways shouldbe mentioned separately. Compounds 22 [44] and 23 [45] were obtained during evaporation at room
temperature. The ﬁrst compound was obtained from the reaction of UO2 (NO3 )2 ·6H2 O with selenicacid (H2 SeO4 ) in aqueous medium for 1 year, which could be explained by the reduction of Se(VI)to Se(IV) in the solution during the experiment. Moreover, as it was recently shown, the hydronium
ions usually enter the structure at the very latest crystallization stages, when there are no more othercations in the solution [52,59,60]. The Na-bearing compound was obtained in the presence of sodium
oxalate, which probably could be regarded as a catalyst of the uranyl selenite crystallization process.
Another ﬁve compounds, 11 [38], 21 [43], 27 [33], 28 [38], and 32 [47], were obtained in the gas–solid orhydrothermal reactions using sealed tubes. In the case of the last three compounds, the temperaturereached over 425 ◦ C.
The majority of the organically templated compounds are actually uranyl selenites-selenates.
The selenite anions that are arranged in the structures of such compounds are in minor amounts withrespect to the selenate groups. Such a tendency comes from the experimental conditions, in which thesource of Se was the selenic acid. Selenic acid is less stable in environmental conditions than selenous
acid, and it reduces to the latter during storage. Initially, pure H2 SeO4 reagent after a few months ofstorage contains signiﬁcant amount of [SeO3 ]2− and [HSeO3 ]− ions, which participate in the structureformation along with [SeO4 ]2− groups. There are only ﬁve structures of organically templated uranyl
selenites known without [SeO4 ]2− oxyanions (Table 2), three of which (34 [49], 35 [49], and 46 [58]) wereobtained during mild hydrothermal experiments (130–150 ◦ C) when the source of Se was again selenic
acid. Here, the temperature and amine molecules or ionic liquids [58] acted as a reduction agent for
H2 SeO4 , since it is known that the selenic acid is easily reduced to H2 Se4+ O3 and oxygen upon heating
above 160 ◦ C [61]. The other two organically templated uranyl selenites (33 [48] and 36 [50]) wereobtained during evaporation at room temperature from the aqueous solution of UO2 (NO3 )2 ·6H2 O,
SeO2 , and respective amine. Since SeO2 transforms to weak selenous acid in water, the low dissociation
ability of the latter [62] and the presence of [NO3 ]− groups in the structures of both compounds explains
the long crystallization process of 1 to 2 months. It is likely that dissociation of uranyl nitrate and thepresence of amine ﬁnally helped to create an environment suﬃcient for the selenous acid dissociation,and thus to start the crystallization of uranyl selenites. Nitrate groups, in these cases, act as additional
oxyanions involved in structure formation with a shortage of [SeO3 ]2− groups.
3.3. Topological Analysis
The vast majority of the uranyl selenite crystal structures are based on the layered complexes ofvarious topologies (Tables 1 and 2), and only nine compounds have chain-based crystal structures.
However, among those nine compounds, two are uranyl selenite minerals.
Crystal structures of derriksite and another two synthetic compounds (Tables 1 and 2, Figure 1a–c),which are actually the same but were reﬁned in diﬀerent space groups, are based on the 1D unitsof the cc1–1:2–1 topological type (graph is an inﬁnite chain of four-membered vertex-sharing rings).
The graph corresponds to the type of chains, which were observed in the kröhnkite [63]. This topology
is one of the most common and simplest chain topologies among U(VI)-bearing compounds with the[TOm ]n− groups (m = 3,4; T = S, Se, P, As). It was observed in the structures of uranyl-sulfate minerals
as svornostite, K2 Mg[(UO2 )(SO4 )2 ]2 (H2 O)8 [64], rietveldite, Fe(UO2 )(SO4 )2 (H2 O)5 [65], and their
Mg-bearing synthetic analogues Mg[(UO2 )(TO4 )2 (H2 O)](H2 O)4 (T = S, Se) [66]. Although the topology
of chains is the same, their structures are remarkably diﬀerent, representing two diﬀerent isomers.

13

Crystals 2019, 9, 639

In the case of derriksite (Figure 1c), U6+ atoms present in the tetragonal bipyramidal coordination,
where all four equatorial O atoms are shared with the [SeO3 ]2− groups, and each selenite group in
turn has only two O atoms shared with two neighbors’ Ur. Uranyl selenite chains in the structureof derriksite are directed along [001] and the equatorial planes of uranyl bipyramids are arranged
parallel to the (101). In between the chains, Cu-centered tri-octahedral layers are observed as being
arranged parallel to (010), in which each Cu atom has four OH− groups shared with the neighbor
Cu atoms and two more vertices in the trans-orientation are the third vertices of selenite pyramids,
non-shared with U-centered bipyramids. Selenite groups are arranged in such a way that lone electron
pairs from one side of the U-Se chain are directed in one way, and from the other side, in the opposite
direction (up or down), relative to the equatorial planes of uranyl bipyramids. Thus, the sequenceof orientation symbols could be written as (u)(d). The latter has been termed an orientation matrix.
In the structures of synthetic [(UO2 )(HSeO3 )2 (H2 O)] [35,36] compounds, U6+ atoms are arranged in
the center of pentagonal bipyramids, in which four equatorial O atoms are shared with the [HSeO3 ]−groups and the ﬁfth vertex is occupied by the H2 O molecule. Hydrogen selenite groups also have
two O atoms shared with two neighboring Ur and the third vertex is attributed to the OH− group.
The linkage of chains into the 3D structure is carried out by the means of H-bonding between theneighbor chains only. The arrangement of lone electron pairs relative to the equatorial planes of uranyl
bipyramids is staggered on both sides of the chain, so the orientation matrix for the current geometricalisomer is (ud)(du).

Figure 1. (a–k) 1D complexes in the crystal structures of natural and synthetic uranyl selenites (a–k:
see text for details). Legend: U-bearing coordination polyhedra = yellow; Se atoms = orange; O atoms= red; H atoms = white; N atoms = light blue; black nodes = U atoms, white nodes = Se atoms. SeIV O3
trigonal pyramids and NO3 groups are shown in a ball-and-stick mode.

The crystal structure of demesmaekerite is based on the chains of the cc1–1:3–2 topology(Figure 1d,e), which is very similar to the previous type. The graph is a vertex-sharing inﬁnite chain of14

Crystals 2019, 9, 639

four-membered rings with additional one-connected selenite group to each Ur. This topology is quiterare and has been observed in the structures of two synthetic uranyl chromates Na4 [(UO2 )(CrO4 )3 ] [67]and K5 [(UO2 )(CrO4 )3 ](NO3 )(H2 O)3 [68], two uranyl molybdates Na3 Tl5 [(UO2 )(MoO4 )3 ]2 (H2 O)3 andNa13 Tl3 [(UO2 )(MoO4 )3 ]4 (H2 O)5 [69], and one uranyl selenate (C2 H8 N)3 [(UO2 )(SeO4 )2 (HSeO4 )] [52].
U6+ atoms are arranged in the centers of pentagonal bipyramids, so that four equatorial verticesof which are shared with two-connected selenite groups (as in previous type), and the ﬁfth vertex
that was occupied by H2 O molecule, now is replaced by another one-connected [SeO3 ]2− pyramid.
Uranyl selenite chains are passing along the (101), and stacked one above the other, forming blocks
parallel to (010). These blocks are separated by the sheets of edge-shared Cu- and Pb-centeredcoordination polyhedra. There are three types of Cu2+ -centered octahedra in the structure ofdemesmaekerite, [CuO4 (OH)2 ]8− , [CuO3 (OH)3 ]7− , and [CuO2 (OH)3 (H2 O)]6− , and the single type ofninefold [Pb2+ O6 (OH)3 ]13− complexes. Lone electron pairs of one- and two-connected selenite groups
from one side of the U-Se chain are oriented in the same direction, while on the other side the directionis the opposite, thus the orientation matrix could be written as (u)(d).
The crystal structure of the organically templated compound 33 [48] is based on the uranyl selenite
nitrate 1D complexes that belong to the cc1–1:2–12 topological type (Figure 1f,g). This topology has
been observed in the structures of several uranyl and neptunyl sulfates and selenates, for example,see [70–73], and represents an inﬁnite chain of edge-shared four-memebered cycles, in which eachuranyl polyhedron has three equatorial vertices shared with three selenite groups while the left pair of
O atoms is edge-shared with the [NO3 ]− group. Being three-connected to the neighbor Ur, [SeO3 ]2−pyramids have a lone electron pair oriented either up or down relative to the equatorial planes of uranyl
bipyramids in the (ud)∞ sequence.
The crystal structures of Ca- [37] and Sr-bearing [38] isotypic uranyl selenites are based on 1D
complexes of the cc1–1:2–14 topological type (Figure 1h,i), which are built by the dimers of edge-sharing
uranyl pentagonal bipyramids that are interlinked by the pair of edge- and vertex-sharing selenitegroups with another one-connected selenite group decorating the ﬁfth non-shared equatorial vertices ofU polyhedra from both sides of such a double-wide chain. It should be noted that [SeO3 ]2− pyramids,
which are involved in the linkage of U dimers, have lone electron pairs oriented up from one sideof the chain, and down from the other side, thus illustrating the (ud)∞ sequence. This type of chainsoccurs in the structures of two uranyl minerals: Parsonite, Pb2 [(UO2 )(PO4 )2 , [74] and hallmondite,Pb2 [(UO2 )(AsO4 )2 ](H2 O)n , [75].
The crystal structures of two more Sr- [37] and Na-hydronium-bearing [39] compounds are basedon the uranyl selenite chains with an edge-sharing motif, similar to the previous one. Chains belong
to the cc1–1:2–15 topological type (Figure 1j,k), and are built by the dimers of edge-sharing uranyl
pentagonal bipyramids, which, in contrary to the aforementioned topology, are interlinked by a pairof only vertex-sharing selenite groups, while edge-sharing selenite pyramids in this case decorate
both sides of the chain. Both compounds represent two diﬀerent geometrical isomers, assuming theorientation of lone electron pairs. Thus, Sr uranyl selenite possesses the same (ud)∞ sequence, as in aprevious case, while the Na-bearing compound has a (u)∞ sequence. This type of topology has been
observed in several synthetic uranyl chromates, phosphates, and arsenates, as well as in lakebogaite,
CaNa(Fe3+ )2 [(H(UO2 )2 (PO4 )4 (OH)2 ](H2 O)8 [76].
The crystal structures of 17 synthetic uranyl selenites are based on the layers, which belong to
the cc2–1:2–4 topological type (Figure 2a,b), the most common among the uranyl selenite compounds
and among the layered uranyl compounds, generally. The topology consists of dense four-memberedcycles and large hollow eight-membered rings. It is worth noting, that almost all sheets of this
topology contain protonated [HSeO3 ]− groups with the H-bonds arranged inside the eight-memberedcycles. Although the topology of the sheets remains the same, their real architecture is quite diverse,
which occurs due to various blocks involved in the structure formation. Thus, the structures ofthese compounds are formed via combination of the [UO7 ]8− , [HSeO3 ]− , [SeO3 ]2− , and [SeO4 ]2−
coordination polyhedra through common oxygen atoms. Uranyl pentagonal bipyramids share all15

Crystals 2019, 9, 639

of ﬁve equatorial O atoms with the selenite or selenate groups, while Se-bearing oxyanions act as
two- or three-connected units. Such a diversity of building blocks opens up the possibility of a large
number of geometric isomers’ existence. Within the uranyl selenite and selenite-selenate compoundsof cc2–1:2–4 topology, three isomers are distinguished: Layers, containing only selenite groups; those,having selenite and hydrogen selenite groups; and those with hydrogen selenite groups and selenate
tetrahedra. However, what is the most interesting, is that all three isomers have a similar orientationof lone electron pairs and fourth non-shared vertices (for tetrahedra), which is described by the very
simple (ud) matrix. Only except for the compound 36, which has the (ud)(du) matrix.

Figure 2. (a–j) 2D complexes based on corner-sharing linkage in the crystal structures of synthetic
uranyl selenites and selenite-selenates (a–j: see text for details). Legend: see Figure 1; SeVI O4
groups = orange tetrahedra.

The crystal structures of Ag-bearing uranyl selenite [41] is based on the layered complex of
cc2–1:2–5 topological type (Figure 2c,d). This type of topology has been observed in the structures ofseveral synthetic uranyl and neptunyl molybdates as Na2 (UO2 )(MoO4 )2 [77] and K3 NpO2 (MoO4 )2 [78].
Topological types cc2–1:2–4 and cc2–1:2–5 have nearly identical chemical composition and looks
quite similar. Those graphs are built from the similar four- and eight-membered rings, and evenhave the same connectivity of black and white vertices (U and Se polyhedra, respectively), but the
topologies are diﬀerent due to diﬀerences in coordination sequence [18]. Such chemically identical, but
topologically diﬀerent structural units are called topological or structural isomers. It should be notedthat the cc2–1:2–4 topology is much more representative among the inorganic oxysalt compounds than16

Crystals 2019, 9, 639

cc2–1:2–5. If the lone electron pair of the selenite pyramid would be equated to the fourth non-sharedvertex of the selenate tetrahedron, the current isomer can be described by the (uddu)(dduu) matrix.
The crystal structure of 41 [55] is based on the 2D complexes, possessing unprecedented topology
for both the structural chemistry of uranium and the chemistry of inorganic oxysalts in general, of
the cc2–1:2–14 type (Figure 2e,f). U atoms are arranged in the centers of pentagonal bipyramids.
Each [SeO4 ]2− group is three-connected, coordinating three uranyl ions, whereas protonated selenitegroups coordinate one uranyl ion each. The topology is remarkable due to the presence of one-connectedbranches inside eight-membered cycles, which are actually selenous acid groups.
The crystal structure of 25 [41] is based on the layered complexes of cc2–1:2–19 topologicaltype (Figure 3a,b), which is a derivative of the autunite topology [18], where each uranyl
pentagonal bipyramid has only one edge shared with the selenite group. The graph of thelayer consists of eight-membered rings only. The current isomer can be described by the(uudd)(uddu)(dduu)(duud) matrix.

Figure 3. (a–n) 2D complexes based on edge-sharing linkage in the crystal structures of natural andsynthetic uranyl selenites and selenite-selenates (a–n: see text for details). Legend: see Figures 1 and 2.

Compound 26 [37] is the only known uranyl selenite, which crystal structure is based on thelayered complexes of cc2–1:2–21 topological type (Figure 3c,d). The graph of the U-bearing sheet
17

Crystals 2019, 9, 639

consists of dense 4-membered and large 12-membered rings. Double links between the black and whitevertices in a graph indicate sharing of an edge between uranyl coordination polyhedra and the selenite
pyramid. Despite the fact that [SeO3 ]2− groups are two-connected, edge-sharing coordination generates
a possibility for an orientational isomerism of the lone electron pair arrangement. Current isomercan be described by the (ud) matrix. It is of interest that interlayer Ba2+ cations are actually arranged
within the layer, inside the 12-membered rings.
Next compound, 42 [56], got into the review with a large tolerance. There are three nonequivalent
positions of Se in the structure, only one of which was occupied by both Se(VI) and Se(IV), and theamount of the latter is very small (~0.07 per formula unit). The current topology of the cc2–2:3–4 type
(Figure 2g,h) is one of the most common among synthetic uranyl sulfates, chromates, and selenates(>30 structures are known), but it has not been observed for any compound with a higher content ofselenite ions than here.
The crystal structures of three organically templated compounds, 43 [57], 44, and 45 [56],are based upon the layers with U:Se = 3:5 formed as a result of condensation of the [UO2 ]2+ ,[UO2 (H2 O)]2+ , [SeVI O4 ]2− , and [HSeIV O3 ]− coordination polyhedra by sharing common oxygen
atoms. The corresponding graph of cc2–3:5–3 topology is built by four- and six-membered rings(Figure 2i,j). This topology of inorganic complexes is typical for uranyl selenite-selenates but has also
been observed in some pure uranyl selenates, for instance, Rb4 [(UO2 )3 (SeO4 )5 (H2 O)] [79]. The presenceof two-connected selenite trigonal pyramids and three-connected selenate tetrahedra gives rise togeometric isomerism. Thus, the orientation matrices can be written as (ududud)(uddu) for the ﬁrst
and second, and (duuudd)(uddu) for the third compound, respectively.
The simplest uranyl selenite, at least from the chemical point of view, [(UO2 )(SeO3 )] [33], has alayered structure (Figure 3e,f). According to Lussier et al. [80], the anionic topology of the layer ofthis compound belongs to the topology consisting of triangles and hexagons. The topology of thelayer in this compound is the same as in mineral rutherfordine, [(UO2 )(CO3 )] [81,82], which is why
it is called a rutherfordine anion topology. This topology consists of parallel chains of edge-sharing
hexagons divided by dimers of edge-sharing triangles. Each of the hexagons is occupied by Ur,
and one triangle per dimer is occupied by the [SeO3 ]2− group. The other half of the triangles is vacant.
Electroneutral sheets are linked together by van der Waals interactions only. It should be noted thatrecently, an isotypic neptunyl compound has been reported [34].
One of the most remarkable topological types within the uranyl selenite family of compounds is thephosphuranylite topology (Figure 3g,h): The crystal structures of marthozite, guilleminite, and larisaite
are based on such layers, while haynesite and piretite (although their structures are still unknown) aresupposed to have topologically the same architecture due to the similarity of their unit-cell parameters.
Except for minerals, two more Li- and Sr-bearing synthetic uranyl selenites have structures based on
the 2D units belonging to the phosphuranilite anion topology. The phosphuranilite topology contains
two types of alternating inﬁnite chains: Edge-sharing dimers of pentagons that are further linked
by edge-sharing hexagons, and zig-zag chains of edge-sharing triangles and squares [80,83]. The
topology can be described by the 61 52 42 32 ring symbol with pentagons and hexagons occupied by Ur,
triangles are occupied by selenite anions, while squares stay vacant. In the crystal structures of naturaland synthetic compounds, additional mono-, divalent cations, and H2 O molecules are arranged in
between the layers forming covalent and H-bonding systems to build the 3D structure. In the structureof marthozite, there are Cu2+ cations arranged in between the layers and octahedrally coordinatedby two O atoms of uranyl ions from the above and underlying layers and four O atoms of H2 O
molecules from the interlayer space. There are also four ‘zeolite’-like H2 O molecules arranged in theinterlayer space, which are not covalently bonded to cations and held in the structure by H-bonds only.
Na+ and K+ sites in the structure of larisaite are characterized by partial occupancies, as well as H2 O
molecules and hydronium cations, which are statistically distributed over six sites within the interlayer
space. Thus, there are also two types of H2 O molecules, those which coordinate alkali cations and‘zeolite’-like, as in the structure of marthozite. Na+ and K+ cations in the crystal structure of larisaite

18

Crystals 2019, 9, 639

alternately occupy neighbor cavities in the interlayer space, while in the structure of guilleminite, thosecavities are equivalent and occupied only by Ba2+ cations. It is of interest that according to previous
works [1,27], only two sites of H2 O molecules coordinating Ba2+ cations have been determined in the
structure of guilleminite, leaving rather a large cavity to be vacant. Our single crystal XRD studies at
low temperatures allowed us to determine the third site arranged within the void and occupied by the
H2 O molecule, which suggests a change to the formula of guilleminite to Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 .
Such ambiguity allows reference to the variable character of H2 O molecules’ amount within thesestructures, which could depend on the chemical composition and conditions, and the temperatureand humidity storage of samples. Another interesting feature is that the structures of natural andsynthetic compounds belong to diﬀerent geometrical isomers. The (ud)(du) isomer was determined in
the structures of Li- and Sr-bearing synthetic uranyl selenites, while (ud)(ud) isomer was observedin the crystal structures of all three minerals. It should be noted that implementation of the (ud)(du)isomer results in formation of stepped layers, in which each subsequent chain of edge-sharing uranyl
polyhedra is located above the level of the previous chain, whereas the (ud)(ud) isomer results in theformation of zig-zag uranyl selenite layers, in which the chains of edge-sharing uranyl polyhedra arealternately located above or below the mean plane of the layer (Figure 4).

Figure 4.
(a–d) The crystal structure projections along the layers, uranyl selenite layers,
symmetry elements, and the respective layer symmetry groups for guilleminite (a,b) andSr[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 (c,d). Legend: see Figure 1.

Another topology that consists of hexagons, pentagons, squares, and triangles can be describedby the 61 53 46 35 ring symbol (Figure 3i,j), and is quite rare. There are only three compounds known,
whose structures are based on the layers of this type. Two of them are Cs-bearing [46] and organicallytemplated [58] uranyl selenites, and the third one is a very exotic Cs2 [(UO2 )4 (Co(H2 O)2 )2 (HPO4 )(PO4 )]uranyl phosphate compound [84]. Layers are formed by the speciﬁc heptamers, and the uranylhexagonal bipyramid is in the center, sharing each even equatorial edge with three uranyl pentagonal
bipyramids, while the odd edges are shared with [SeO3 ]2− groups. The linkage of these heptamers
occurs via the third non-shared vertex of the selenite group and by the two additional selenitegroups of each pentagonal bipyramid, which share all three O atoms with three neighbor heptamers.
Thus, all pentagons and hexagons in the anion topology are occupied by the uranyl ions, triangles, andby the selenite groups while squares are vacant. It should be noted, that the arrangement of the loneelectron pair in the structures of both uranyl selenites is diﬀerent. In the structure of Cs-bearing uranyl
19

Crystals 2019, 9, 639

selenite, the orientation of the lone electron pairs around the core of uranyl bipyramids is uneven andcan be described by the (uuudduuuuudd) matrix, while that in the structure of organically templated
uranyl selenite is uniform (uududuuddudd), but it does not result in any visible diﬀerences in the
distortion or undulations between the layers.
The crystal structure of the Cs-bearing uranyl selenite-selenate 31 [46] phase is based on the layersof a highly remarkable anion topology with the 61 56 46 36 ring symbol (Figure 3k,l), which could beassumed as the modular structure, composed of blocks from both the phosphuranylite and zippeite
anion topologies. The latter, for instance, is one of the most common topologies among the naturaluranyl sulfates [25]. The zippeite fragment of the topology includes selenate tetrahedra, and thephosphuranylite fragment contains selenite groups.
The crystal structure of the only uranyl diselenite compound [47] is based on the sheets of
miscellaneous anion topology of the 81 52 38 type (Figure 3m,n), consisting of octagons, pentagons,
and triangles. The layered complex is built by the dimers of edge-sharing uranyl pentagonal
bipyramids, which are arranged similarly as in the structures of such minerals as deliensite,
Fe[(UO2 )2 (SO4 )2 (OH)2 ](H2 O)7 [85] or plášilite, Na(UO2 )(SO4 )(OH)(H2 O)2 [86], but the linkage
character is remarkably diﬀerent. Instead of isolated groups, uranyl dimers are interlinked lengthand side-ways through the vertex-sharing diselenite groups; besides, lone electron pairs within
the [Se2 O5 ]2− oxyanions are co-directed. In those diselenite groups, which are arranged along theextension of the uranyl dimers, lone electron pairs are oriented towards one side relative to the planeof the sheet, and in those groups, arranged side-ways, the direction of the lone electron pair is the
opposite. The crystal structure of 32 is anhydrous and free of additional ions, thus electroneutral
layered complexes are linked into the 3D structure by the means of electrostatic interactions involving
lone electron pairs only.
3.4. Structural and Topological Complexity
Calculation was performed in several stages and the main results are summarized in Table 3 andFigures 5 and 6. First, the topological complexity (Tl), according to the maximal rod (for chains) orlayer symmetry group, was calculated, since these are the basic structural units. Second, the structuralcomplexity (Sl) of the units was analyzed taking into account its real symmetry. The next contribution
to information comes from the stacking (LS) of chained and layered complexes (if more than one
layer or chain is in the unit cell). The fourth contribution to the total structural complexity is given by
the interstitial structure (IS). The last portion of information comes from the interstitial H bonding
system (H). It should be noted that the H atoms related to the U-bearing chains and layers wereconsidered as a part of those complexes but not within the contribution of the H-bonding system.
Complexity parameters for the whole structures were calculated using ToposPro package [87].

20

P−1

[(UO2 )(HSeO3 )2 (H2 O)][(UO2 )(HSeO3 )2 ](H2 O)

Pb2 Cu5 [(UO2 )2 (SeO3 )6 (OH)6 ](H2 O)2 /demesmaekerite[C4 H12 N][(UO2 )(SeO3 )(NO3 )]Ca[(UO2 )(SeO3 )2 ]Sr[(UO2 )(SeO3 )2 ]Sr[(UO2 )(SeO3 )2 ](H2 O)2Na3 [H3 O][(UO2 )(SeO3 )2 ]2 (H2 O)

8
9

2

33

10
11

12
13

21

36

35

34

[NH4 ][(UO2 )(SeO3 )(HSeO3 )]K[(UO2 )(HSeO3 )(SeO3 )]Rb[(UO2 )(HSeO3 )(SeO3 )]Cs[(UO2 )(HSeO3 )(SeO3 )]Cs[((U,Np)O2 )(HSeO3 )(SeO3 )]Tl[(UO2 )(HSeO3 )(SeO3 )]Cs[(UO2 )(SeO3 )(HSeO3 )](H2 O)3
Na[(UO2 )(SeO3 )(HSeO3 )](H2 O)4[C6 H14 N2 ]0.5 [(UO2 )(HSeO3 )(SeO3 )](H2 O)0.5 (CH3 CO2 H)0.5[C4 H12 N2 ]0.5 [(UO2 )(HSeO3 )(SeO3 )][(C2 H8 N2 )H2 ][(UO2 )(SeO3 )(HSeO3 )](NO3 )(H2 O)0.5

15
16
17
18
19
20
21
22

Layers with corner-linkage[NH4 ]2 [(UO2 )(SeO3 )2 ](H2 O)0.5

14

cc2–1:2–4

cc1–1:2–15

cc1–1:2–14

cc1–1:2–12

cc1–1:3–2

A2/a
C2/c

cc1–1:2–1

ChainsCu4 [(UO2 )(SeO3 )2 ](OH)6 /derriksite

1

264

84

P21 /c
Pbca

4.991

228

Pnma

5.044

4.392

4.087
3.700
3.700
4.700
3.700
3.700
4.459
4.644

94
68
52
52
104
52
52
88
100

4.576

4.170
5.000

3.585

4.236

4.800

3.125

4.236

IG

P21 /c

36
64

24

56

55

32

54

ν

P21 /nP21 /nP21 /nP21 /cP21 /nP21 /nP21 /nP21 /n

P−1

P−1

C2/m

Pn21 m

Sp. Gr.

TopologyNo.

Formula1331.720

368.955

1137.899

277.947
192.423
192.423488.846
192.423
192.423392.430
464.386430.131150.117320.000

86.039237.212263.975

100.000

228.764

IG,totalComplexity Parameters of the
Crystal Structure48
48

p21 /b48

p21 /bp21 /b

48
48
48
48
48
48
48
48

44

22

p21 /bp21 /bp21 /bp21
p21 /bp21 /bp21 /bp21 /bp21 /b

þ-1

22

22

þa 21 /m11
þ-1

30

32

11

ν

þ-1

þa 12/a1

þc m11

Layer or Rod
Sym. Gr.

3.585

3.585

3.585

3.585
3.585
3.585
4.585
3.585
3.585
3.585
3.585

3.459

3.459

3.459

3.096

3.907

3.125

3.096

IG

172.080172.080172.080172.080172.080172.080172.080172.080172.080172.080172.080

152.196

76.10876.10868.108117.207100.000

34.054IG,total

Structural Complexity of the U-Se Unit

p21 /bþ-1

þ-1

þa 21 /m11

þa 21 /m11

þa 12/a1þa 2/m11

Layer or Rod
Sym. Gr.

48

44

22

22

22

30

32

11

ν

3.585

3.459

3.459

3.459

3.096

3.374

3.125

2.187

IG

172.080

152.196

76.10876.10868.108101.207100.000

24.054IG,total

Topological Complexity of the U-Se
Unit

Table 3. Structural and topological complexity parameters for the uranyl selenite and selenite-selenate compounds.

Crystals 2019, 9, 639

22[C4 H15 N3 ][H3 O]0.5 [(UO2 )2(SeO4 )2.93 (SeO3 )0.07 (H2 O)](NO3 )0.5

[C5 H14 N]4 [(UO2 )3 (SeO4 )4(HSeO3 )(H2 O)](H2 SeO3 )(HSeO4 )[C2 H8 N]3 (C2 H7 N)[(UO2 )3 (SeO4 )4 (HSeO3 )(H2 O)][C2 H8 N]2 [H3 O][(UO2 )3(SeO4 )4 (HSeO3 )(H2 O)](H2 SeO3 )0.2

41

42

43

Sr[(UO2 )3 (SeO3 )2 O2 ](H2 O)4
Li2 [(UO2 )3 (SeO3 )2 O2 ](H2 O)6

28
29

5
C2/m
P21 /c

P11m

Pmn21

3.450
4.311

5.395

73
28
78

4.386

5.459

2.236

3.585

3.835

5.200

5.629

7.011

5.728

5.672

3.700

4.087
5.044
4.585
4.644
6.700

IG

70

176

4

Pbn21
61 52 42 32

Cu[(UO2 )3 (SeO3 )2 O2 ](H2 O)8 /marthozite
Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 /guilleminite
Na(H3 O)[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 /larisaite

3

14

P21 /m

61 32[(UO2 )(SeO3 )]

27

48
48

134

P21 /m

P21 /ccc2–1:2–19

364

258

212

Pnma

P−1

P21 /c

Pmc21

Ba[(UO2 )(SeO3 )2 ]26

52

P21 /n
204

68
132
96
100
208

P21 /nP21 /nP21 /nP21 /nP-1

P21 /nν

Sp. Gr.

96.606336.261393.857

307.050960.860

31.303172.078

184.078696.8562048.837

1808.897

1214.319

1157.175

192.423277.947665.860
440.156464.3861393.691

IG,totalComplexity Parameters of the
Crystal Structure

cc2–1:2–21

Pb[(UO2 )(SeO3 )2 ]25

45

Layers with Edge-Linkage

cc2–2:3–4[C2 H8 N][(H5 O2 )(H2 O)][(UO2 )2 (SeO4 )3 (H2 SeO3 )](H2 O)

44

cc2–1:2–14

Ag2 [(UO2 )(SeO3 )2 ]24

cc2–3:5–3

cc2–1:2–5[H3 O][(UO2 )(SeO4 )(HSeO3 )][C5 H14 N][(UO2 )(SeO4 )(HSeO3 )][C2 H8 N][(UO2 )(SeO4 )(HSeO3 )][C5 H6 N][(UO2 )(SeO4 )(HSeO3 )][C9 H24 N2 ][(UO2 )(SeO4 )(HSeO3 )](NO3 )

23
37
38
39
40

Topology

FormulaNo.

Table 3. Cont.

c2/m
p21 /apm

p21 mnpn

p21 /mp21 /ap21 map21 /mp–1

p21

p21 /b

19
38

38

38

38

14

44

44

74

74

48

104

3.090
3.301

4.511

3.511

4.248

2.236

3.459

3.641

4.399

5.209

4.585

4.755

3.459

4.700

52
44

p–1
p21 /b

3.700

IG

52

ν

p21 /bLayer or Rod
Sym. Gr.

58.711125.421171.421133.421161.42131.303152.215160.215325.500

385.500

220.078513.528

152.215

244.423

192.423

IG,total

Structural Complexity of the U-Se Unit

c2/m

pmmn

p21 /mp21 /ap21 map21 /mp21

p21 /bp21 /bLayer or Rod
Sym. Gr.

19

38

14

44

44

74

48

104

44

52

ν

3.090

3.195

2.236

3.459

3.641

4.399

4.585

4.755

3.459

3.700

IG

58.711121.42131.303152.215160.215325.500

220.078513.528

152.215

192.423

IG,total

Topological Complexity of the U-Se
Unit

Crystals 2019, 9, 639

P21 /mP−1

61 56 46 36
81 52 38

Cs2 [(UO2 )7 (SeO4 )2(SeO3 )2 (OH)4 O2 ](H2 O)5UO2 Se2 O5

31

32

40

132

160
328

P21 /nPnma

61 53 46 35

Cs2 [(UO2 )4 (SeO3 )5 ](H2 O)2[C8 H15 N2 ]2 [(UO2 )4 (SeO3 )5 ]

30
46

ν

Sp. Gr.

4.322

5.226

5.322
5.455

IG

172.877

689.860

851.508
1789.277

IG,totalComplexity Parameters of the
Crystal Structure

FormulaNo.

Topology

Table 3. Cont.

p1

p–1

pn
p21 mnLayer or Rod
Sym. Gr.

20

49

64
64

ν

4.322

4.635

5.000
4.250

IG

86.439227.121320.000
272.000

IG,total

Structural Complexity of the U-Se Unit

p2

p–1

p21 mnLayer or Rod
Sym. Gr.

20

49

64

ν

3.422

4.635

4.250

IG

68.439227.121272.000

IG,total

Topological Complexity of the U-Se
Unit

Crystals 2019, 9, 639

23

Crystals 2019, 9, 639

Figure 5. Ladder diagrams showing contributions of various factors to structural complexity in termsof bits per unit cell for the structures based on chains (a), layers with edge-linkage of polyhedra (b),layers with corner-linkage of polyhedra (c) and organically templated compounds (d). Legend: TI =topological information; CI = cluster information (valid for Prw and Nsb: See text for details); SI =structural information; LS = layer stacking; IS = interstitial structure; HB = hydrogen bonding. See
Table 3 and text for details.

Figure 6. Ladder diagrams showing normalized contributions (in %) of various factors to structuralcomplexity for the structures based on chains (a), layers with edge-linkage of polyhedra (b), layerswith corner-linkage of polyhedra (c) and organically templated compounds (d). Legend: see Figure 5.
See Table 3 and text for details.

24Crystals 2019, 9, 639

4. Discussion
Structural features of natural uranyl selenites make one think about the conditions of theirformation in nature. Analogies with synthetic compounds, which have a similar structure, allow someof the most probable pathways to be suggested. The formation of structural units with edge-sharing
polyhedra in most cases indicates their hydrothermal origin, and the synthetic uranyl selenites 28 and 29,
whose structures are built upon the layers with a phosphuranylite topology (Figure 3g), are no exception.
Both compounds were obtained from the aqueous medium at temperatures above 220 ◦ C. In the caseof compounds with structures based upon 1D units, the situation is somewhat more complicated.
Topological type cc1–1:2–1, which is one of the most common among the U(VI)-bearing oxysalts, wasrepeatedly observed in the structures of compounds obtained at room temperature. However, synthetic
uranyl selenite 9 was grown at slightly higher temperatures of 80 ◦ C. Moreover, the presence ofrather speciﬁc uranyl tetragonal bipyramids in the structure of derriksite refers to a family of isotypic
uranyl phosphate [88], molybdate [89], and tellurite [90] compounds, which were obtained duringhydrothermal (above 180 ◦ C) or high temperature solid state (above 650 ◦ C) syntheses. Analogously, thecrystal structures of synthetic uranyl chromates [67,68] and molybdates [69], which are isotypic to that
one of demesmaekerite, were obtained at hydrothermal conditions (above 120 ◦ C) or solid state reactions(at 300 ◦ C). Nevertheless, based on laboratory [91,92] and ﬁeld observations, namely of the mineral
association from Zálesí (Czech Republic) [8], it is clear that demesmaekerite and piretite (and several
other unnamed or poorly identiﬁed U-Se phases) formed as a result of supergene alteration processes,
which exclude hydrothermal activity. These observations are supported by the radioanalytical datingof demesmaekerite.
The crystal structure of derriksite is built on the 1D uranyl selenite complexes, whose symmetry is
described by the þc m11 rod symmetry group. However, its highest (topological) symmetry is describedby the centrosymmetric þa 2/m11 rod group (Figure 7a). Stacking of chains doubles the complexity
contribution of the uranyl selenite block (68.107 bits/cell) into the whole structure, but is still less thanthe contribution of the Cu-O interstitial block (96.370 bits/cell) and nearly equal to the contributionof the interstitial H-bonding system (64.287 bits/cell; Figure 5 and 6). Alteration of uranyl tetragonal
bipyramids by pentagonal ones with the additional H2 O molecule in the equatorial plane of Ur
preserves the topology, but it doubles the size of the reduced segment of a chain and changes its
maximal symmetry to the þa 2/m11 rod group (Figure 7b). The absence of the interstitial substructure
makes the topological complexity parameters be equal to those for the whole structure of 8 and 9.

Figure 7. 1D uranyl selenite units and their highest rod symmetry groups for derriksite (a),[(UO2 )(HSeO3 )2 (H2 O)] (b) and demesmaekerite (c). Legend: see Figure 1.

25

Crystals 2019, 9, 639

The topological symmetry of the uranyl selenite chain in the structure of demesmaekerite is
monoclinic þa 21 /m11 and is higher than its real triclinic þ-1 symmetry (Figure 7c). In this case, the uranyl
selenite substructure (117.207 bits/cell) makes the largest contribution to the complexity of the whole
structure. The interstitial complex contributes a slightly lower amount of information (85.926 bits/cell),and even less is accounted for in the H-bonding system (60.842 bits/cell; Figure 5 and 6).
The crystal structures of 17 uranyl selenites and selenite-selenates are built upon the layersof cc1–1:2–4 topological type and those are distributed almost equally between pure inorganic andorganically templated compounds having various monovalent inorganic ions and protonated amine
molecules of diﬀerent shapes and sizes as an interstitial block. Moreover, this topology preserveschanges in the chemical composition of uranyl-bearing layers, which involves the occurrence ofuranyl selenites, selenite-hydrogen selenites, and hydrogen selenite-selenates. It is of interest that
all isomers within this family of compounds, including chemical substitutions and two geometricalisomers (see Chapter 3.3.), have the highest symmetry of the layer described by the p21 /b layergroup (Figure 8). Furthermore, the topological symmetry is preserved in the structures of almost allcompounds, except for two of them (Table 3). All three aforementioned cases point to the fact that thecurrent topological type is unusually resistant and one of the most preferable in the systems with the
U:T ratio = 1:2. As for the complexity calculations, certainly, those will primarily depend on the numberof orbits (atoms). Thus, the H-free uranyl selenite layer has the lowest amount of information (152.196
bits/cell), next in a row would be the uranyl selenite-hydrogen selenite complex (172.080 bits/cell), andﬁnally those containing selenate oxyanions (192.423 bits/cell). Analogously, complexity parameters forthe whole structure majorly depend on the size of the aliphatic part of organic molecules.

Figure 8. (a–d) Uranyl selenite layers, symmetry elements, and the respective layer symmetry groups
for various isomers of the cc1–1:2–4 topological type (a–d: see text for details). Legend: see Figures 1
and 2.

26

Crystals 2019, 9, 639

The crystal structures of three uranyl selenite minerals and two synthetic compounds are basedon dense layers with a phosphuranylite anion topology. It is of interest that natural and synthetic
compounds are described by the diﬀerent orientation matrices (Figure 4). The (ud)(du) orientation of
the lone electron pairs in the structures of synthetic uranyl selenites 28 and 29 resulted in the formationof layers with the c2/m topological symmetry (58.711 bits/cell), whereas the highest symmetry of thosein natural compounds is described by the (ud)(ud) matrix and orthorhombic pmmn layer symmetrygroup (121.421 bits/cell). It should be noted that only Sr-bearing synthetic compound 28 has the real
symmetry of the layer equal to the topological one. In the cases of marthozite, guilleminite, larisaite,and Li-bearing synthetic compound, topological symmetry is signiﬁcantly reduced by the interstitialcations and H2 O molecules (Table 3, Figures 4 and 5). The contribution of each of the components
makes the marthozite the most complex inorganic uranyl selenite (960.860 bits/cell; Figures 5 and 6).
It is of interest that the formation of a particular isomer causes the speciﬁc arrangement of the layers,
and it appears that the (ud)(ud) isomer of the posphuranylite topology, which results in the formationof zig-zag layers, is more stable and most likely thermodynamically preferable among the others, since
it has only been observed in the structures of natural layered uranyl selenites.
5. Conclusions
The reﬁnement of the demesmaekerite crystal structure makes it possible to determine the Hatoms positions belonging to the interstitial H2 O molecules. The reﬁnement of the guilleminite crystal
structure allows the determination of one additional site arranged within the void of the interlayer
space and occupied by the H2 O molecule, which suggests the formula of guilleminite to be written as
Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 instead of Ba[(UO2 )3 (SeO3 )2 O2 ](H2 O)3 .
Our numerous attempts to determine the crystal structure of haynesite, [(UO2 )3 (SeO3 )2 (OH)2 ](H2 O)5 ,were unsuccessful. However, several assumptions could be made. First, the presence of (OH)− groups
within such a dense layer is doubtful. At least, there is no other evidence for O atoms’ protonation in
the structures of all the other natural and synthetic uranyl selenites, whose structures are built uponlayers of the same topology. Moreover, the structure of haynesite has to be electroneutral, and H2 O
molecules from the interlayer space should be replaced by H3 O+ cations, which points to the similarity
and, probably, to the common genesis or even the closest relationships between the haynesite and larisaite,
Na(H3 O)[(UO2 )3 (SeO3 )2 O2 ](H2 O)4 .
Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl
selenite mineral formation requires heating, which most likely, keeping in mind their surface ornear-surface occurrence conditions, can be attributed to the radioactive decay.
Structural complexity studies revealed an interesting tendency in that the majority of synthetic
compounds have the topological symmetry of uranyl selenite building blocks equal to the structuralsymmetry, which means that the highest symmetry of uranyl complexes is preserved regardlessof the interstitial ﬁlling of the structures. Whereas the real symmetry of chained and layered
complexes in the structures of uranyl selenite minerals is lower than their topological symmetry, whichmeans that interstitial cations and H2 O molecules signiﬁcantly aﬀect the structural architecture ofnatural compounds. At the same time, structural complexity parameters for the whole structure areusually higher for the minerals than that for synthetic compounds of a similar or close organization,
which probably indicates the preferred existence of such natural-born architectures.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/9/12/639/s1:
Cif ﬁles for guilleminite and demesmaekerite.
Author Contributions: Conceptualization, V.V.G. and J.P.; Methodology, V.V.G., V.M.K. and I.V.K.; Investigation,
V.V.G., I.V.K., V.M.K., M.N.M., A.V.K. and J.P.; Writing-Original Draft Preparation, V.V.G., I.V.K., V.M.K., M.N.M.,
A.V.K. and J.P.; Writing-Review & Editing, V.V.G. and J.P.; Visualization, V.V.G. and I.V.K.
Funding: This research was funded by the Russian Science Foundation (grant 18-17-00018 to V.V.G. and I.V.K.)
and through the project of the Ministry of Education, Youth and Sports National sustainability program I of the
Czech Republic (project No. LO1603 to J.P.).

27

Crystals 2019, 9, 639

Acknowledgments: The XRD measurements of guilleminite have been performed at the X-ray Diﬀraction Centreof the St. Petersburg State University. We are grateful to reviewers for useful comments.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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a new uranyl sulfate mineral from Jáchymov, Czech Republic. J. Geosci. 2015, 60, 113–121. [CrossRef]
Kampf, A.R.; Sejkora, J.; Witzke, T.; Plášil, J.; Čejka, J.; Nash, B.P.; Marty, J. Rietveldite, Fe(UO2 )(SO4 )2 (H2 O)5 ,a new uranyl sulfate mineral from Giveaway-Simplot mine (Utah, USA), Willi Agatz mine (Saxony, Germany)and Jáchymov (Czech Republic). J. Geosci. 2017, 62, 107–120. [CrossRef]
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Krivovichev, S.V.; Burns, P.C. Crystal chemistry of K uranyl chromates: Crystal structures
K5 [(UO2 )(CrO4 )3 ](NO3 )(H2 O)3 ,K4 [(UO2 )3 (CrO4 )5 ](H2 O)8andof
K8 [(UO2 )(CrO4 )4 ](NO3 )2 ,K2 [(UO2 )2 (CrO4 )3 (H2 O)2 ](H2 O)4 . Z. Kristallogr. 2003, 218, 725–732.
Krivovichev, S.V.; Burns, P.C. Crystal chemistry of uranyl molybdates.
VIII. Crystal
structures of Na3 Tl3 [(UO2 )(MoO4 )4 ], Na13 Tl3 [(UO2 )(MoO4 )3 ]4 (H2 O)5 , Na3 Tl5 [(UO2 )(MoO4 )3 ]2 (H2 O)3 andNa2 [(UO2 )(MoO4 )2 ](H2 O)4 . Can. Mineral. 2003, 41, 707–720. [CrossRef]
Grigor’ev, M.S.; Fedoseev, A.M.; Budantseva, N.A.; Yanovskii, A.I.; Struchkov, Y.T.; Krot, N.N. Synthesis,
crystal and molecular structure of complex neptunium(V) sulfates (Co(NH3 )6 )(NpO2 (SO4 )2 )·2H2 O and(Co(NH3 )6 ) H8 O3 (NpO2 (SO4 )3 ). Sov. Radiokhem. 1999, 33, 54–60.
Norquist, A.J.; Doran, M.B.; Thomas, P.M.; O’Hare, D. Structural diversity in organically templated sulfates.
Dalton Trans. 2003, 1168–1175. [CrossRef]
Forbes, T.Z.; Burns, P.C. Structures and syntheses of four Np5+ sulfate chain structures: Divergence from U6+
crystal chemistry. J. Solid State Chem. 2005, 178, 3455–3462. [CrossRef]
Gurzhiy, V.V.; Tyumentseva, O.S.; Krivovichev, S.V.; Tananaev, I.G. Novel type of molecular connectivity in
one-dimensional uranyl compounds: [K@(18-crown-6)(H2 O)][(UO2 )(SeO4 )(NO3 )], a new potassium uranyl
selenate with 18-crown-6 ether. Inorg. Chem. Commun. 2014, 45, 93–96. [CrossRef]
Burns, P.C. A new uranyl phosphate chain in the structure of parsonsite. Am. Mineral. 2000, 85, 801–805.
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Locock, A.J.; Burns, P.C.; Flynn, T.M. The role of water in the structures of synthetic hallimondite,Pb2 [(UO2 )(AsO4 )]2 (H2 O)n and synthetic parsonsite, Pb2 [(UO2 )(PO4 )2 ](H2 O)n , 0 ≤ n ≤ 0.5. Am. Mineral.
2005, 90, 240–246. [CrossRef]
Mills, S.J.;
Birch, W.D.;
Kolitsch, U.;
Mumme, W.G.;
Grey, I.E. Lakebogaite,
CaNaFe2 3+ H(UO2 )2 (PO4 )4 (OH)2 (H2 O)8 , a new uranyl phosphate with a unique crystal structurefrom Victoria, Australia. Am. Mineral. 2008, 93, 691–697. [CrossRef]
Krivovichev, S.V.; Finch, R.; Burns, P.C. Crystal chemistry of uranyl molybdates. V. Topologically diﬀerent
uranyl molybdate sheets in structures of Na2 [(UO2 )(MoO4 )2 ] and K2 [(UO2 )(MoO4 )2 ](H2 O). Can. Mineral.
2002, 40, 193–200. [CrossRef]
Grigor’ev, M.S.; Charushnikova, I.A.; Fedoseev, A.M.; Budantseva, N.A.; Yanovskii, A.I.; Struchkov, Y.T.
Crystal and molecular structure of neptunium(V) complex molybdate K3 NpO2 (MoO4 )2 . Sov. Radiokhem.
1992, 34, 7–12.
Krivovichev, S.V.; Kahlenberg, V. Structural diversity of sheets in Rb uranyl selenates: Synthesis and crystal
structures of Rb2 [(UO2 )(SeO4 )2 (H2 O)](H2 O), Rb2 [(UO2 )2 (SeO4 )3 (H2 O)2 ](H2 O)4 , Rb4 [(UO2 )3 (SeO4 )5 (H2 O)].
Z. Anorg. Allg. Chem. 2005, 631, 739–744. [CrossRef]
Lussier, A.J.; Lopez, R.A.K.; Burns, P.C. A revised and expanded structure hierarchy of natural and synthetic
hexavalent uranium compounds. Can. Mineral. 2016, 54, 177–283. [CrossRef]
Christ, C.L.; Clark, J.R.; Evans, H.T., Jr. Crystal structure of rutherfordine, UO2 CO3 . Science 1955, 121, 472–473.
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Finch, R.J.; Cooper, M.A.; Hawthorne, F.C.; Ewing, R.C. Reﬁnement of the crystal structure of rutherfordine.
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Demartin, F.; Diella, V.; Donzelli, S.; Gramaccioli, C.M.; Pilati, T. The importance of accurate crystal structure
determination of uranium minerals. I. Phosphuranylite KCa(H3 O)3 (UO2 )7 (PO4 )4 O4 ·8H2 O. Acta Crystallogr.
1991, B47, 439–446. [CrossRef]
Shvareva, T.Y.; Albrecht-Schmitt, T.E. General route to three-dimensional framework uranyl transition metal
phosphates with atypical structural motifs: The case examples of Cs2 {(UO2 )4 [Co(H2 O)2 ]2 (HPO4 )(PO4 )4 }and Cs3+x [(UO2 )3 CuH4−x (PO4 )5 ]·H2 O. Inorg. Chem. 2006, 45, 1900–1902. [CrossRef] [PubMed]
Plášil, J.; Hauser, J.; Petříček, V.; Meisser, N.; Mills, S.J.; Škoda, R.; Fejfarová, K.; Čejka, J.; Sejkora, J.; Hloušek, J.;et al. Crystal structure and formula revision of deliensite, Fe[(UO2 )2 (SO4 )2 (OH)2 ](H2 O)7 . Mineral. Mag.
2012, 76, 2837–2860. [CrossRef]
Kampf, A.R.; Kasatkin, A.V.; Čejka, J.; Marty, J. Plášilite, Na(UO2 )(SO4 )(OH)·2H2 O, a new uranyl sulfate
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Guesdon, A.; Chardon, J.; Provost, J.; Raveau, B. A copper uranyl monophosphate built up from CuO2
inﬁnity chains: Cu2 UO2 (PO4 )2 . J. Solid State Chem. 2002, 165, 89–93. [CrossRef]
Krivovichev, S.V.; Burns, P.C. Synthesis and crystal structure of Li2 [(UO2 )(MoO4 )2 ], a uranyl molybdate withchains of corner-sharing uranyl square bipyramids and MoO4 tetrahedra. Solid State Sci. 2003, 5, 481–485.
[CrossRef]
Almond, P.M.; Albrecht-Schmitt, T.E. Expanding the remarkable structural diversity of uranyl tellurites:
Hydrothermal preparation and structures of KUO2 Te2 O5 (OH), Tl3 {(UO2 )2 Te2 O5 (OH)(Te2 O6 )}·2H2 O,
β-Tl2 (UO2 (TeO3 ))2 , and Sr3 ((UO2 )(TeO3 ))2 (TeO3 )2 . Inorg. Chem. 2002, 41, 5495–5501. [CrossRef]
Charykova, M.V.; Krivovichev, V.G. Mineral systems and the thermodynamics of selenites and selenates in
the oxidation zone of sulﬁde ores—A review. Mineral. Petrol. 2017, 111, 121–134. [CrossRef]
Krivovichev, V.G.; Charykova, M.V.; Vishnevsky, A.V. The thermodynamics of selenium minerals innear-surface environments. Minerals 2017, 7, 188. [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/).

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crystals
ArticleCrystallographic Insights into Uranyl Sulfate
Minerals Formation: Synthesis and Crystal Structuresof Three Novel Cesium Uranyl Sulfates
Olga S. Tyumentseva 1 , Ilya V. Kornyakov 1,2 , Sergey N. Britvin 1,2 , Andrey A. Zolotarev 1 andVladislav V. Gurzhiy 1, *1

2*

Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University Emb.
7/9, St. 199034 Petersburg, Russian; o-tyumentseva@mail.ru (O.S.T.); ikornyakov@mail.ru (I.V.K.);
sbritvin@gmail.com (S.N.B.); a.zolotarev@spbu.ru or aazolotarev@gmail.com (A.A.Z.)
Kola Science Centre, Russian Academy of Sciences, Fersmana 14, 184209 Apatity, Russia
Correspondence: vladislav.gurzhiy@spbu.ru or vladgeo17@mail.ru

Received: 16 November 2019; Accepted: 5 December 2019; Published: 9 December 2019

Abstract: An alteration of the uranyl oxide hydroxy-hydrate mineral schoepite [(UO2 )8 O2 (OH)12 ](H2 O)12 at mild hydrothermal conditions was studied. As the result, four diﬀerent crystalline phases
Cs[(UO2 )(SO4 )(OH)](H2 O)0.25 (1), Cs3 [(UO2 )4 (SO4 )2 O3 (OH)](H2 O)3 (2), Cs6 [(UO2 )2 (SO4 )5 ](H2 O)3(3), and Cs2 [(UO2 )(SO4 )2 ] (4) were obtained, including three novel compounds. The obtained Cs
uranyl sulfate compounds 1, 3, and 4 were analyzed using single-crystal XRD, EDX, as well as
topological analysis and information-based structural complexity measures. The crystal structureof 3 was based on the 1D complex, the topology of which was unprecedented for the structuralchemistry of inorganic oxysalts. Crystal chemical analysis performed herein suggested that the
majority of the uranyl sulfates minerals were grown from heated solutions, and the temperature
range could be assumed from the manner of interpolyhedral linkage. The presence of edge-sharing
uranyl bipyramids most likely pointed to the temperatures of higher than 100 ◦ C. The linkage ofsulfate tetrahedra with uranyl polyhedra through the common edges involved elevated temperatures
but of lower values (~70–100 ◦ C). Complexity parameters of the synthetic compounds were generally
lower than that of uranyl sulfate minerals, whose structures were based on the complexes with thesame or genetically similar topologies. The topological complexity of the uranyl sulfate structuralunits contributed the major portion to the overall complexity of the synthesized compounds, while
the complexity of the respective minerals was largely governed by the interstitial structure andH-bonding system.
Keywords: uranyl; hydroxy-hydrate; sulfate; cesium; schoepite; crystal structure; topology;
structural complexity

1. Introduction
Uranyl-oxide hydroxy-hydrate minerals are regarded to be the products of the ﬁrst stages ofuraninite alteration under oxidizing conditions [1–5]. Being formed as the result of the reaction of
bedrock with aqueous ﬂuids, these natural compounds obviously play an important, if not a key, rolein the uranium transfer to the environment. In addition, uranyl-oxide hydroxy-hydrate phases canbe regarded as the precursors of the formation of other secondary uranium-bearing minerals, underreaction with waters enriched by various cations (usually mono- and divalent) and oxyanions (CO3 2− ,SO4 2− , PO4 2− , etc.). The description of the new mineral species is rarely followed by the experiments,
which could shed some light on the conditions of their genesis. Due to the complexity of uranyl-bearing
complexes forming in aqueous solutions in the presence of a certain cation/oxyanion combination as a
Crystals 2019, 9, 660; doi:10.3390/cryst9120660

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Crystals 2019, 9, 660

function of pH, the exact formation mechanisms of uranyl minerals are not fully understood. In thisstudy, we presented the results of uranyl sulfate synthesis experiments that might elucidate some of
the formation behavior of natural uranyl sulfates.
Herein, we reported on the alteration experiment of the synthetic analog of uranyl-oxidehydroxy-hydrate mineral schoepite, [(UO2 )8 O2 (OH)12 ](H2 O)12 [6,7]. As the result, four different crystalline
phases Cs[(UO2 )(SO4 )(OH)](H2 O)0.25 (1), Cs3 [(UO2 )4 (SO4 )2 O3 (OH)](H2 O)3 (2) [8], Cs6 [(UO2 )2 (SO4 )5 ](H2 O)3 (3), and Cs2 [(UO2 )(SO4 )2 ] (4) were obtained, including three novel compounds. The obtained Cs
uranyl sulfate compounds were analyzed using single-crystal X-ray diffraction (SC XRD), energy-dispersive
X-ray analysis (EDX), as well as topological analysis and information-based structural complexity measures.
2. Materials and Methods
2.1. Synthesis
An analog of the hydroxy-hydrated uranyl oxide mineral schoepite ([(UO2 )8 O2 (OH)12 ](H2 O)12 [6,7])
was synthesized according to the procedure discussed in [9], and its purity was checked using powder
XRD; H2 SO4 (Sigma-Aldrich, 98%) and Cs2 SO4 (Vekton, Russia, 99%) were used as received. The 0.2 gof synthetic schoepite (0.03 mmol), 0.12 g of cesium sulfate (0.33 mmol), and 0.01 (0.19 mmol) ml ofsulfuric acid were dissolved in 10 mL of deionized water. The solution was stirred and loaded in a
23 mL Teﬂon-lined steel autoclave, which was placed in a box furnace and heated to 110 ◦ C. After
24 h, the furnace was cooled at ~20 ◦ C/h to room temperature. A ﬁne crystalline precipitate coveredby a translucent light-yellow solution was found at the bottom of the Teﬂon capsule. Afterward,
the product was poured into a watch glass. The detailed examination under the optical microscope
revealed the presence of two types of crystals (Figure 1): The bulk of the precipitant was tiny yellowish
isometric crystals with nearly diamond luster (compound 1), interspersed with larger in size, but ofmuch worse quality orange rhombus lamina crystals (compound 2). After picking several crystals forfurther diagnostics, the product was left in a watch glass to evaporate at room temperature. In a few
days, two more kinds of crystals were detected in a small amount at the edge of the solution (Figure 1):
green rosettes of thin plates (compound 3) and light green ﬂattened rhombic crystals (compound 4).

Figure 1. Crystals of 1 (a,b), 2 (a,b), 3 (c) and 4 (d) formed in the described synthetic experiment.

34

Crystals 2019, 9, 660

2.2. Chemical Analysis
Small pieces of single crystals of 1–4 veriﬁed on the diﬀractometer were crushed, pelletized, andcarbon-coated. The chemical composition of the samples was determined using a TM 3000 scanningelectron microscope equipped with an Oxford EDX spectrometer, with an acquisition time of 15 s per
point in energy-dispersive mode (acceleration voltage 15 kV). The following analytical standards wereused: CsBr (CsK), barite (SK), and U3 O8 (UK).
Analytical calculations. Compound 1: atomic ratio from structural data Cs 1.00, U 1.00, S 1.00;
found by EDX: Cs 1.03, U 1.05, S 0.92. Compound 2: atomic ratio from structural data Cs 3.00, U 4.00,
S 2.00; found by EDX: Cs 2.93, U 4.06, S 2.01. Compound 3: atomic ratio from structural data Cs 6.00,
U 2.00, S 5.00; found by EDX: Cs 5.96, U 2.07, S 4.97. Compound 4: atomic ratio from structural data Cs
2.00, U 1.00, S 2.00; found by EDX: Cs 1.95, U 1.05, S 2.00.
2.3. Single-Crystal X-Ray Diﬀraction Study
Single crystals of 1–4 were selected under binocular, coated in oil-based cryoprotectant, andmounted on cryoloops. Diﬀraction data were collected using a Bruker Kappa Duo diﬀractometer(Bruker AXS, Madison, WI, USA) equipped with a CCD (charge-coupled device) Apex II detector
operated with monochromated microfocused MoKα radiation (λ[MoKα] = 0.71073 Å) at 45 kV and
0.6 mA. Diﬀraction data were collected at room temperature with frame widths of 0.5◦ in ω and ϕ, andexposures of 40 to 120 s per frame. Diﬀraction data were integrated and corrected for polarization,
background, and Lorentz eﬀects. An empirical absorption correction based on spherical harmonics
implemented in the SCALE3 ABSPACK algorithm was applied in the CrysAlisPro program [10].
The unit-cell parameters (Table 1) were reﬁned using the least-squares techniques. The structure wassolved by a dual-space algorithm and reﬁned using the SHELX programs [11,12] incorporated in the
OLEX2 program package [13]. The ﬁnal models include coordinates and anisotropic displacement
parameters for all non-H atoms. The H atoms of OH groups and H2 O molecules were localized from
diﬀerence Fourier maps and were included in the reﬁnement with Uiso (H) set to 1.5Ueq (O) and O–H
bond-length restraints to 0.95 Å. Selected bond lengths and angles are listed in Tables 2–4. Checking of
the unit-cell parameters along with the results of the chemical analyses showed that the crystals of 2
were the cesium uranyl sulfate phase, which was reported previously [8]. Due to the small size andlow quality of the crystals of 3, reﬁnement parameters were rather high, but the structural model wasquite reliable.
Table 1. Crystallographic data for 1, 3, and 4.
Compound1

3

4

Crystal System
a (Å)
b (Å)
c (Å)
α (◦ )β (◦ )γ (◦ )
V (Å3 )
Molecular weightSpace groupμ (mm−1 )Temperature (K)Z
Dcalc (g/cm3 )Crystal size (mm3 )Diﬀractometer
Radiation

Orthorhombic
9.2021(3)13.2434(5)12.5610(3)9090901530.77(8)520.51
Pnma
26.156

Triclinic
7.5829(3)14.4441(11)14.6458(14)93.737(7)99.535(5)99.614(5)1552.6(2)1871.87
P–1
17.779293(2)
2
4.004
0.04 × 0.03 × 0.005
Bruker Kappa Apex II Duo
MoKαMonoclinic
10.8351(5)9.0317(5)11.8494(6)90110.7510(10)901084.35(10)727.97
P21 /n22.0048
4.517
0.06 × 0.04 × 0.02

35

4
4.459
0.08 × 0.05 × 0.02

Crystals 2019, 9, 660

Table 1. Cont.
Compound1

3

4

Total reﬂections
Unique reﬂections
Angle range 2θ (◦ )Reﬂections with |Fo | ≥ 4σF
Rint
Rσ
R1 (|Fo | ≥ 4σF )wR2 (|Fo | ≥ 4σF )R1 (all data)wR2 (all data)Sρmin , ρmax , e/Å3
CSD

13617
1824
4.47–55.00
1560
0.05030.02010.02610.06070.0336
0.06481.054−1.097, 2.145
19658198814
5151
3.84–50.00
3806
0.07840.09490.08290.20500.10770.2215
1.068−4.179, 3.819
196581727773
2493
4.38–55.00
2308
0.03580.01540.01310.02780.01570.02871.040−0.604, 0.692
1965818

R1 = Σ||Fo | − |Fc ||/Σ|Fo |; wR2 = {Σ[w(Fo 2 − Fc 2 )2 ]/Σ[w(Fo 2 )2 ]}1/2 ; w = 1/[σ2 (Fo 2 ) + (aP)2 + bP], where P = (Fo 2 + 2Fc 2 )/3;s = {Σ[w(Fo 2 − Fc 2 )]/(n − p)}1/2 where n is the number of reﬂections, and p is the number of reﬁnement parameters.

Table 2. Selected bond lengths (Å) and angles (◦ ) in the structure of 1.
Bond

Bond

U1–O1
U1–O2
< U1–OUr >U1–O3
U1–OH4
U1–OH4
U1–O5

1.768(5)1.768(5)1.768
2.351(4)2.330(5)2.322(5)2.421(5)

S1–O6
S1–O7
< S1–O >1.481(5)1.426(6)1.465< Cs1–O > CN * = 8< Cs2–O > CN * = 10

3.313
3.302

U1–O6

2.358(5)Angle< U1–Oeq >

2.356

S1–O3
S1–O5

1.478(4)1.473(5)

U1–O4–U1
S1–O5–U1S1–O6–U1

113.0(2)140.3(3)140.3(3)*-Coordination numbers (CN) at the 3.6 Å limit, for the average bond length value.

Table 3. Selected bond lengths (Å) and angles (◦ ) in the structure of 3.
Bond

Bond

U1–O1
U1–O2
< U1–OUr >
U1–O3
U1–O4
U1–O5
U1–O6
U1–O7
< U1–Oeq >1.770(19)1.80(2)
1.785
2.336(19)2.40(2)2.428(17)2.347(19)2.35(2)
2.37

U2–O15
U2–O16< U2–OUr >U2–O14U2–O17
U2–O21
U2–O22

1.76(2)
1.77(2)
1.765
2.349(19)2.42(2)2.35(3)2.38(3)S3–O14< S3–O >1.50(2)
1.47

S4–O17
S4–O18S4–O19S4–O20< S4–O >1.51(2)
1.48(2)
1.43(2)
1.42(3)
1.46

S5–O21
S5–O22
S5–O23
S5–O24< S5–O >1.48(3)1.43(3)
1.46(3)1.43(3)
1.45< Cs1–O > CN * = 12
< Cs2–O > CN * = 10

3.26
3.32

36

Crystals 2019, 9, 660

Table 3. Cont.
Bond

Bond
< Cs3–O > CN * = 9< Cs4–O > CN * = 10
< Cs5–O > CN * = 9< Cs6–O > CN * = 10

U2–O22A< U2–Oeq >2.26(10)
2.35

S1–O3
S1–O7

1.492(19)1.50(2)

S1–O8

1.42(2)

Angle

S1–O9
< S1–O >1.43(3)
1.46

S2–O4
S2–O5
S2–O10S2–O11< S2–O >1.505(19)1.510(19)1.42(2)
1.42(2)
1.46

S3–O6
S3–O12S3–O13

1.497(19)1.494(19)1.39(2)

U1–O3–S1
U1–O4–S2
U1–O5–S2U1–O6–S3U1–O7–S1
U2–O14–S3
U2–O17–S4
U2–O18–S4
U2–O21–S5
U2–O22–S5
U2–O22A–S5

3.33
3.33
3.27
3.28

138.4(13)100.9(9)99.6(9)141.8(12)140.0(14)135.7(12)100.5(10)101.3(11)146.0(16)146(2)
157(7)*-Coordination numbers (CN) at the 3.6 Å limit, for the average bond length value.

Table 4. Selected bond lengths (Å) and angles (◦ ) in the structure of 4.
Bond

Bond

U1–O1
U1–O2
< U1–OUr >
U1–O4
U1–O5
U1–O7
U1–O9
U1–O10< U1–Oeq >1.775(2)1.768(2)1.772
2.301(2)2.319(2)2.322(2)2.478(2)2.482(2)
2.380

S1–O3
S1–O4
S1–O5
S1–O6
< S1–O >1.446(2)1.502(2)1.502(2)1.440(2)
1.473

S2–O7
S2–O8
S2–O9
S2–O10< S2–O >1.477(2)1.430(3)1.486(2)1.492(2)1.471< Cs1–O > CN * = 10
< Cs2–O > CN * = 10

3.286
3.311

Angle
S1–O4–U1
S1–O5–U1S2–O7–U1S2–O9–U1
S2–O10–U1

142.45(14)133.36(14)148.09(16)99.54(11)99.22(11)*-Coordination numbers (CN) at the 3.6 Å limit, for the average bond length value.

Supplementary crystallographic data (see online Supplementary Materials) for 1, 3, and4 were deposited in the Inorganic Crystal Structure Database and could be obtained from
Fachinformationszentrum Karlsruhe via https://www.ccdc.cam.ac.uk/structures/.
3. Results
3.1. Structure Descriptions
The crystal structure of 1 contained one crystallographically nonequivalent U6+ atom with two
short U6+ ≡ O2− bonds (1.768(5) Å), forming nearly linear UO2 2+ uranyl ion (Ur), which was coordinatedby another ﬁve oxygen atoms < U1–Oeq > = 2.356 Å that were arranged in the equatorial plane of theUO7 pentagonal bipyramid. Three of those Oeq atoms belonged to sulfate tetrahedra, while the othertwo cis-O atoms made an edge shared with the neighbor pentagonal bipyramid, thus forming a dimer.
In addition, both O atoms from the shared edge were protonated to form OH− groups. There was one
37

Crystals 2019, 9, 660

crystallographically nonequivalent S6+ atom tetrahedrally coordinated by four O2− atoms. U dimers
and S-centered tetrahedra shared common vertices to form a layered [(UO2 )(SO4 )(OH)]− complexes(Figure 2a). Being 3-connected, sulfate tetrahedra had the fourth non-shared vertex oriented either
up or down relative to the plane of the layer, which gave rise to geometric isomerism with various
orientations of the sulfate tetrahedra. To distinguish the isomers, their orientation matrices wereassigned using symbols u (up) and d (down). There were two nonequivalent Cs+ atoms in the structureof 1, occupying special positions on a mirror plane and arranged in the interlayer space along with
substantially vacant and disordered over two sites H2 O molecule.

Figure 2. 1D and 2D complexes and their respective topologies in the structures of 1 (a,b), 2 (c,d), 3 (e,f),
and 4 (g,h). Legend: U-bearing coordination polyhedra = yellow; S-centered tetrahedra = orange;
O atoms = red; H atoms = light grey; black nodes = U atoms, white nodes = S atoms.

There were two crystallographically nonequivalent U6+ atoms in the structure of 3 with two short
U6+ ≡ O2− bonds (1.76(2)–1.80(2) Å), forming Ur. The Ur cations were coordinated by ﬁve oxygen
atoms < Ur-Oeq > = 2.35 and 2.37 Å (for U1 and U2, respectively), which belonged to sulfate tetrahedrathat were arranged in the equatorial plane of the UO7 pentagonal bipyramid. There were ﬁve S6+ atoms
in the structure of 3, tetrahedrally coordinated by four O2− atoms each. All sulfate tetrahedra were2-connected, having only two vertices shared with the uranyl bipyramids, while the other two vertices
were left non-shared. But if S1-, S3-, and S5-centered tetrahedra shared two of their vertices with two38

Crystals 2019, 9, 660

Ur polyhedra, S2- and S4-centered tetrahedra linked to the single bipyramid via sharing a commonedge. U and S coordination polyhedra shared common vertices and edges to form wave-like inﬁnite
chains of [(UO2 )2 (SO4 )5 ]6− composition (Figure 2e). Besides, the linkage of coordination polyhedra
inside the chain occurred only via corner-sharing, while edge-shared sulfate tetrahedra decorated theexterior of the chain. Uranyl sulfate chains were interlinked into the pseudo layered structure via an
H-bonding system involving H2 O molecules. The negative charge of the uranyl sulfate complex wascompensated by six Cs+ cations arranged in between the pseudo layers. It should be noted that threesites occupied by H2 O molecules were localized only in the voids arranged in the plane of the chainsbut not between the pseudo-2D complexes.
There was one crystallographically nonequivalent U6+ atom in the structure of 4 with two short bonds(< U1–OUr > = 1.772 Å) forming Ur, which was coordinated by five oxygen atoms < Ur-Oeq > = 2.38 Å,
belonging to sulfate tetrahedra and arranged in the equatorial plane of the pentagonal bipyramid.
Two nonequivalent S6+ atoms were tetrahedrally coordinated by four O2− atoms each. There were
two types of sulfate oxyanions: [S1O4 ]2− were 2-connected, sharing two vertices with two adjacent
Ur polyhedra, and [S2O4 ]2− were 3-connected (chelating-bridging), sharing vertex with one uranyl
bipyramid and an edge with the neighbor one. The structure of 4 was based on the layered complexesof [(UO2 )(SO4 )2 ]2− composition (Figure 2g), interlinked by the two non-equivalent Cs+ cations.
3.2. Topological Analysis
The anion topology of the U-S layer in 1 (Figure 2b) was determined using the approach describedin [14]. It belonged to the 54 41 32 -I topological type according to the classiﬁcation suggested in [15],and it has been observed in the structures of several synthetic uranyl sulfates [16,17], chromates [18],and phosphates [19]. Topology consisted of chains of pentagons, half of which were occupied by
the Ur edge-sharing dimers, separated by the groups of one square and two triangles. Each triangle
corresponded to the 3-connected face of the sulfate tetrahedra, while the squares were vacant. The
orientation of non-shared vertex in the structure of 1 alternated by rows (Figure 2a): in the ﬁrst row,
all vertices were oriented down, in the second–up, then again down, etc. Thus, the geometric isomer
represented in 1 was described by the (u)(d) matrix. It should be noted that the layered complex
in the structure of 1 was very similar to those found in the structures of uranyl sulfate minerals as
deliensite, Fe[(UO2 )2 (SO4 )2 (OH)2 ](H2 O)7 [20], plášilite, Na(UO2 )(SO4 )(OH)·2H2 O [21], and others [22].
But their topologies were signiﬁcantly distinct due to various arrangements of the uranyl dimers
within the layer. In the structure of 1, dimers were stacked in a ladder fashion, while in the structuresof aforementioned minerals, dimers were arranged parallel to each other, thus forming, typical forminerals, so-called phosphuranylite topology [23]. Diﬀerences between these topological isomers havebeen recently described in [17].
The crystal structure of 2 [8] was based on the layered complexes of the [(UO2 )4 (SO4 )2 O3 (OH)]6−
composition (Figure 2c,d). Its topology was described by the 52 43 32 ring symbol and was related to oneof the most common topological types among the natural uranyl sulfates [22], the so-called zippeite
topology. Topology consisted of zig-zag inﬁnite chains of edge-sharing pentagons separated by chainsof squares and triangles.
The topology of the uranyl sulfate chain in the structure of 3 (Figure 2f) could be visualized using
the theory of graphical representation [24]. Double links between the black and white vertices in agraph indicate the sharing of an edge between uranyl coordination polyhedra and sulfate oxyanion.
The chain topology in 3 was unprecedented for the structural chemistry of inorganic oxysalts andbelonged to the novel cc1-2:5-1 type.
The graph of the uranyl sulfate layered complex in the structure of 4 belonged to the cc2–1:2–21
topological type (Figure 2h) and consisted of dense 4-membered and large 12-membered rings. This
topology is rather rare but has been observed in the structures of a few actinide-bearing compounds.
It has been described at ﬁrst in the structure of isotypic Cs neptunyl sulfate compound [25], and laterin the structures of Ba uranyl selenite [26] and organically templated uranyl sulfate [27].

39Crystals 2019, 9, 660

3.3. Structural Complexity
The information-based complexity parameters for 1–4 are given in Table 5. This approach,recently developed by S.V. Krivovichev [28–30] and successfully implemented in [22,31,32], allowed tocompare the structures in terms of their information content and to analyze contributions of various
substructural building blocks into the complexity of the whole structure. Structural complexity is anegative contribution to the conﬁgurational entropy of a crystalline compound and thus could help tounderstand and describe the processes of various structures formation in the laboratory and, what is of
special interest, in nature.
Table 5. Structural and topological complexity parameters for the uranyl sulfate compounds.

Compound

1
2
3
4
K4 [UO2 (SO4 )3 ]Marecottite
Peligotite
Lussierite
Klaprothite
Ottohahnite

Complexity Parameters of the
Crystal Structure
Sp. Gr.

νIGIG,total

Pnma
P–1
P–1
P–1
Pnma
P21 /nP–1
Cc
P21 /c
P–1

100
78
92
141
176
60
82
104
164
126

3.844
5.285
5.524
6.161
4.641
3.907
5.382
5.700
5.382
6.000

384.386412.261508.168868.677
816.860
234.413441.319592.846
882.639768.000

Structural Complexity of
the U-S Unit
Layer/Rod
Gr.

ν

IG

IG,total

p21 /a
p–1

þ –1
p–1
pm
p21 /a

40
54
62
54
36
52

3.322
4.755
4.954
4.755
4.337
3.700

132.877
256.764307.160
256.764156.117
192.423

p–1

23

4.524

p–1

62

4.954

Topological Complexity of
the U-S Unit
Layer/Rod
Gr.

ν

IG

IG,total

p21 /ap–1
p–1
pmm2
p21 /a

40
54
62
54
36
52

3.322
4.755
3.599
4.755
3.892
3.700

132.877
256.764223.160
256.764140.117
192.423104.042

pm

23

3.915

90.042307.160

p–1

62

4.954

307.160

þ a mam

The general trend in evolution of crystallization was recently summarized as follows [33–36]:
complexities of structures formed on the latter stages of crystallization are higher than those for thephases growing on the primary stages, wherein very complex structures may form as transitionalarchitectures prior or between phases with relatively small amounts of structural information.
The evaluation was performed in several steps (Figure 3). First, the topological complexity (TI),
according to the maximal rod (for chains) or layer symmetry group, was calculated since these are thebasic structural units. Second, the structural complexity (SI) of the units was analyzed, taking into
account its real symmetry. The next contribution to information came from the stacking (LS) of chained
and layered complexes (if more than one layer or chain is in the unit cell). The fourth contribution
to the total structural complexity was given by the interstitial structure (IS). And the last portion of
information came from the interstitial H bonding system (H). It should be noted that the H atoms
related to the U-bearing chains and layers were considered as a part of those complexes, but not within
the contribution of the H-bonding system. For instance, contribution of H was equal to the diﬀerence
between complexity parameters for the whole structure and those for the structural model with no H
atoms; contribution of IS was equal to the diﬀerence between complexity parameters for the structuralmodel with no H atoms and those for the structural model with no interstitial substructure at all; etc.
Complexity parameters for the aforementioned contributions were calculated manually using thegeneral formulae [28–30]. Complexity parameters for the whole structures were calculated using theToposPro package [37]. Complexity calculations showed that the crystal structures of 1, 2, and 4 should
be described as intermediate, possessing the values below 500 bits/cell, while compound 3 just slightly
passed through this border (508.168 bits/cell) and should be described as complex.

40

Crystals 2019, 9, 660

Figure 3. Ladder diagrams showing contributions and normalized contributions (in %) of various
factors to structural complexity in terms of bits per unit cell. Legend: TI = topological information;
CI = cluster information; SI = structural information; LS = layer stacking; IS = interstitial structure;
HB = hydrogen bonding. See Table 5 and text for details.

4. Discussion
The crystals of 1–4 could be distributed over two genetically distinct groups. The crystals of
1 and 2 were formed during the ﬁrst stage of synthetic schoepite alteration. Their structures werebased on the layered complexes with the edge-sharing linkage of uranyl pentagonal bipyramids,
which reﬂected the heating of the reaction solution during the growth processes. It should be notedthat original schoepite was obtained from nearly neutral solutions, whereas the aqueous medium
in our experiment was signiﬁcantly more acidic (pH ~ 2). Acidic conditions and the presence ofadditional Cs+ cations destroyed the dense layer in the structure of schoepite, but high temperatureallowed preserving an edge-sharing complexation of Ur coordination polyhedra. An initial solution in
the experiment contained the Cs:U:S molar ratio ~ 1:1:1, which explained the predominance of theCs[(UO2 )(SO4 )(OH)](H2 O)0.25 (1) phase in the precipitate. The lower amount of crystals of 2 could
be explained by the lower temperature of the experiment that is preferable for the formation of thezippeite-type structures.
The crystals of 3 and 4 could be attributed to the later genetic type because they were grownafter cooling the system at room temperature conditions. It is of interest that both phases had in theirstructures uranyl pentagonal bipyramids that shared an edge with the sulfate tetrahedra. Similararrangement of Ur and sulfate oxyanions were found in the structures of four natural uranyl sulfates:
klaprothite, Na6 (UO2 )(SO4 )4 (H2 O)4 , its polymorph peligotite, Na6 (UO2 )(SO4 )4 (H2 O)4 , ottohahnite,
Na6 (UO2 )2 (SO4 )5 (H2 O)8.5 [38], and lussierite Na10 [(UO2 )(SO4 )4 ](SO4 )2 ·3H2 O [39]. The same clusters

41Crystals 2019, 9, 660

that were observed in the structures of klaprothite and peligotite (Figure 4) were previously describedin a few synthetic compounds [40–43], which were grown using low temperature (70 ◦ C) hydrothermal
experiments. The presence of such unusual arrangements of edge-sharing uranyl bipyramid and sulfate
tetrahedra have never been observed during regular evaporation experiments at room temperature.
Thus, we could assume that these clusters were formed on the ﬁrst, hydrothermal stage of ourexperiment. Moreover, the presence of such S-enriched [(UO2 )(SO4 )4 ]6− clusters in the heated solution
could explain the local disturbance of the Cs:U:S ~ 1:1:1 concentration, which induced the formation of
S-“depleted” zippeite-like compound 2. There is one more known synthetic K-bearing uranyl sulfate,
whose structure is based on the double klaprothite-type clusters [44]. This row could be continued by
the further doubling of the cluster in K4 (UO2 (SO4 )3 ) [44] to get the quadruple 0D unit in the structureof ottohahnite (Figure 4). Further increase of the cluster size led to the arrangement of the inﬁnite
chains in the structure of 3, which in turn, during the dehydration process [34], would transform
into the layer in the structure of 4. The absence of the structures based on the 0D structural units
in our experiment might come from the requirements for longer storage at elevated, but not high,temperatures, and higher concentrations of Cs+ cations and [SO4 ]2− oxyanions in the initial solution.

Figure 4. Scheme of structural evolution for the 1D and 2D uranyl sulfate complexes, which have anedge-sharing uranyl bipyramid and sulfate tetrahedron.

5. Conclusions
Summarizing our crystal’s chemical observations, we could suggest that the majority of the
discovered natural uranyl sulfates were grown from heated solutions, and the temperature range
could be assumed from the manner of interpolyhedral linkage. The presence of edge-sharing
uranyl bipyramids (phosphuranylite anion topology [22]), most likely pointed to the temperaturesof higher than 100 ◦ C, and the crystal growth should apparently occur directly in hydrothermal
conditions. The linkage of sulfate tetrahedra with Ur through the common edges also involved
elevated temperatures, but of less values (~70–100 ◦ C), which could be achieved by cooling the system.
Moreover, in the second case, crystallization might start much later at environmental conditions but
from the initially heated solutions. The enriched solution might pass some way along the cracks in the
bedrock, transferring klaprothite-like clusters in the dissolved form. It is of interest that complexity
parameters of the synthetic compounds were generally lower than that of minerals, whose structures42

Crystals 2019, 9, 660

were based on the complexes with the same or genetically similar topologies. Furthermore, thetopological complexity of the uranyl sulfate structural units contributed the major portion to the overall
complexity of the synthesized compounds, while the complexity of the respective minerals was largely
governed by the interstitial structure and H-bonding system.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/9/12/660/s1:
Cif ﬁles for 1, 3, and 4.
Author Contributions: Conceptualization, O.S.T. and V.V.G.; Methodology, O.S.T. and I.V.K.; Investigation, O.S.T.,
I.V.K., S.N.B., and A.A.Z.; Writing-Original Draft Preparation, O.S.T. and V.V.G.; Writing-Review and Editing,
V.V.G.; Visualization, V.V.G. and I.V.K.
Funding: This research was funded by the Russian Science Foundation (grant 18-17-00018).
Acknowledgments: The XRD and EDX measurements have been performed at the X-ray Diﬀraction Center andCenter for Microscopy and Microanalysis of the St. Petersburg State University.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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45

crystals
ArticleKrasnoshteinite, Al8[B2O4(OH)2](OH)16Cl4·7H2O,
a New Microporous Mineral with a Novel Type of
Borate Polyanion

Igor V. Pekov 1, *, Natalia V. Zubkova 1 , Ilya I. Chaikovskiy 2 , Elena P. Chirkova 2 ,Dmitry I. Belakovskiy 3 , Vasiliy O. Yapaskurt 1 , Yana V. Bychkova 1 , Inna Lykova 4 ,Sergey N. Britvin 5,6 and Dmitry Yu. Pushcharovsky 1
1

2
3
4
5
6

*

Faculty of Geology, Moscow State University, Vorobievy Gory, 119991 Moscow, Russia;
n.v.zubkova@gmail.com (N.V.Z.); yvo72@geol.msu.ru (V.O.Y.); yanab66@yandex.ru (Y.V.B.);
dmitp@geol.msu.ru (D.Y.P.)
Mining Institute, Ural Branch of the Russian Academy of Sciences, Sibirskaya str., 78a, 614007 Perm, Russia;
ilya@mi-perm.ru (I.I.C.); zaitseva59@mail.ru (E.P.C.)
Fersman Mineralogical Museum, Russian Academy of Sciences, Leninsky Prospekt 18-2, 119071 Moscow,
Russia; dmzvr@mail.ru
Canadian Museum of Nature, 240 McLeod St, Ottawa, ON K2P 2R1, Canada; ilykova@nature.ca
Department of Crystallography, St. Petersburg State University, Universitetskaya nab. 7/9,
199034 St. Petersburg, Russia; sbritvin@gmail.com
Kola Science Centre, Russian Academy of Sciences, Fersman Street 14, 184209 Apatity, Russia
Correspondence: igorpekov@mail.ru

Received: 6 April 2020; Accepted: 12 April 2020; Published: 15 April 2020

Abstract: A new mineral, krasnoshteinite (Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O), was found in the
Verkhnekamskoe potassium salt deposit, Perm Krai, Western Urals, Russia. It occurs as transparent
colourless tabular to lamellar crystals embedded up to 0.06 × 0.25 × 0.3 mm in halite-carnallite rock andis associated with dritsite, dolomite, magnesite, quartz, baryte, kaolinite, potassic feldspar, congolite,
members of the goyazite–woodhouseite series, ﬂuorite, hematite, and anatase. Dmeas = 2.11 (1) andDcalc = 2.115 g/cm3 . Krasnoshteinite is optically biaxial (+), α = 1.563 (2), β = 1.565 (2), γ = 1.574 (2),and 2V meas = 50 (10)◦ . The chemical composition (wt.%; by combination of electron microprobe
and ICP-MS; H2 O calculated from structure data) is: B2 O3 8.15, Al2 O3 46.27, SiO2 0.06, Cl 15.48,
H2 Ocalc. 33.74, –O=Cl –3.50, totalling 100.20. The empirical formula calculated based on O + Cl = 33
apfu is (Al7.87 Si0.01 )Σ7.88 [B2.03 O4 (OH)2 ][(OH)15.74 (H2 O)0.26 ]Σ16 [(Cl3.79 (OH)0.21 ]Σ4 ·7H2 O. The mineral ismonoclinic, P21 , a = 8.73980 (19), b = 14.4129 (3), c = 11.3060 (3) Å, β = 106.665 (2)◦ , V = 1364.35 (5) Å3 ,and Z = 2. The crystal structure of krasnoshteinite (solved using single-crystal data, R1 = 0.0557) is
unique. It is based upon corrugated layers of Al-centered octahedra connected via common vertices.
BO3 triangles and BO2 (OH)2 tetrahedra share a common vertex, forming insular [B2 O4 (OH)2 ]4−groups (this is a novel borate polyanion) which are connected with Al-centered octahedra via common
vertices to form the aluminoborate pseudo-framework. The structure is microporous, zeolite-like,with a three-dimensional system of wide channels containing Cl- anions and weakly bonded H2 O
molecules. The mineral is named in honour of the Russian mining engineer and scientist ArkadiyEvgenievich Krasnoshtein (1937–2009). The diﬀerences in crystal chemistry and properties between
high-temperature and low-temperature natural Al borates are discussed.
Keywords: krasnoshteinite; zeolite-like borate; hydrous aluminum chloroborate; new mineral; crystal
structure; microporous crystalline material; evaporitic salt rock; Verkhnekamskoe potassium salt
deposit; Perm Krai

Crystals 2020, 10, 301; doi:10.3390/cryst10040301

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Crystals 2020, 10, 301

1. Introduction
Boron is a rare chemical element in nature; its average content in the upper continental crustof the Earth is 0.0011 wt.% [1]. Despite its rarity, boron demonstrates diverse and complicatedmineralogy and mineral crystal chemistry. Three hundred minerals with species-deﬁning B are known,
including 160 borates and oxoborates [2], and some of these minerals form huge and sometimes
extremely rich deposits. Unusual geochemical and mineralogical features of boron are due to its very
bright crystal chemical individuality which causes strong ability to separate from other elements incrystal structures and form very speciﬁc, unique structural units [3,4]. Unlike boron, aluminum is oneof the most abundant elements in the lithosphere, however, natural Al borates are not numerous (onlytwelve borate and oxoborate minerals with species-deﬁning Al are known: see Discussion) and areclassiﬁed as rare minerals.
In the present article, we characterize the new mineral species krasnoshteinite (Cyrillic:крaснoштейнит), a hydrous aluminum chloroborate, and its unusual crystal structure. The mineral isnamed in honour of the Russian mining engineer and scientist, corresponding member of the Russian
Academy of Sciences, Arkadiy Evgenievich Krasnoshtein (1937–2009), an outstanding specialist in themining of potassium salts who made a great contribution to the exploitation of underground mines at
the Verkhnekamskoe deposit. Dr. Krasnoshtein was the founder (1988) and ﬁrst director of the Mining
Institute of the Ural Branch of the Russian Academy of Sciences in Perm. Both the new mineral and its
name have been approved by the Commission on New Minerals, Nomenclature and Classiﬁcation of
the International Mineralogical Association, IMA No. 2018-077.
The type specimen of krasnoshteinite was deposited in the systematic collection of the Fersman
Mineralogical Museum of the Russian Academy of Sciences (Moscow, Russia), under the catalogue
number 96274.
2. Materials and Methods
2.1. Occurrence, General Appearance, Physical Properties and Optical Data
Krasnoshteinite was found in the core of the borehole #2001, with a depth of 247.6–248 m, drilled in
the Romanovskiy area (30 km south of the city of Berezniki) of the Verkhnekamskoe potassium salt
deposit, Perm Krai, Western Urals, Russia. The general data on this well-known. Huge deposits are
given in monographs [5,6]. Krasnoshteinite occurs in halite-carnallite rock and is associated with dritsite(Li2 Al4 (OH)12 Cl2 ·3H2 O) [7], dolomite, magnesite, quartz, Sr-bearing baryte, kaolinite, potassic feldspar,congolite, members of the goyazite SrAl3 (PO4 )(PO3 OH)(OH)6 –woodhouseite CaAl3 (PO4 )(SO4 )(OH)6
series, ﬂuorite, hematite, and anatase. The new mineral was probably formed as a result of
diagenetic or post-diagenetic processes in halite-carnallite evaporitic rock of the Layer E of the
Verkhnekamskoe deposit.
Krasnoshteinite occurs as separate tabular to lamellar crystals of up to 0.06 × 0.25 × 0.3 mm(Figure 1a,b) and their parallel intergrowths (Figure 1c) embedded in carnallite and halite. In somecases, tiny crystals of krasnoshteinite overgrow its larger crystal in random orientations to form acrystal cluster (Figure 1d). Samples shown in Figure 1 were separated after dissolution of a hosthalite-carnallite rock in water.
Crystals of krasnoshteinite are ﬂattened on the ab plane. The pedions {010} and {0-10} and thepinacoid {100} are major lateral faces of the tabular crystals. The surface of the most developed “face”
of a crystal is typically complicated, rough, and demonstrating coarse or/and ﬁne striation along {100}(Figure 1); it is usually composed by several poorly formed faces belonging to the 0kl zone.
Krasnoshteinite is a transparent colorless mineral with a white streak and vitreous luster. It isbrittle, with a Mohs hardness is ca. of 3. Krasnoshteinite demonstrates perfect cleavage on {010} andan imperfect cleavage on {100}. A fracture is stepped (observed under the microscope). The mineral is
non-ﬂuorescent in the ultraviolet light. The density measured by ﬂotation in heavy liquids (bromoform

48

Crystals 2020, 10, 301+ dimethylformamide) is 2.11 (1) g/cm3 , and the density calculated using the empirical formula andthe unit-cell parameters determined from single-crystal X-ray diﬀraction data is 2.115 g/cm3 .
In plane polarized light, krasnoshteinite is colorless and non-pleochroic. It is optically biaxial (+),
α = 1.563 (2), β = 1.565 (2), γ = 1.574 (2) (589 nm). 2V (meas.) = 50 (10)◦ and 2V (calc.) = 51◦ .
Dispersion of optical axes is distinct, r > v. Optical orientation is: Y = b, and X = a.

Figure 1. Separate crystals (a,b) and crystal clusters ((c,d): small crystals overgrow large crystal) of
krasnoshteinite. Blocky crystals of dolomite and distorted quartz crystal are observed in (b).

2.2. Chemical Composition
The chemical composition of krasnoshteinite was studied using a Jeol JSM-6480LV scanningelectron microscope equipped with an INCA-Wave 500 wavelength-dispersive spectrometer (Laboratoryof Analytical Techniques of High Spatial Resolution, Dept. of Petrology, Moscow State University).
Electron microprobe analyses were obtained in the wavelength-dispersive spectroscopy mode (20 kV
and 20 nA; the electron beam was rastered to the 5 × 5 μm area to avoid damage of the highly hydrated
mineral) and gave contents of Al, Si, O, and Cl. The standards used were: Al2 O3 (Al), wollastonite (Si),YAl3 (BO3 )4 (O), and NaCl (Cl). The contents of other elements with atomic numbers higher than 8 are
below detection limits.
The presence of signiﬁcant amount of chlorine in krasnoshteinite prevents the quantitative
determination of boron by electron microprobe, due to the overlap of X-ray emission lines of the K
series of B with L lines of Cl. The boron content was determined using ICP-MS. The measurements
were carried out with the Element-2 (Thermo Fisher Scientiﬁc) instrument which has high resolution(that avoids interference of components) and sensitivity. Several crystals of the mineral were dissolvedin 10 cm3 of 3% HNO3 solution (Merck, Suprapur® ) in deionized water (EasyPure). Since the mass ofthe mineral was too small for accurate weighing, we have determined contents of B and Al in relative
units and further used averaged Al content, obtained by electron microprobe, for B content calculation.
The obtained value is in good agreement with the boron content determined from the crystal structure
reﬁnement. Contents of Li and Be in krasnoshteinite are below detection limits.
49

Crystals 2020, 10, 301

H2 O was not analysed because of the paucity of material. Hydrogen (H2 O) content was calculated
based on the structure data (see below) and taking into account the charge balance requirement.
The analytical total is close to 100 wt.% (Table 1) that demonstrates a good agreement between electron
microprobe data for Al, Si, O, and Cl, ICP-MS data for B and calculated value for H. The correctnessof the obtained chemical data was also conﬁrmed by the superior value of the Gladstone–Dalecompatibility index [8]: 1 − (Kp /Kc ) = 0.003 (superior) with measured density value, or 0.006 (superior)with calculated density value.
CO2 was not analysed because the structure data showed the absence of this constituent.
The absence of gas release in HCl (see below) also indicated that krasnoshteinite does not containcarbonate groups.
2.3. Single-Crystal X-ray Diﬀraction and Crystal Structure Determination
Single-crystal X-ray diﬀraction data were collected by means of an Xcalibur S CCD diﬀractometer(Dept. of Crystallography and Crystal Chemistry, Faculty of Geology, Moscow State University)
operated at 40 kV and 50 mA using MoKα radiation. A full sphere of three-dimensional data wascollected. Data reduction was performed using CrysAlisPro Version 1.171.37.35 [9]. The data werecorrected for Lorentz factor and polarization eﬀects. The crystal structure was solved and reﬁned with
the ShelX program package using direct methods [10].
2.4. Powder X-ray Diﬀraction
Powder X-ray diﬀraction data were collected by means of a Rigaku R-Axis Rapid II diﬀractometer(XRD Resource Center, St. Petersburg State University) equipped with a rotating anode X-ray source
and a curved image plate detector (Debye-Sсherrer geometry, d = 127.4 mm, CoKα, λ = 1.79021 Å).
The data were integrated using the software package Osc2Tab/SQRay [11]. The unit-cell parameters
were reﬁned from the powder data using the Pawley method and Topas software [12].
3. Results
3.1. Chemical Data
Chemical composition of krasnoshteinite is given in Table 1. The empirical formula calculatedon the basis of O + Cl = 33 atoms per formula unit is H32.47 Al7.87 Si0.01 B2.03 Cl3.79 O29.71 or,after recalculation of the anionic part and taking into account crystal-structure data,(Al7.87 Si0.01 )Σ7.88 [B2.03 O4 (OH)2 ][(OH)15.74 (H2 O)0.26 ]Σ16 [(Cl3.79 (OH)0.21 ]Σ4 ·7H2 O. The ideal formula isAl8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O, which requires Al 24.65, B 2.47, Cl 16.19, H 3.69, O 53.00, total 100 wt.%,or, in oxides, Al2 O3 46.58, B2 O3 7.95, H2 O 32.93, Cl 16.19, –O=Cl –3.65, total 100 wt.%.
Table 1. Chemical composition (in wt.%) of krasnoshteinite in elements and in oxides (Al, Si, Cl, and O:
average data for 7 spot electron-microprobe analyses).
Data in Elements, with Measured O Content
Constituent

Wt.%

B

2.53

Al

24.49

23.79–24.96

Si

0.03

0.02–0.05

Cl

15.48

15.01–16.69

0.59

H(calc.)

3.75

O

53.92

Total

100.20

Range

Stand. Dev.

52.43–56.62

Constituent

Wt.%

B 2 O38.15

0.41

Al2 O3

46.27

0.01

SiO2

0.06

Cl

15.48

H2 O (calc.)

33.74

1.42

50

Data Recalculated in Oxides

–O=Cl

−3.50

Total

100.20Crystals 2020, 10, 301

Krasnoshteinite is insoluble in water and slowly dissolves in cold diluted HCl without eﬀervescence.
The obtained solution shows characteristic color reaction, with quinalizarin clearly indicatingboron presence.
3.2. Single-Crystal X-ray Diﬀraction and Crystal Structure Determination
The single-crystal X-ray diﬀraction data were indexed in the P21 space group with the following
unit-cell parameters: a = 8.73980 (19), b = 14.4129 (3), c = 11.3060 (3) Å, β = 106.665 (2)◦ , and V =1364.35 (5) Å3 (Table 2). Details on data collection and structure reﬁnement are also given in Table 2.
The ﬁnal structure reﬁnement converged to R1 = 0.0557 for 6142 unique observed reﬂections with
I > 2σ(I). The H atoms of OH groups and H2 O molecules were located from the diﬀerence Fourier
synthesis. The studied crystal is microtwinned with the inversion center as a twin operation: twinningby merohedry Class I [13] with the twin domain ratio of 68/32. Coordinates and equivalent thermal
displacement parameters of atoms are given in Table 3, selected interatomic distances in Table 4,and H-bonding in Table 5. Other crystal structure information for krasnoshteinite has been deposited
with the Editors and is available as Supplementary Materials (see below): anisotropic displacement
parameters of non-hydrogen atoms in the structure are presented in Table S1 and bond valencecalculations in Table S2; crystallographic information ﬁle (CIF) is given as a separate Supplementary
Material. Bond-valence parameters for Al-O and B-O were taken from [14] and for H-bonding
from [15,16].
Table 2. Crystal data, data collection information and structure reﬁnement details for krasnoshteinite.
FormulaFormula weight
Temperature, K
Radiation and wavelength, Å
Crystal system, space group, Z
Unit cell dimensions, Å/◦V, Å3
Absorption coeﬃcient μ, mm−1
F000
Crystal size, mm
Diﬀractometerθ range for data collection, ◦ /Collection mode
Index ranges
Reﬂections collected
Independent reﬂections
Independent reﬂections with I > 2σ(I)Data reduction
Absorption correction
Structure solution
Reﬁnement method
Number of reﬁned parameters
Final R indices [I > 2σ(I)]R indices (all data)
GooF
Largest diﬀ. peak and hole, e/Å3

Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O
875.52293 (2)
MoKα; 0.71073
Monoclinic, P21 , 2
a = 8.73980 (19)b = 14.4129 (3) β = 106.665 (2)c = 11.3060 (3)1364.35 (5)
0.809
892
0.06 × 0.16 × 0.17
Xcalibur S CCD
2.81 – 28.28/full sphere−11 ≤ h ≤ 11, −19 ≤ k ≤ 19, −15 ≤ l ≤ 15
23,8076773 (Rint = 0.0759)
6142
CrysAlisPro, Agilent Technologies, v. 1.171.37.35 [9]multi-scan
Empirical absorption correction using spherical harmonics,
implemented in SCALE3 ABSPACK scaling algorithm
direct methods
full-matrix least-squares on F2
485
R1 = 0.0557, wR2 = 0.1157
R1 = 0.0633, wR2 = 0.1196
1.107
0.60 and −0.5651

Crystals 2020, 10, 301

Table 3. Coordinates and equivalent displacement parameters (Ueq , in Å2 ) of atoms in krasnoshteinite.
Site

X

Y

Z

U eq *

Al1

0.16497(17)0.29061(10)0.29761(12)0.0100(3)Al2−0.15081(17)0.71563(9)−0.04927(12)0.0097(3)Al30.11906(16)0.67894(10)−0.16964(12)0.0098(3)Al4−0.15717(16)0.11133(9)−0.07805(12)0.0089(3)Al50.50122(18)0.16162(11)−0.01146(13)0.0101(2)Al6

0.11383(16)0.13955(10)−0.19674(12)0.0103(3)Al70.16655(16)0.52591(10)0.31878(12)0.0094(3)Al80.00144(18)0.65411(11)0.51556(12)0.0112(3)B10.2145(5)0.4153(4)0.0964(4)0.0102(9)B20.7283(6)−0.0017(4)0.0864(5)0.0123(10)Cl10.62010(14)0.41682(12)0.28131(13)0.0305(3)Cl20.24073(14)0.91650(11)0.04320(14)0.0318(3)Cl30.61509(17)0.82518(11)0.35183(13)0.0321(3)Cl40.5240(2)0.57208(11)0.64519(13)0.0371(4)O10.1226(4)0.1723(2)0.2021(3)0.0096(6)H10.182(5)0.122(2)0.247(4)

0.012

O2

0.3923(4)0.2700(3)0.3509(3)0.0175(8)H2A

0.444(5)0.2119(17)0.369(5)

0.021

H2B

0.451(5)0.302(3)0.305(4)

0.021

O3

0.1966(3)0.4058(3)0.3715(3)0.0145(6)H30.232(5)0.402(4)0.4585(10)

0.017

O4

0.0817(4)0.7234(2)−0.0141(3)0.0103(6)H40.128(6)0.7797(19)0.022(4)

0.012

O5

0.3314(4)0.7079(2)−0.1198(3)0.0142(7)H50.341(6)0.7694(14)−0.144(4)

0.017

O6

−0.1336(4)0.7504(2)0.1100(3)0.0112(6)H6−0.149(6)0.8122(14)0.133(4)

0.013

O7

0.0760(4)0.1003(2)−0.0385(3)0.0109(7)H70.137(5)0.047(2)−0.008(4)

0.013

O8

0.6348(4)0.1519(3)−0.1168(3)0.0143(7)H80.588(6)0.165(4)−0.2011(16)

0.017

O9

0.3603(4)0.1780(2)0.0857(3)0.0104(6)H90.396(6)0.146(3)0.161(2)

0.012

O10

−0.0590(4)0.2960(2)0.2401(3)0.0119(7)H10−0.113(5)0.3394(7)0.179(2)

0.014

O11

−0.1825(4)0.8288(2)−0.1385(3)0.0124(7)O120.5632(4)0.0394(2)0.0507(3)0.0130(7)H120.479(4)−0.003(3)0.043(5)

0.016

O13

0.8084(4)−0.0053(2)−0.1569(3)0.0092(6)O14

0.1514(4)0.2235(2)0.4336(3)0.0124(7)52Crystals 2020, 10, 301

Table 3. Cont.
Site

X

Y

Z

H14

0.223(4)0.246(3)0.5072(14)U eq *
0.015

O15

0.7339(3)−0.0820(3)0.0047(3)0.0143(6)O16

0.5396(4)0.7888(2)0.0675(3)0.0165(7)H16A

0.625(4)0.829(3)0.065(5)

0.020

H16B

0.478(5)0.824(3)0.108(4)

0.020

O17

0.0990(4)0.6243(2)0.6804(3)0.0120(7)H170.144(5)0.5646(16)0.6811(15)0.0140.0201(7)O18

0.7769(4)−0.0354(3)0.2134(3)H18

0.706(5)−0.070(3)0.245(4)

0.024

O19

−0.1261(4)0.1448(2)0.7681(3)0.0092(6)H19−0.172(6)0.198(2)0.723(4)

0.011

O20

−0.1619(4)0.0707(2)0.0752(3)0.0088(6)O210.3296(4)0.1147(2)−0.1411(3)0.0121(7)H210.364(6)0.0579(19)−0.166(4)

0.015

O22

0.1002(4)0.1859(2)0.6494(3)0.0147(7)H220.180(4)0.228(3)0.642(5)

0.018

O23

0.0542(4)0.0215(2)0.7411(3)0.0123(7)H230.106(5)−0.030(2)0.717(4)

0.015

O24

0.1359(5)0.0438(3)0.4965(3)0.0214(8)H24A

0.214(5)0.022(4)0.567(2)

0.026

H24B

0.150(6)0.010(3)0.429(3)

0.026

O25

−0.1540(4)0.0819(2)0.5351(3)0.0145(7)H25−0.248(4)0.088(4)0.470(3)

0.017

O26

0.3966(4)0.5521(2)0.3674(3)0.0173(8)H26A

0.458(5)0.502(2)0.352(4)

0.021

H26B

0.444(5)0.566(3)0.4523(16)

0.021

O27

−0.1362(4)0.2617(3)0.4649(3)0.0230(8)H27A−0.134(6)0.308(3)0.408(4)

0.028

H27B

−0.223(4)0.274(3)0.496(4)

0.028

O28

0.7250(5)0.2946(3)0.6403(4)0.0354(10)H28A

0.617(3)0.289(4)0.641(6)

0.043

H28B

0.760(6)0.351(3)0.684(5)

0.043

O29

0.1131(6)0.9559(4)0.2765(5)0.0571(15)H29A

0.127(8)0.952(5)0.197(3)

0.069

H29B

0.010(5)0.930(6)0.269(6)0.069* The positions of H atoms were located from the diﬀerence Fourier map and reﬁned with O-H and H-H distances
softly restrained to 0.95(1) and 1.50(1) Å, respectively, to hold near-optimal geometry. Uiso (H) = 1.2 Ueq (O).

53

Crystals 2020, 10, 301

Table 4. Selected interatomic distances (Å) in the structure of krasnoshteinite.
Al1 - O3
- O14
- O10
- O2
- O11
- O1
<Al1 – O>
Al2 - O6
- O9
- O11
- O1
- O7
- O4
<Al2 – O>
Al3 - O5
- O17
- O20
- O10
- O4
- O1
<Al3 – O>
Al4 - O8
- O20
- O13
- O19
- O4
- O7
<Al4 - O>
B1 - O15
- O13
- O11
<B1 – O>1.843(4)1.849(3)1.879(3)1.927(4)1.929(3)1.996(3)1.904
1.833(3)1.839(3)1.896(3)1.918(3)1.951(3)1.959(3)1.899
1.827(4)1.831(3)1.867(3)1.875(4)1.984(3)2.040(3)1.904
1.839(3)1.841(3)1.886(3)1.898(3)1.932(3)1.963(3)1.893
1.344(5)1.377(7)1.392(6)1.371

Al5 - O5
- O9
- O21
- O8
- O12
- O16
<Al5 – O>
Al6 - O22
- O21
- O6
- O23
- O7
- O19
<Al6 – O>
Al7 - O3
- O23
- O25
- O19
- O13
- O26
<Al7 – O>
Al8 - O17
- O22
- O14
- O25
- O27
- O24
<Al8 – O>
B2 - O20
- O18
- O15
- O12
<B2 – O>1.882(4)1.885(3)1.896(3)1.897(3)1.916(4)1.939(4)1.903
1.835(3)1.844(3)1.857(3)1.858(4)1.992(3)2.021(3)1.901
1.825(4)1.853(3)1.869(3)1.956(3)1.956(3)1.963(4)1.904
1.864(3)1.878(3)1.886(3)1.905(4)1.921(4)1.973(4)1.905
1.448(6)1.459(6)1.490(6)1.504(6)1.475

Table 5. Hydrogen-bond geometry (Å,◦ ) in the structure of krasnoshteinite.
D–H

H···A

D···A

∠(D – Н···A)

O1 - H1···O29
O1 - H1···Cl40.946(10)2.52(4)2.61(3)

3.237(7)3.398(3)

133(4)
141(4)

O2 - H2A···Cl4
O2 - H2B···Cl1

0.947(10)0.947(10)2.05(2)2.28(2)2.942(4)3.156(4)

157(5)
153(4)

O3 - H3···Cl30.945(10)2.44(3)3.300(3)

151(5)

O4 - H4···Cl2

0.947(10)2.185(19)3.096(4)

161(4)

O5 - H5···Cl10.940(10)2.71(2)

3.609(4)

161(4)

O6 - H6···O15
O6 - H6···O18

0.946(10)2.15(4)2.533(15)2.794(5)3.468(5)

124(4)
170(4)
171(5)

D – H···A

O7 - H7···Cl2

0.945(10)2.093(14)3.030(4)O8 - H8···O26

0.942(10)2.52(5)

3.119(5)

121(4)

O9 - H9···Cl40.943(10)2.352(11)3.294(3)

177(4)

O10 - H10···Cl2

0.948(10)2.684(17)3.589(3)

160(3)

O12 - H12···Cl2

0.946(10)2.380(15)3.309(3)

168(4)

O14 - H14···Cl3

0.943(10)2.134(12)3.059(3)

167(3)

O16 - H16A···O15

0.949(10)1.84(2)2.747(5)

158(5)

54

Crystals 2020, 10, 301

Table 5. Cont.
D – H···A

D–H

H···A

D···A

∠(D – Н···A)

O16 - H16A···O6
O16 - H16B···Cl2
O16 - H16B···Cl30.949(10)0.952(10)0.952(10)2.32(4)2.40(4)2.67(5)2.812(5)3.143(4)3.135(4)111(3)
135(4)
111(3)

O17 - H17···O18

0.944(10)1.873(13)2.677(5)141.5(19)O18 - H18···Cl30.945(10)2.22(2)

3.123(4)

160(5)

O19 - H19···O28

0.946(10)1.770(12)2.714(5)

175(5)

O21 - H21···Cl1

0.941(10)2.441(18)3.353(4)

163(4)

O22 - H22···Cl30.946(10)2.259(12)3.200(4)

173(4)

O23 - H23···Cl1
O23 - H23···O10

0.944(10)2.51(3)2.61(4)3.295(3)3.257(4)

140(4)
126(4)

O24 - H24A···Cl1
O24 - H24B···O29

0.940(10)0.942(10)2.43(2)
1.835(16)3.337(4)2.747(6)

161(4)
162(4)

O25 - H25···Cl40.941(10)2.40(2)

3.294(4)160(4)

O26 - H26A···Cl1
O26 - H26B···Cl4

0.951(10)0.952(10)2.187(18)2.092(16)

3.107(4)3.030(4)

163(5)
168(5)

O27 - H27A···O10
O27 - H27A···Cl1
O27 - H27B···O28

0.935(10)0.940(10)2.19(4)2.71(4)
1.84(3)2.853(5)3.363(4)2.648(5)

128(4)
128(3)
142(4)

O28 - H28A···Cl3
O28 - H28B···O29

0.951(10)0.951(10)2.12(2)
1.86(3)3.031(4)2.745(7)

161(5)
154(5)

O29 - H29A···Cl2
O29 - H29B···O18

0.946(10)0.951(10)2.29(3)2.02(6)

3.196(6)2.820(6)160(6)
141(7)

3.3. Powder X-ray Diﬀraction
The indexed powder X-ray diﬀraction data are given in Table S3 in Supplementary Materials (seebelow). The powder X-ray diﬀraction pattern of krasnoshteinite is unique and can be used as a good
diagnostic tool of the mineral. The parameters of a monoclinic unit cell reﬁned from the powder data
are as follows: a = 8.740 (4), b = 14.409 (4), c = 11.316 (4) Å, β = 106.58 (3)◦ , and V = 1366 (1) Å3 .
4. Discussion
The crystal structure of krasnoshteinite (Figure 2) is unique. It is based upon the (010) corrugatedlayers of Al-centered octahedra connected via common vertices to form a pseudo-framework.
There are eight crystallographically non-equivalent octahedrally coordinated Al sites: Al(1) andAl(7) cations center octahedra AlO(OH)4 (OH2 ), Al(2,3) – AlO(OH)5 , Al(4) – AlO2 (OH)4 , Al(5) –Al(OH)5 (OH2 ), Al(6) – Al(OH)6 , and Al(8) – Al(OH)4 (OH2 )2 . These Al-centered octahedra play
diﬀerent structural roles. Al(1–4)- and Al(6,7)-centered octahedra share edges to form six-memberedclusters. Al(8)-centered octahedra link adjacent clusters along the c axis sharing two corners with eachcluster, while Al(5)-centered octahedra play the same role linking the clusters along the a axis to form
octahedral layers (Figure 3a). Adjacent layers are connected via the common O(3) vertex of Al(7)- andAl(1)-centered octahedra, forming the three-dimensional octahedral motif.
Boron atoms occupy two crystallographically non-equivalent sites and center B(1)O3 triangles
and B(2)O2 (OH)2 tetrahedra, which share a common vertex to form insular [B2 O4 (OH)2 ]4− groups(Figure 3b). According to the classiﬁcation of fundamental building blocks (FBB) in borates [17,18],
FBB in krasnoshteinite is 1Δ:Δ, i.e., the block with one triangle and one tetrahedron sharing
corner. Krasnoshteinite is the ﬁrst borate with such FBBs. These groups are connected with clusters
of Al-centered octahedra via common vertices. Thus, a BO3 triangle shares one O vertex with a55

Crystals 2020, 10, 301

B-centered tetrahedron, one vertex with two Al-centered octahedra [Al(4) and Al(7)] of the layer,
and one vertex with Al(1)- and Al(2)-centered octahedra of an adjacent layer (thus reinforcing thelinkage between neighboring octahedral layers). A BO2 (OH)2 tetrahedron shares one O vertex with a
BO3 triangle, one O vertex with two Al-centered octahedra [Al(3) and Al(4)], and one O=OH vertex
with a Al(5)-centered octahedron; all Al(3,4,5) octahedra belong to the same layer. The resultant
aluminoborate pseudo-framework contains three-membered [2B + Al] rings. Such Al-B-O units
are known in the porous aluminoborate frameworks as being crucial to stabilizing them [19–21].
The same conﬁguration of the three-membered [2B + Al] ring was described in the crystal structuresof satimolite, KNa2 (Al5 Mg2 )[B12 O18 (OH)12 ](OH)6 Cl4 ·4H2 O [22], and synthetic porous Al borates,
PKU-3 H24.3 Al9 B18 O51 Cl3.3 ·6.8H2 O [23] and PKU-8 (H18 Al7 B12 O36 )Cl3 (NaCl)2.4 ·6.5H2 O [19].
The aluminoborate pseudo-framework in krasnoshteinite is microporous and zeolite-like (Figure 2).
The three-dimensional system of wide channels contains Cl− anions and H2 O molecules. Together with
OH groups and H2 O molecules belonging to Al- and B-centered polyhedra, they form a complicatedsystem of hydrogen bonds (Table 5).
Among 160 natural borates and oxoborates, known to date as valid mineral species,
only twelve minerals contain species-deﬁning Al, namely aluminomagnesiohulsite, (Mg,Fe2+ )2(Al,Mg,Sn)(BO3 )O2 ; jeremejevite, Al6 (BO3 )5 (F,OH)3 ; johachidolite, CaAlB3 O7 ; krasnoshteinite,
Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O; londonite, CsAl4 Be4 B12 O28 ; mengxianminite, (Ca,Na)2 Sn2 (Mg,Fe)3
Al8 [(BO3 )(BeO4 )O6 ]2 ; painite, CaZrAl9 (BO3 )O15 ; peprossiite-(Ce), CeAl2 B3 O9 ; pseudosinhalite,
Mg2 Al3 O(BO4 )2 (OH); rhodizite, KAl4 Be4 B12 O28 ; satimolite, KNa2 (Al5 Mg2 ) [B12 O18 (OH)12 ](OH)6 Cl4 ·4H2 O; and sinhalite, MgAl(BO4 ) [2]. Satimolite and krasnoshteinite are low-temperature(LT) borates formed in evaporitic rocks, whereas the other ten minerals are known only inhigh-temperature (HT) geological formations: granitic pegmatites, HT metamorphic or metasomatic
rocks, or post-volcanic HT assemblages. These HT Al borates and oxoborates do not contain H2 O
molecules and have compact crystal structures that cause high hardness and mechanical and chemical
stability in the majority of them. Data on the Mohs hardness of peprossiite-(Ce) and pseudosinhalite
are absent in literature, aluminomagnesiohulsite has the Mohs hardness value of 6, and jeremejevite,
johachidolite, londonite, mengxianminite, painite, rhodizite, and sinhalite demonstrate the Mohs
hardness values between 7 to 8 [24]. All ten minerals have crystal structures with only one typeof B-centered polyhedra, BO3 triangles [aluminomagnesiohulsite, jeremejevite, mengxianminite,and painite], or BO4 tetrahedra [johachidolite, londonite, peprossiite-(Ce), pseudosinhalite, rhodizite,
and sinhalite], without OH groups coordinating B [25]. In LT formations, the crystal chemistry
and properties of borate minerals with species-deﬁning Al change drastically. Satimolite andkrasnoshteinite are highly hydrated chloroborates which have low Mohs hardness values, 2 (satimolite)
or 3 (krasnoshteinite), and dissolve even in diluted HCl. They contain complex borate polyanions
composed of both B-centered triangles and tetrahedra with OH groups which participate in thetetrahedra. The structures of both satimolite and krasnoshteinite are microporous and zeolite-like.
Thus, under LT conditions, aluminum octahedra in borates became a building unit of open-workaluminoborate structure motifs.

56

Crystals 2020, 10, 301

Figure 2. The crystal structure of krasnoshteinite in three projections. The unit cell is outlined.

57

Crystals 2020, 10, 301baFigure 3. Octahedral layer (a) and insular [B2 O4 (OH)2 ]4− group (b) in the structure of krasnoshteinite.
For legend, see Table 3.

5. Conclusions
This paper is devoted to the new mineral species, krasnoshteinite. No mineral or synthetic
compound related to it, in terms of crystal structure, has been found in literature and databases.
Krasnoshteinite contains an earlier unknown borate polyanion, the insular [B2 O4 (OH)2 ]4− groupconsisting of one BO3 triangle and one BO2 (OH)2 tetrahedron sharing corner. It was a surprise thatsuch a simple anionic complex turned out novel for borates, both natural and synthetic, which is oneof the most structurally diverse and best-studied classes of chemical compounds.
Krasnoshteinite (Al8 [B2 O4 (OH)2 ](OH)16 Cl4 ·7H2 O) is the second, after jeremejevite (Al6 (BO3 )5(F,OH)3 ), natural borate with only Al as a metal cation; and the second, after satimolite (KNa2 (Al5 Mg2 )[B12 O18 (OH)12 ](OH)6 Cl4 ·4H2 O), mineral with a zeolite-like aluminoborate framework motif in the
structure. Due to the presence of a three-dimensional system of wide channels containing Cl− anions
and weakly bonded H2 O molecules, krasnoshteinite is of interest as a potential prototype of a novel
family of microporous crystalline materials without large cations.
Borate minerals with species-deﬁning Al formed in high-temperature and low-temperature
geological formations are strongly diﬀerent in crystal chemistry and physical and chemical properties.
The high-temperature Al borates and oxoborates do not contain H2 O molecules, have compact
crystal structures, and are typically characterized by high hardness and mechanical and chemical
stability. Their crystal structures contain only one type of B-centered polyhedral, BO3 triangles, or BO4
tetrahedra. Unlike them, the low-temperature Al borates are highly hydrated, have low hardness,
and are chemically unstable. They contain complex borate polyanions composed of both triangular andtetrahedral borate polyhedra with OH groups which participate in boron tetrahedra. Their structuresare zeolite-like, being based upon open-work aluminoborate motifs.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/4/301/s1.
Author Contributions: Conceptualization, I.V.P., N.V.Z., I.I.C., and D.Y.P.; Methodology, I.V.P., N.V.Z., and S.N.B.;
Investigation, I.V.P., N.V.Z., I.I.C., E.P.C., D.I.B., V.O.Y., Y.V.B., and I.L.; Writing—Original Draft Preparation, I.V.P.,
N.V.Z., I.I.C., and D.Y.P.; Writing—Review and Editing, I.V.P. and N.V.Z.; Visualization, N.V.Z. All authors haveread and agreed to the published version of the manuscript.
Funding: This work was supported by the Russian Foundation for Basic Research, grants 18-29-12007-mk (I.V.P.,
N.V.Z., and V.O.Y. for electron microprobe, XRD, and crystal structure studies) and 18-05-00046 (I.I.C. and E.P.C.
for ﬁeldwork and SEM studies).

58

Crystals 2020, 10, 301

Acknowledgments: The research has been carried out using facilities at the XRD Research Center of St. Petersburg
State University in part of powder XRD study.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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crystals
ArticleCrystal Chemistry of Stanﬁeldite, Ca7M 2Mg9(PO4)12(M = Ca, Mg, Fe2+), a Structural Base of
Ca3Mg3(PO4)4 Phosphors
Sergey N. Britvin 1,2, *, Maria G. Krzhizhanovskaya 1 , Vladimir N. Bocharov 3 andEdita V. Obolonskaya 4
1
2
3
4

*

Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University,
Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia; mariya.krzhizhanovskaya@spbu.ru
Nanomaterials Research Center, Kola Science Center of Russian Academy of Sciences, Fersman Str. 14,
184209 Apatity, Russia
Centre for Geo-Environmental Research and Modelling, Saint-Petersburg State University,
Ulyanovskaya ul. 1, 198504 St. Petersburg, Russia; bocharov@molsp.phys.spbu.ru
The Mining Museum, Saint Petersburg Mining University, 2, 21st Line, 199106 St. Petersburg, Russia;
musmet11@yandex.ru
Correspondence: sergei.britvin@spbu.ru

Received: 1 May 2020; Accepted: 25 May 2020; Published: 1 June 2020

Abstract: Stanﬁeldite, natural Ca-Mg-phosphate, is a typical constituent of phosphate-phosphideassemblages in pallasite and mesosiderite meteorites. The synthetic analogue of stanﬁeldite isused as a crystal matrix of luminophores and frequently encountered in phosphate bioceramics.
However, the crystal structure of natural stanﬁeldite has never been reported in detail, and thedata available so far relate to its synthetic counterpart. We herein provide the results of a study of
stanﬁeldite from the Brahin meteorite (main group pallasite). The empirical formula of the mineralis Ca8.04 Mg9.25 Fe0.72 Mn0.07 P11.97 O48 . Its crystal structure has been solved and reﬁned to R1 = 0.034.
Stanﬁeldite from Brahin is monoclinic, C2/c, a 22.7973(4), b 9.9833(2), c 17.0522(3) Å, β 99.954(2)◦ ,V 3822.5(1)Å3 . The general formula of the mineral can be expressed as Ca7 M2 Mg7 (PO4 )12 (Z = 4),
where the M = Ca, Mg, Fe2+ . Stanﬁeldite from Brahin and a majority of other meteorites correspondto a composition with an intermediate Ca≈Mg occupancy of the M5A site, leading to the overall
formula ~Ca7 (CaMg)Mg9 (PO4 )12 ≡ Ca4 Mg5 (PO4 )6 . The mineral from the Lunar sample “rusty rock”
66095 approaches the M = Mg end member, Ca7 Mg2 Mg9 (PO4 )12 . In lieu of any supporting analytical
data, there is no evidence that the phosphor base with the formula Ca3 Mg3 (PO4 )4 does exist.
Keywords: stanﬁeldite; phosphate; crystal structure; merrillite; meteorite; pallasite; mesosiderite;
luminophore; bioceramics; powder diﬀraction; Raman spectroscopy

1. Introduction
It is known that the speciation of chemical elements in meteoritic substance signiﬁcantly
diﬀers from their speciation in contemporary terrestrial lithosphere [1]. Concerning phosphorus,
the main geochemical factors governing the diversity of terrestrial phosphorus-bearing minerals
are (1) highly oxidative conditions typical of the present Earth and (2) the aquatic environment,
which dramatically multiplies the number of possible pathways for phosphate geosynthesis. Contrary to
Earth, the reductive and (in general) water-free conditions that accompanied the formation andearly evolution of celestial bodies determined the limited number of meteoritic phosphorus-bearing
minerals [2].
The most common meteoritic phosphates are the minerals related to the join
merrillite–ferromerrillite, Ca9 NaMg(PO4 )7 –Ca9 NaFe2+ (PO4 )7 [3,4]. They are the typical accessories of
Crystals 2020, 10, 464; doi:10.3390/cryst10060464

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Crystals 2020, 10, 464

ordinary chondrites, lunar rocks, martian meteorites, iron and stony-iron meteorites [3–6]. Chlorapatite,
Ca5 (PO4 )3 Cl, is the second abundant phosphate in meteorites [2]. A series of Mg-rich phosphates arecharacteristic of stony-iron meteorites—pallasites and mesosiderites [6,7]. These minerals usually occur
in association with schreibersite, (Fe,Ni)3 P, and because of that, they are used for the assessment ofphosphide–phosphate redox equilibria [8]. Stanﬁeldite, ~Ca4 Mg5 (PO4 )6 , is the most common phosphatein the given assemblages [6,7]. The mineral was discovered in the Estherville mesosiderite [9] andis recognized in all well-studied pallasites [6,7,9–13], several mesosiderites [14,15] and even in the
Lunar samples [16]. Being one of a very few Ca-rich phases occurring in pallasites, stanﬁeldite acts as acarrier of rare-earth elements substituting for Ca, and thereby is used in the studies of REE distribution
among these meteorites [10,17]. Based on the overall observations and experimental data, stanﬁeldite
can be regarded a late-stage cumulate of silicate-phosphate melts [18]. It is noteworthy that, in spite ofan ordinary, “rock-forming” set of elements in the chemical composition, stanﬁeldite has never beenencountered in terrestrial rocks. However, the phosphate identiﬁable as stanﬁeldite was reported
as a constituent of prehistoric slags in Tyrol, Austria [19], and phosphorus-doped basaltic melts [20].
Stanﬁeldite of technogenic origin was detected as a component of bone-repairing bioceramics [21–28]
and incinerated phosphate-based fertilizers [29–32].
In view of the notable role of stanﬁeldite in the mineralogy of pallasites, it looks unusualthat the data on its chemical composition are rather ambiguous. The formula was ﬁrst reported
as Ca4 Mg5 (PO4 )6 [9], but later on, the variations in the Ca/Mg ratio were shown to exist [12],and in many cases, stanﬁeldite formula is oversimpliﬁed as Ca3 Mg3 (PO4 )4 , e.g., Reference [33].
Synthetic Ca3 Mg3 (PO4 )4 was reported as a phase in the Ca3 (PO4 )2 –Mg3 (PO4 )2 system [34],and nowadays, the compounds having the same inferred formula are widely explored as luminophores(e.g., References [35–39]). However, the powder XRD (X-ray diﬀraction) data given in theseworks [35–39] refer not to Ca3 Mg3 (PO4 )4 but to the compound Ca7 Mg9 (Ca,Mg)2 (PO4 )12 [40],which was erroneously mislabeled as “Ca3 Mg3 (PO4 )4 ” both in ICSD and ICDD databases. Synthetic
Ca7 Mg9 (Ca,Mg)2 (PO4 )12 was shown to have the same cell metrics as natural stanﬁeldite but it
crystallizes in a diﬀerent space group [9,40]. The latter discrepancy was discussed by Steele andOlsen in the abstract devoted to a crystal structure of natural stanﬁeldite [41]. However, no further
structural data were provided by these authors, and as a consequence, no crystal structure of natural
stanﬁeldite is available so far. In the course of a research of phosphate–phosphide assemblages of ironand stony-iron meteorites, we have found well-crystallized stanﬁeldite in the Brahin meteorite andcarried out the detailed study of this mineral. We herein present the results and try to resolve someambiguities related to a crystal chemistry of natural stanﬁeldite and its synthetic analogues.
2. Stanﬁeldite in the Brahin Pallasite
Brahin is a meteorite related to the main-group pallasites. It was ﬁrst found in 1810 as two
fragments (masses) of total weight ~80 kg at the Kaporenki village, Brahin district, Belarus. Since then,a few larger masses were recovered in the same district in 1968 and 2002. Nowadays, the total known
weight of Brahin exceeds 800 kg [42]. Like other main-group pallasites, Brahin consists of round, nut-likeand fragmented olivine crystals embedded into the Fe-Ni metal matrix. The less-common minerals are
represented by schreibersite-nickelphosphide (Fe3 P-Ni3 P), chromite, troilite and daubreelite, FeCr2 O4 .
The speciﬁc feature of oxygen-bearing minerals of Brahin is their depletion in Mn [12]. Phosphates
are comprised by merrillite, Ca9 NaMg(PO4 )7 , and stanﬁeldite. The crystal structure of iron-freemerrillite from this meteorite has been recently reported [4]. Stanﬁeldite in Brahin was studied
with respect to the occurrence of ﬁssion tracks [43]. A 1 × 2 mm grain of colorless stanﬁeldite wasfound in the centimeter-sized Brahin fragment kindly provided for the study by the Mining Museum,Saint Petersburg Mining University (specimen M65/2, which originates from the ﬁrst ﬁnd in 1810).
Stanﬁeldite and merrillite ﬁll up the pocket bound by the fragmented olivine grains, schreibersite and(Fe, Ni) metal (Figure 1).

62Crystals 2020, 10, 464

Figure 1. Stanﬁeldite (St) in the Brahin pallasite. Associated minerals: Ol, olivine; Mr, merrillite;
Crt, chromite; Schr, schreibersite; Km, kamacite, α-(Fe,Ni); Pl, plessite (ﬁne-grained aggregate of α- andγ-(Fe, Ni)). Polished section. SEM image of backscattered electrons. Image made by S.N.B.

3. Materials and Methods
A piece of the Brahin pallasite was polished and coated with a carbon ﬁlm for electron microprobe
study. SEM imaging (Figure 1) and microprobe analysis for the main elements were conductedby means of a CamScan 4 scanning electron microscope (SEM) (Cambridge, UK) equipped with a
LINK AN1000 energy-dispersive analyzer (LINK Analytical, CA, USA). The following standards
were used: chlorapatite (Ca-K, P-K), enstatite (Mg-K), hematite (Fe-K). The analysis was carried out
at 20 kV acceleration voltage, 0.8 nA beam current, 1 μm estimated beam diameter and 60 s liveacquisition time per spot. The check-up for minor constituents was performed with a Microspec
WDX-2 wavelength-dispersive X-ray spectrometer (Microspec Corporation, CA, USA) attached to thesame SEM. The Mn content was determined using Mn-Kα line (MnCO3 standard) at 20 kV and 15 nA,
whereas the contents of Ni, Co, Na, K and Si were found to lie below the detection limit (less than
0.05 wt.%).
For the purposes of the X-ray structural study, the grain of stanﬁeldite was extracted from thesection and crushed into a few fragments, which were examined under a polarizing microscopein the immersion oil. Several optically homogeneous grains were checked using a Rigaku Oxford
diﬀraction Xcalibur single-crystal diﬀractometer equipped with a ﬁne-focus sealed tube and graphite
monochromator (MoKα, 50 kV, 40 mA). It was found that all checked fragments are optically irresolvableintergrowths, each of them being composed of two or more domains misoriented within 5–10◦ . The best
selected two-domain grain (0.15 × 0.10 × 0.10 mm) was glued onto a plastic loop and subjected tofurther data collection. A hemisphere of reciprocal space was collected up to 70◦ at room temperature,and the details are provided in Table 1. Subsequent data processing routines (integration, scaling and
SHELX ﬁles setup) were performed by means of a CrysAlisPro software (Rigaku Oxford diﬀraction) [44].
The crystal structure has been solved using an intrinsic phasing approach and reﬁned by means of a
SHELX-2018 set of programs [45] incorporated into the Olex2 operation environment [46]. The detailsof structure reﬁnement are given in Table 1 and in the crystallographic information ﬁle (CIF) attachedto the Supplementary Materials (S1).

63

Crystals 2020, 10, 464

Table 1. Crystal data, single-crystal and Rietveld reﬁnement details for stanﬁeldite from Brahin.
Crystal Data:

Single Crystal

Rietveld Method

Crystal system, space group
Crystal size (mm)
a (Å)
b (Å)
c (Å)
β (◦ )
V (Å3 )Z
Dx (g cm−1 )Monoclinic, C2/c
0.15 × 0.10 × 0.10
22.7973(4)9.9833(2)17.0522(3)99.954(2)3822.5(1)4
2.990

Monoclinic, C2/cball Ø 0.15
22.8036(2)9.9832(1)17.0558(2)99.964(1)3824.2(1)4
2.988

Data collection and reﬁnement: Single Crystal
Diﬀractometer
Radiation
μ (mm−1 )No. of meas., independent and obs. [I > 2σ(I)] reﬂectionsh, k, l range
Rint , Rσ
R1 (|Fo |≥4σF ), wR2 , GoF
Δσmin, Δσmax (e Å−3 )Rigaku Oxford Diﬀraction Xcalibur EoS
MoKα
2.12
18,163, 5420, 3949
–27→32; –14→13; –23→23
0.044, 0.048
0.034, 0.082, 0.92
–0.56, 0.86

Data collection and reﬁnement: Rietveld method
Diﬀractometer
Radiation
μ (mm−1 )Exposure time (s)Calculation step (◦ )2Θ range (◦ )Peak shape description
Background subtraction
Rp , Rwp , RB (%), GOF

Rigaku RAXIS Rapid II (imaging plate)
CoKα1 /CoKα2
26.11
1800
0.02
6–132
Modiﬁed Pseudo-Voigt
28-coeﬃcient Chebyshev polynomial
0.39, 0.69, 0.41, 1.79

The X-ray powder diﬀraction pattern (Table 2) was obtained with a Rigaku R-AXIS Rapid II
difractometer (Rigaku Corporation, Tokyo, Japan) equipped with a curved (semi-cylindrical) imaging
plate. A ~150 μm ball was prepared from the stanﬁeldite powder mixed with an epoxy resin and was
picked onto a glass ﬁber. The image acquisition conditions were: CoKα-radiation, rotating anode withmicrofocus optics, 40 kV, 15 mA, Debye-Scherrer geometry, r = 127.4 mm, exposure 30 min. The imaging
plate was calibrated against Si standard. The image-to-proﬁle data conversion was performed with an
osc2xrd program [47]. The unit-cell parameters and occupancies of Mg1–Mg5 sites were reﬁned by
the Rietveld method (Table 1, Figure 2) using Bruker TOPAS v. 5.0 software (Bruker Inc., Wisconsin).
The occupancies at the M5A site were ﬁxed at the values determined by single-crystal reﬁnement.
The atomic coordinates were not reﬁned but were ﬁxed according to single-crystal data. The XPRD
pattern (Table 2) was indexed on the basis of theoretical values calculated with STOE WinXPOW
v. 1.28 software (Stoe & Cie GmbH, Darmstadt, Germany).
The micro-Raman spectrum was recorded from a random powder sample using a Horiba
Jobin-Yvon LabRam 800 instrument (HORIBA Jobin Yvon GmbH, Bensheim, Germany), equippedwith a 50× confocal objective. The instrument was operated with a 514 nm Ar+ laser at a 1 nm
lateral resolution and 2 cm−1 spectral resolution. The optics were preliminarily calibrated using a Si
reﬂection standard.

64Crystals 2020, 10, 464

Table 2. X-ray powder diﬀraction data (d in Å) for stanﬁeldite from the Brahin meteorite.
Imeas

dmeas

Icalc 1

dcalc

hkl

Imeas

dmeas

Icalc 1

dcalc

hkl

1
1
23
<1
23
<1
3
3
4
16
2
1
4
2
3
5
3
42

11.31
9.13
8.32
6.26
6.00
5.625
5.424
5.247
5.108
5.006
4.800
4.662
4.604
4.406
4.334
4.186
4.104
3.849

2
<1
21
1
21
<1
2
2
3
13
1
1
3
0.5
2
4
2
36

2.500

4
6
2
4
3
5
6
8
2
5
9

2.423
2.401
2.379
2.343
2.319
2.295
2.283
2.259
2.240
2.205
2.161

3.738

100

4
9
3
2
2
7

2.138
2.127
2.077
2.066
2.028
2.006

10
3
2
9

3.688
3.592
3.282
3.248

4
2
3
15

1.960
1.901
1.884
1.867

3
2
10
18
7
2
10

3.217
3.118
3.087
3.042
2.955
2.873
2.834

83

2.807

12
6
13
2
2
3

2.731
2.706
2.685
2.639
2.621
2.601

8
3
2
7
2
1
1
7,2
15
6
2
7
12
26
27
24
3,5
5
11
2
2
4

200
110
−111
202
112
400
311
−312
−402
020
021
113
312
−313
221
−204
510
511
600
−421
420
204
023
−404
−513
422
−131
314
024
404
115, 513
132
423
−133
−206
−515
−802
800
−225
−116, 712
622
−316
424
−532
531

48

100

11.23
9.12
8.26
6.23
5.97
5.614
5.409
5.230
5.091
4.992
4.785
4.629
4.585
4.405
4.326
4.177
4.096
3.842
3.742
3.733
3.730
3.727
3.726
3.681
3.586
3.272
3.246
3.234
3.213
3.114
3.083
3.039
2.952
2.870
2.834
2.825
2.812
2.807
2.800
2.730
2.705
2.682
2.642
2.627
2.599

7
3
4
2
3
2
3
2
3
3
6
3
3
3

1.840
1.827
1.801
1.789
1.753
1.742
1.697
1.652
1.621
1.605
1.597
1.543
1.516
1.481

30
20
4
6
2
3
1, 1
5
1, 3
6
1, 1
3
3
2
2
1, 1
5, 4
2
1, 1
1
2
2
2
5
1, 1
2
4
8
4
4
7
1, 1
4
2
1, 2
1, 1
2
2
2, 2
2
6
3
2
2

2.500
2.496
2.420
2.398
2.377
2.341
2.322
2.293
2.281
2.257
2.240
2.202
2.163
2.160
2.159
2.138
2.126
2.076
2.066
2.029
2.008
2.003
2.001
1.960
1.901
1.883
1.871
1.867
1.864
1.863
1.841
1.828
1.800
1.787
1.751
1.742
1.697
1.653
1.622
1.605
1.597
1.543
1.515
1.481

623
040
910
241
−823
911
135, 533
624−441, 440−716
−442, 441
−626
027
−136
732
−734, −825
823, 426640−10.2.2, 227−10.2.3
−245
045
−518
914
−736, 318
−153
12.0.0
−842
408
046
−808
841, 915
551−338−429, 138
−247, −538
11.1.4−555−356, 10.0.6
−829
429
339
−13.1.7−848

1 Intensities were normalized to ΣI[(600) + (–421) + (420) + (204) + (023)] = 100. Reﬂections having relative intensity

less than 2 at d < 4.00 Å have been omitted.

65

Crystals 2020, 10, 464

Figure 2. Rietveld reﬁnement plot of stanﬁeldite from the Brahin meteorite.

4. Results and Discussion
4.1. Stanﬁeldite: A Complete Structure-Composition Dataset
The X-ray examination of Brahin stanﬁeldite was carried out by two diﬀerent methods.
Both single-crystal and Rietveld reﬁnements of the unit cell had led to almost identical parameters(deviation between the unit-cell volumes is 0.04%, Table 1). The reﬁned Mg/Fe occupancies were also
well converged (Table 3). The chemical composition of studied stanﬁeldite (wt.%, average of 3 points):
CaO 26.21, MgO 21.66, FeO 3.02, MnO 0.27, P2 O5 49.38, total 100.54 can be recalculated to the formulaCa8.04 Mg9.25 Fe0.72 Mn0.07 P11.97 O48 . The latter is close to the composition determined by the structure
reﬁnement, Ca7.97 Mg9.47 Fe0.56 P12 O48 . Based on these results, one can state that the bond lengths andderivative bond-valence sums (Table 3) are herein calculated with a good conﬁdence.
Fuchs [9], in 1969, could reliably determine the unit-cell metrics, but misrecognized the spacegroup of the mineral (Table 4), perhaps due to the same pseudo-twinning of the crystals [40,41] which
we observed on our studied stanﬁeldite.
Table 3. Selected structural parameters of cation sites in natural and synthetic stanﬁeldite 1 .
Site

CN 2

Length 3

BVS 4

Mg 5

8
7
8
8
4
6
5
5
6
6
4
4
4
4
4
4

2.543
2.445
2.566
2.496
1.995
2.096
2.068
2.049
2.132
2.257
1.535
1.535
1.537
1.532
1.534
1.530

1.86
2.09
1.69
2.02
1.79
2.08
1.83
1.92
1.89
1.89
4.83
4.83
4.81
4.85
4.83
4.89

BVS 4

Ca7 (Ca,Mg)Mg9 (PO4 )12Synthetic [40]Brahin (Present Work)
Ca1 (4e)Ca2 (8f )Ca3 (8f )Ca4 (8f )Mg1 (4e)Mg2 (8f )Mg3 (8f )Mg4 (8f )Mg5 (8f )M5A (8f )P1 (8f )P2 (8f )P3 (8f )P4 (8f )P5 (8f )P6 (8f )Length 3

0.934(5)/0.91
0.945(3)/0.94
0.985(4)/0.98
0.983(3)/1.00
0.919(3)/0.90
See Table 5

2.547
2.444
2.569
2.498
1.999
2.095
2.071
2.054
2.130
2.270
1.540
1.538
1.541
1.538
1.536
1.532

1.85
2.10
1.68
2.01
1.75
2.07
1.81
1.89
1.88
1.92
4.76
4.79
4.75
4.78
4.81
4.86

Length 3

Mg 5

Imilac [41]
2.542
2.445
2.573
2.498
2.097

0.928(4)0.942(3)0.969(3)0.980(3)0.903(3)2.282

1Complete listings of atomic coordinates, thermal displacement parameters and bond lengths are given
in Supplementary Tables S1–S3. 2 CN, coordination number. 3 Average cation-oxygen bond lengths (Å).

4 BVS, bond-valence sums (in valence units), calculated by summation of individual element contributions

based on parameters reported by Brese and O’Keeﬀe [48]. 5 Reﬁned Mg occupancies for Mg1–Mg5 sites. Data for
Brahin include both single-crystal and Rietveld reﬁnement results separated by slash.

66Crystals 2020, 10, 464

Table 4. Unit cell parameters of natural and synthetic stanﬁeldite reﬁned from single-crystal data.
SourceBrahinSynthetic

ImilacEsthervilleSpace group
a(Å)
b(Å)
c(Å)
β(◦ )
V(Å3 )
Reference

C2/c
22.7973(4)9.9833(2)17.0522(3)99.954(2)3822.5This work

C2/c
22.841(3)9.994(1)17.088(5)99.63(3)3845.8
[40]

C2/c
22.81
9.993
17.09
99.96
3836.8[41]

P2/c or Pc 1
17.16(3)10.00(2)22.88(4)100.3(2)3862.9[9]

1 Space group assignment and cell axes permutation are discussed in References [40,41].

Dickens and Brown [40] have synthesized the synthetic, Fe-free analogue of stanﬁeldite andthoroughly described its crystal structure. However, the latter authors did not perform independent
determination of the chemical composition of synthesized material—as one will see, this is an essential
requirement in view of the widely varying composition of at least one structural site of stanﬁeldite.
Steele and Olsen [41] have reported the preliminary results of structural examination of natural
stanﬁeldite from the Imilac pallasite. They gave the analytical chemical formula of the mineral, but did
not provide full structural data, conﬁning the results to unit cell metrics, average bond lengths andselected site occupancies (Tables 3 and 4). As a consequence, no complete structure-composition
dataset for stanﬁeldite is available so far, and the data provided herein are the ﬁrst report of that type.
4.2. General Features of Stanﬁeldite Structure and Its Formula
The crystal structure of stanﬁeldite is a complex framework composed of 10 metal sites and6 phosphate groups (Table 3, Figure 3, Supplementary Table S1). Dickens and Brown [40] gave thedetailed description of each site in the structure of synthetic analogue of the mineral, and the present
paragraph aims to overview stanﬁeldite structure and highlight its features. The most interesting one
is a pseudo-hexagonal character of the framework which can be best viewed via the arrangementof [PO4 ] tetrahedra along the [10–2] axis (Figure 3A). In principle, stanﬁeldite, being presented by
the oversimpliﬁed formula M3 (PO4 )2 (M = Mg, Ca; Z = 24), can be regarded as a derivative of
the well-known glaserite structure type, K3 Na(SO4 )2 [49,50]. Dickens and Brown [40] discuss therelationships between stanﬁeldite and glaserite-related phosphates belonging to α- and β-Ca3 (PO4 )2
structural types. The latter is known as a basement of whitlockite-group mineral structures [51],two of which, merrillite and ferromerrillite, are of fundamental importance in the mineralogy of
meteorites [3,4]. In view of the common and intimate association of stanﬁeldite and merrillite inpallasite meteorites (Figure 1), these relationships could be of particular interest. However, contrary
to Dickens and Brown [40], we would not overestimate the similarity of stanﬁeldite and merrillite
structures. The unusual face-sharing of adjacent [MO6 ] octahedron and [PO4 ] tetrahedron characteristicof merrillite [3] does not occur in stanﬁeldite structure.
The reﬁnement of occupancies of four Ca-sites in the stanﬁeldite structure showed no evidence
for either Mg or Fe substitution. However, reﬁnement of four Mg sites using both single-crystal andRietveld methods concordantly leads to a partial substitution of Mg for Fe, with iron being preferentiallyconcentrated in Mg1 (tetrahedral) and Mg5 (octahedral) positions (Table 3). The tetrahedral coordinationof Mg1 is highly unusual; however, it is sometimes encountered in mineral structures such as åkermanite,
Ca2 MgSi2 O7 (melilite structure type), and spinel. The M5A site allows mixed occupancy by Ca, Mg
and Fe, and thus will be discussed in the next section. Based on the structural data, the overall formulaof stanﬁeldite can be written as Ca7 M2 Mg9 (PO4 )7 , where M = Ca, Mg or Fe2+ .

67

Crystals 2020, 10, 464

Figure 3. Crystal structure of stanﬁeldite. (A) Pseudo-hexagonal stacking of [PO4 ] tetrahedra (yellow),
projection along the [10–2] axis. The cations were hidden for clarity. (B) A slice of the structureon {010}. Arrangement of Mg-polyhedra and [PO4 ] tetrahedra. Ca atoms reside in the spaces andhave been hidden for clarity. (C) A view of a pair of distorted [M5AO6] octahedra corner-linked by[PO4 ] tetrahedra.

4.3. The M5A Site, A Key to A Flexibility of Stanﬁeldite Composition
This cation site (which was previously referred to as Ca5 [40]) deserves a special discussion as it
determines the variability of stanﬁeldite composition which, in turn, has led to misinterpretations of
the chemical formula of the mineral and its synthetic analogues. The central atom resides in the general
8f position and coordinates to six oxygen atoms to form a highly distorted octahedron (Figure 3C),
with the bond distances varying from 2.1 to almost 2.5 Å (Table 5).
Table 5. Bond lengths (Å), bond-valence sums (BVS, valence units) and occupancy factors for the M5A
site of stanﬁeldite and its synthetic analogue 1 .

M5A—O4
M5A—O5
M5A—O18
M5A—O19
M5A—O22
M5A—O23
Mean M5A—O
BVS
Ca
Mg
Fe2+
Reference

BrahinSynthetic2.194(2)2.291(2)2.494(2)2.200(2)2.099(3)2.262(3)2.257
1.89
0.48(4)0.43(2)0.08(2)This work

2.212(3)2.302(3)2.493(3)2.223(4)2.107(4)2.282(4)
2.270
1.92
0.51
0.49
[40]

Imilac2.282
0.55
0.353(3)0.097(3)[41]

1 Bond-valence sums were calculated by summation of individual contributions for each element, based onparameters reported by Brese and O’Keeﬀe [48].

68Crystals 2020, 10, 464

The M5A octahedra form paired clusters in the structure via corner-linking by phosphate tetrahedra(Figure 3C). Based on previous reports [40,41,52] and our data (Table 5), M5A may accommodate Ca,
Mg, Fe2+ and Ni in diﬀerent proportions, with the total occupancy equal to unity. Natural stanﬁeldite
is a Mg-dominant mineral, and the reﬁnement of M5A occupancy leads to a dominance of Mg overFe2+ as well (Table 5). The latter is supported by calculation of the bond-valence sum, which is almost
identical to that of synthetic analogue of stanﬁeldite (Table 3). The variability of M5A occupancy
substantiates the existence of solid solution between hypothetical Mg and Ca end members.
The former would have the composition corresponding to Ca7 Mg2 Mg9 (PO4 )12 . The latter
member would correspond to Ca7 Ca2 Mg9 (PO4 )12 , that is equal to Ca3 Mg3 (PO4 )4 . The intermediate
composition having Ca = Mg in M5A results in a formula Ca7 (CaMg)Mg9 (PO4 )12 , or, in a simpliﬁed
form, Ca4 Mg5 (PO4 )4 . One can see that the latter perfectly fulﬁls the ideal composition of stanﬁeldite
proposed by Fuchs [9]. It is noteworthy that stanﬁeldite from Brahin described herein, the previously
reported mineral from Imilac [41] and the synthetic analogue of stanﬁeldite [40] have M5A occupancies
almost equally shared between Ca and (Mg + Fe) (Table 5). This could lead to the assumptionthat the ordering between Ca and Σ(Mg, Fe) might exist in the M5A site. However, neither ourobservations nor previously reported data reveal superstructure reﬂections which would evidence theCa/Mg ordering. In this respect, an overview of reported compositions of stanﬁeldite-like minerals
and compounds would be of special interest. We have collected the chemically relevant data which
are gathered in Table 6 and plotted in Figure 4. It can be seen that the overwhelming majority of
stanﬁeldite compositions fall within the range corresponding to Ca ≈ (Mg + Fe) in the M5A site.
Therefore, the above assumption on the possible Ca/Mg ordering, albeit speculative, has a statistically
substantiated basis.

Figure 4. Plot of ΣCa versus ΣMg group element contents in natural, technogenic and synthetic
stanﬁeldite. Left and bottom scale: formula amounts recalculated on the basis of 48 oxygen atoms per
formula unit. Right and upper scale: expected occupancy factors for the M5A site. The grey straightline shows the linear ﬁt for the depicted analytical data. The red dots denote theoretical (calculated)compositions corresponding to (1) Ca7 Mg2 Mg9 (PO4 )12 , (2) Ca7 (CaMg)Mg9 (PO4 )12 ≡ Ca4 Mg5 (PO4 )6 ,(3) Ca7 Ca2 Mg9 (PO4 )12 ≡ Ca3 Mg3 (PO4 )4 . The blue dots and labels mark the compositions of particularinterest which are discussed in the paper. References and source data are given in Table 6.

69Crystals 2020, 10, 464

Table 6. Formula amounts of cations in stanﬁeldite and its analogues grouped by elements 1 .
Source 2

ΣCa 3

ΣMg 4

P + Si

Reference

Albin
Antofagasta
Antofagasta
BrahinBrahin 5Eagle Station
Eagle Station
Eagle Station
Estherville
ImilacImilacLunar
Mt. Vernon
Mt. Vernon
Mt. Vernon
Ollague
Rawlinna
Rawlinna
Santa Rosalia
Santa Rosalia
Santa Rosalia
Springwater
Springwater
Springwater
Springwater
Vaca Muerta
Slags
Slags
Slags
Synthetic

8.23
8.48
8.41
7.68
8.04
7.63
7.24
7.78
8.06
7.91
8.11
7.15
8.21
8.09
8.31
8.19
7.66
7.82
7.90
8.12
7.98
7.97
7.74
7.90
7.87
7.93
8.83
7.03
7.68
8.02

9.42
9.80
9.76
10.01
10.03
10.56
11.19
10.02
9.77
10.07
9.85
10.61
9.91
9.56
9.68
10.01
11.07
10.84
9.93
9.58
9.74
9.88
9.95
10.15
10.11
10.29
9.20
10.95
10.27
9.98

12.15
11.89
11.93
12.12
11.97
11.93
11.86
12.08
12.07
12.04
12.02
12.12
11.95
12.14
12.02
11.92
11.79
11.73
12.08
12.13
12.11
12.08
12.14
12.00
12.01
11.97
12.01
12.04
12.05
12.00

[7]
[7]
[7]
[43]
[7]
[7]
[10]
[9]
[7]
[41]
[16]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[7]
[10]
[15]
[19]
[19]
[19]
[40]

1 Atoms per formula unit calculated on the basis of 48 oxygen atoms. 2 Meteorite names. Non-meteoritic sources

are shown in italic type. 3 ΣCa includes (Ca, Na, K). 4 ΣMg includes (Mg, Fe, Mn, Al, Cr, Ti). 5 Present work.

The next interesting point is a signiﬁcant departure of total cationic sums of many analyses
from the ideal value requiring 18 cations per formula unit. These departures are readily revealed by
the shifts of corresponding analytical points from the linear ﬁt in Figure 4. At present, we have no
explanation for the observed departures. They could imply the existence of analytical errors in thereported microprobe data. On the other hand, these shifts might mean the occurrence of vacancies incationic sites of stanﬁeldite structure, and then they deserve a special investigation.
Although the majority of reported data fall within the central area of the plot in Figure 4, there area few points showing signiﬁcant prevalence of (Mg + Fe) sum over total Ca. These include one
analysis from the Eagle Station pallasite [7] and the mineral found in the Lunar sample 66095 returnedby the Apollo 16 mission [16]. These two analyses approach the Ca7 Mg2 Mg9 (PO4 )12 end-memberof the M5A solid solution. At the opposite extreme of the plot, there is a single point approaching
hypothetical Ca3 M3 (PO4 )4 composition. This analysis, along with two more listed in Table 6, relate toa stanﬁeldite-like phosphate described from the ancient slags found in Tyrol [19]. The main feature ofthis compound is wide variations both in Ca/(Fe + Mg) and Fe/Mg ratios, up to nearly Fe-dominantcompositions. Schneider, with co-authors [19], has provided Raman spectrum for this phosphate, but inthe absence of Raman spectra for genuine stanﬁeldite, the comparison was not possible. We herein
provide the Raman spectrum of stanﬁeldite from the Brahin meteorite (Figure 5). A comparison of thisspectrum with that reported by Schneider with co-authors [19] shows that the latter can represent apoorly crystallized Fe-dominant analogue of stanﬁeldite.

70Crystals 2020, 10, 464

Figure 5. The Raman spectrum of stanﬁeldite from the Brahin meteorite. The bands between 900 and
1150 cm−1 correspond to stretching vibrations in [PO4 ] tetrahedra. Bands in the range 400–650 cm−1
relate to bending modes of phosphate anion.

5. Ca3 Mg3 (PO4 )4 Phosphors: Do They Exist?
In this section, we would like to clarify the mistake caused by the incorrect database assertion ofprimary structural data on the synthetic analogue of stanﬁeldite reported by Dickens and Brown [40].
In the title of their article, Dickens and Brown report the formula Ca7 Mg9 (Ca,Mg)2 (PO4 )12 , with Ca= Mg in the Ca5 site, leading to a bulk one Ca4 Mg5 (PO4 )6 . It looks obvious that the mistake wasintroduced in the stage of structural data transfer from the article tables to the ICSD database. The Ca5
site, equally occupied by Ca and Mg [40], was erroneously assigned to be fully occupied by Ca.
The latter had led to a wrong formula, Ca3 Mg3 (PO4 )4 , which still appears in the ICSD database [53](ICSD code 23642). Moreover, the calculated X-ray powder diﬀraction pattern has been further
included into the ICDD (JCPDS) database under the reference number JCPDS-ICDD 73-1182 (Figure 4).
There is a substantial interest to the family of luminophores (phosphors) based on the stanﬁeldite
structure [35–39]. It is erroneous that the mistake in the chemical formula caused by the incorrect
primary data transfer has passed ﬁrst to the ICDD database and then to the papers devoted to a studyof these phosphor materials [35–39]. Unfortunately, neither of the published articles does containquantitative chemical data on synthesized phosphors. Thus, in lieu of any evidence supporting
the existence of Ca3 Mg3 (PO4 )4 , one can state that these phosphors are in fact stanﬁeldite-based,
Ca4 Mg5 (PO4 )4 compounds.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/6/464/s1:
Supplementary crystallographic data for stanﬁeldite from the Brahin meteorite in Crystallographic Information
File (CIF) format. Alternatively, CCDC reference number 1998335 contains the same data, and it can be obtained
free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk. S2, supplementary
crystallographic Tables S1–S3.
Author Contributions: Conceptualization, writing—original draft preparation, S.N.B.; investigation, S.N.B.,
M.G.K. and V.N.B.; data curation, E.V.O. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was ﬁnancially supported by the Russian Science Foundation, grant number 18-17-00079.
Acknowledgments: The authors gratefully acknowledge the curators of the Mining Museum, Saint Petersburg
Mining University, for providing the sample of the Brahin meteorite used in this study. We are thankful toanonymous reviewers for their comments and suggestions. This research was supported by the resource Centre
for X-ray diﬀraction studies and the Geomodel resource centre of Saint Petersburg State University.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

71

Crystals 2020, 10, 464

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74

crystals
ArticleThe High Pressure Behavior of Galenobismutite,
PbBi2S4: A Synchrotron Single Crystal X-ray
Diﬀraction Study
Paola Comodi 1, *, Azzurra Zucchini 1 , Tonci Balić-Žunić 2 , Michael Hanﬂand 3 and Ines Collings 3
1
2
3

*

Dipartimento di Fisica e Geologia, Università di Perugia, 06123 Perugia, Italy; azzurra.zucchini@unipg.it
Department of Geosciences and Natural Resource Management, University of Copenhagen,1017 Copenhagen, Denmark; toncib@ign.ku.dk
European Synchrotron Radiation Facility, 71 avenue des Martyrs, 38000 Grenoble, France;
hanﬂand@esrf.fr (M.H.); ines.collings@esrf.fr (I.C.)
Correspondence: paola.comodi@unipg.it; Tel.: +39-075-5852656

Received: 27 March 2019; Accepted: 16 April 2019; Published: 18 April 2019

Abstract: High-pressure single-crystal synchrotron X-ray diﬀraction data for galenobismutite, PbBi2 S4collected up to 20.9 GPa, were ﬁtted by a third-order Birch-Murnaghan equation of state, as suggested
by a FE -fE plot, yielding V 0 = 697.4(8) Å3 , K0 = 51(1) GPa and K’ = 5.0(2). The axial moduli wereM0a = 115(7) GPa and Ma ’ = 28(2) for the a axis, M0b = 162(3) GPa and Mb ’ = 8(3) for the b axis,
M0c = 142(8) GPa and Mc ’ = 26(2) for the c axis, with reﬁned values of a0 , b0 , c0 equal to 11.791(7)Å, 14.540(6) Å 4.076(3) Å, respectively, and a ratio equal to M0a :M0b :M0c = 1.55:1:1.79. The main
structural changes on compression were the M2 and M3 (occupied by Bi, Pb) movements toward
the centers of their respective trigonal prism bodies and M3 changes towards CN8. The M1 site,
occupied solely by Bi, regularizes the octahedral form with CN6. The eccentricities of all cationsites decreased with compression testifying for a decrease in stereochemical expression of loneelectron pairs. Galenobismutite is isostructural with calcium ferrite CaFe2 O4 , the suggested highpressure structure can host Na and Al in the lower mantle. The study indicates that pressure
enables the incorporation of other elements in this structure, increasing its potential signiﬁcance formantle mineralogy.
Keywords: galenobismutite; high pressure; single-crystal X-ray synchrotron diﬀraction; equation ofstate; calcium ferrite structure type; lone electron pair

1. Introduction
Galenobismutite PbBi2 S4 is a Bi-sulfosalt usually found in hydrothermal veins or associated with
fumarolic deposits [1,2]. Like other Bi-minerals, it has an important role in the reconstruction of theformation of ore deposits as it is sensitive to physical–chemical ﬂuctuations and can constrain thegenesis of ore.
According to sulfosalt classiﬁcation [3] galenobismutite is classiﬁed among commensuratecomposite derivatives of cannizzarite in a sub-group with angelaite, Cu2 AgPbBiS4 [4], nuﬃeldite,
Cu1.4 Pb2.4 Bi2.4 Sb0.2 S7 [5] and weibullite, Ag0.33 Pb5.33 Bi8.33 (S,Se)18 [6]. It has a distinctly diﬀerent
crystal structure from the chemically (stoichiometrically) similar berthierite FeSb2 S4 [7], garavellite
FeSbBiS4 [8] and clerite MnSb2 S4 [9] which form a berthierite isotypic series [3].
Chemical substitution of Sb for Bi and Fe for Pb are common in gelenobismutite. It is illustrated bygalenobismutite from Beiya porphyry- and skarn-type deposits that contain Sb up to 0.39% and Fe up
to 0.42% [10]. Selenium can replace sulfur in galenobismutite. In galenobismutite from Vulcano Island(Italy) [1] heterogeneous distribution of selenium in the Sulphur sites was found, with a total amount

Crystals 2019, 9, 210; doi:10.3390/cryst9040210

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Crystals 2019, 9, 210

of up to 0.13 atoms of Se per formula unit. Moreover, galenobismutite from Vulcano shows an unusualpresence of Cl, according to the coupled heterovalent substitution scheme: Pb2+ + Cl− = Bi3+ + S2− [2].
The crystal structure of galenobismutite is orthorhombic, space group Pnam, and was describedﬁrstly by Wickman [11], then by Iitaka and Nowacki [12], and later classiﬁed by Makovicky [13] asbeing representative of a speciﬁc subgroup of cannizzarite-type structures.
The crystal structure contains three cation positions. M1 has a slightly distorted octahedral
coordination and forms fragments of galena-like structure two octahedra wide by sharing edges witha conjugated M1 octahedron. M2 is surrounded by seven S atoms forming a “lying” (prism axis isperpendicular to the c crystal axis) mono-capped trigonal prism. M3 polyhedron is a “standing” (prism
axis parallel to the c crystal axis) bi-capped trigonal prism (CN8) with one of the capping ligands
relatively distant, so the coordination can also be described as a 7+1 (Figure 1).
The determination of the distribution of Bi and Pb in the three sites by reﬁnement of X-ray
diﬀraction data is practically impossible due to the similar number of electrons (83 and 82, respectively).
It is therefore based on the bond valence calculations. According to Pinto et al. [1], who used bondlengths, bond valences and geometrical characteristics of the coordination polyhedral to interpret theoccupancy on the M1, M2 and M3 positions. M1 is considered fully occupied by Bi, while M2 and M3
are mixed sites dominated by Bi and Pb, respectively.
The conﬁguration of this structure, isotypic with calcium ferrite (CF) CaFe2 O4 , is of a particularinterest for high pressure mineral physics. Finger and Hazen [14] include among the sevenstructure-types with exclusively six-coordinated silicon NaAlSiO4 [15,16], the analogue of CF. This
structure bears a close relationship to hollandite, another VI Si -coordinated structure type. Bothstructures consist of double octahedral chains which are joined to form ‘tunnels’ parallel to c that
accommodate the alkali or alkaline-earth cations. In hollandite four double chains form square tunnels,
whereas in CF four double chains deﬁne triangular tunnels.
Akaogi et al. [17] were the ﬁrst who synthesized NaAlSiO4 with CF structure at pressures higher
than 18–20 GPa. Tutti et al. [18] found that this phase is stable at pressure up to at least 70–75 GPa andtemperatures 800–2200 ◦ C indicating it as an important carrier of Na and Al in the lower Mantle.
The most regular CF-type structure known is that of PbSc2 S4 ([19], Figure 1b). The reported
crystal structure of NaAlSiO4 ([16], Figure 1c), obtained from a powder sample, has a substantiallymore distorted octahedral coordination. Dubrovinsky et al. [20] reported the structure of NaAlSiO4 at
35 GPa, likewise done on a powder sample (Figure 1d). The data suggest that the M1 coordination
becomes more regular without signiﬁcant contraction, whereas M2 and Na coordinations signiﬁcantly
contract keeping their general shapes. Compared to the other CF structures, galenobismutite diﬀers inhaving CN7 coordination of the M2 site and a signiﬁcantly distorted coordination of the M3 site. M1
coordination is eccentric, unlike in PbSc2 S4 or NaAlSiO4 . The increased CN of M2 and distortions of
the other two coordinations in galenobismutite are explained as a stereochemical eﬀect of the loneelectron pair of Bi3+ [21].
The ﬁrst high-pressure study of galenobismutite was done by Olsen et al [21] at pressures up to
8.9 GPa with single crystal X-ray diﬀraction. They found a bulk modulus of K0 equal to 43.9(7) GPa
and a K’ of 6.9 (3). No phase transition was observed in this pressure range and, interestingly, although
the stereochemical activity of Lone Electron Pair (LEP)’s decreased with pressure, the structure did not
approach the CF isotype but moved further away from its typical conﬁguration, keeping its distinct
character [21].
The present paper extends the study of the baric behavior of galenobismutite over a signiﬁcantly
larger pressure range, up to 20.9 GPa, by a synchrotron single crystal X-ray diﬀraction study in orderto obtain a more complete picture of its behavior under high pressure. Really the relevance of highpressure single crystal X-ray diﬀraction data with respect those from high pressure powder diﬀraction
was very recently highlight in several papers i.e. [22,23].

76

Crystals 2019, 9, 210

(a)

(b)

(c)

(d)

Figure 1. Crystal structures of: (a) galenobismutite, projected along [001] with x axis on the verticaland y axis on the horizontal line; (b) PbSc2 S4 [19]; (c) NaAlSiO4 at room pressure [16]; (d) NaAlSiO4 at
35 GPa [20].

2. High Pressure Experiments
The HP synchrotron single-crystal X-ray diﬀraction experiments were carried out at ID-15B
beamline at ESRF (Grenoble) dedicated to the determination of structural properties of solids at highpressure using angle-dispersive-diﬀraction with diamond anvil cells. A membrane-type Diamond
Anvil Cell with an opening angle of +/−32 degrees, equipped with 600 μm diamond culets was used.
Helium was used as a pressure transmitting medium. According to Singh [24] helium is superior inensuring near to hydrostatic conditions at pressures of 20 GPa or over compared to argon.
Ruby sphere was loaded as a ﬂuorescent P calibrant together with the galenobismutite sample(80 × 10 × 20 μm3 ) in the 300 μm hole in the center of a pre-indented stainless steel gasket of 80 μm
thickness. Pressure was measured before and after each data collection.
The sample-to-detector distance was 279.88 mm and calibrated, along with the wavelength, using
Si standard and Fit2D software [25]. The same synthetic sample of galenobismutite used in Olsen et al2007 was selected to collect the present set of measurements, to avoid diﬀerences in compressibility
ascribable to diﬀerent chemical compositions in the samples.
The X-ray beam was monochromatized to a wavelength of 0.41125 Å and focused down to
10 × 10 μm area. Data were collected with a DAC rotating 64◦ around the ω-axis (from −32 to +32◦ )77

Crystals 2019, 9, 210with angular step of 0.5◦ and counting time of 1s per step. The scattered radiation was collected by a
MAR555 ﬂat panel detector, with 430 × 350 mm (555mm diagonal) active area.
The extraction and correction of the intensity data, merging of reﬂections, and the reﬁnements ofthe crystal lattice parameters were done by means of the CrysAlis software (Agilent technologies) [26].
Measurements were performed at diﬀerent pressures from 0.5 to 20.9 GPa on increasing pressure,and at 16.43, 8.12, and 2.1 GPa on decreasing pressure to evaluate the reversibility and hysteresis
phenomena of structural changes. The absorption correction was applied by means of ABSORB-7
software [27].
The structure reﬁnements were carried out with ShelXle [28] on F2 . Scattering curves for neutral
atoms were used. Table 1 summarizes details of data collection and structure reﬁnements up to
20.9 GPa. Final atomic coordinates and isotropic displacement factors are listed in Table 2. Bond
lengths, polyhedral volumes and polyhedral distortion parameters at diﬀerent pressures, are reported
in Table 3. Cif ﬁles with the hkl, i.e., the Miller indices of the collected reﬂections, of individual
reﬁnements are in Table S1 (deposited).

78

−7 ≤ h ≤ 10senθ/λ (Å−1 )2θ maxCompleteness (%) ***Raﬁnement
R1 (|F0 | > 4σ)wR2Goodness of Fit(GooF)# Parameters

79

1.023

44

1.071

44

44

1.044

0.01990.0489−19 ≤ k ≤20
−5 ≤ l ≤ 5
0.85
41.1
29.09

4.94

44

1.016

0.02230.0538−19 ≤ k ≤17
−5 ≤ l ≤ 5
0.86
41.3
29.32

−12 ≤ h ≤ 9

1056
555
514
0.028441.29

11.4505(5)14.1314(4)3.97480(5)643.17(3)
7.780

6.20

44

1.032

0.02130.0539−16 ≤ k ≤19
−5 ≤ l ≤ 5
0.85
41.0
30.13

−9 ≤ h ≤ 12

1052
549
517
0.031540.96

11.3956(6)14.0521(4)3.95717(5)633.67(4)7.897

8.10 *

44

1.027

0.0221
0.0586−20 ≤ k ≤19
−5 ≤ l ≤ 5
0.85
41.2
28.73

−9 ≤ h ≤ 7

0.0265
41.16

1026
510

11.3191(8)13.9355(3)3.93304(5)620.39(5)
8.066

8.80

44

1.084

0.02100.0552−18 ≤ k ≤17
−5 ≤ l ≤ 5
0.85
41.0
29.98

−11 ≤ h ≤ 9

1015
536
518
0.029641.02

11.2841(6)13.8803(3)3.92120(5)614.16(4)
8.148

11.40

44

1.078

0.02210.0553−18 ≤ k ≤16
−5 ≤ l ≤ 5
0.85
41.1
30.18

−11 ≤ h ≤ 9

1052
531
520
0.0321
41.10

11.2038(6)13.7379(4)3.89439(6)599.41(4)
8.348

15.10

44

1.006

0.01930.0442−17 ≤ k ≤18
−5 ≤ l ≤ 5
0.86
41.6
28.53

−9 ≤ h ≤ 11

1022
501
487
0.0263
41.63

11.0890(8)13.5330(5)3.86003(7)579.26(5)8.639

16.43 *

44

1.019

0.01630.0372−18 ≤ k ≤17
−5 ≤ l ≤ 5
0.85
41.2
29.58

−11 ≤ h ≤ 8

1080
500
485
0.024741.16

11.0611(5)13.4491(3)3.84944(5)572.65(3)8.739* Data collected during decompression, ** Observed reﬂections |F0| > 4σ, *** Completeness is reported relative to 2θ max.

0.02130.05370.0246
0.0626−19 ≤ k ≤17
−5 ≤ l ≤ 5
0.85
40.9
30.11

−9 ≤ h ≤ 12

Range hkl

1132
551
516
0.0266
41.05

1091
581
548
0.0314
40.93

1004
536
506
0.031441.84−10 ≤ h ≤12−17 ≤ k ≤19
−6 ≤ l ≤ 5
0.87
41.8
25.76

2.13 *

a (Å)
b (Å)
c (Å)
V (Å3 )
ρ (g/cm3 )Data collection# Meas. reﬂ.
# Unique reﬂ.
# Obs. ref. **Rint
2θmax (◦ )11.6167(7)14.3541(3)4.02572(4)671.28(4)
7.455

1.66

11.6511(4)14.3951(3)4.03492(5)676.73(3)
7.395

0.50

11.7338(7)14.5066(4)4.05910(6)690.93(5)7.243

P (GPa)Table 1. Details of data reﬁnements, lattice parameters, and density at diﬀerent pressures.
17.10

44

1.016

0.01640.0367−17 ≤ k ≤18
−5 ≤ l ≤ 5
0.86
41.3
29.66

−9 ≤ h ≤ 11

1033
504
488
0.023741.26

11.0423(5)13.4115(3)3.84254(5)569.06(3)
8.794

20.90

44

1.037

0.01580.0360−16 ≤ k ≤18
−5 ≤ l ≤ 5
0.86
41.3
30.75

−9 ≤ h ≤ 11

1106
511
491
0.022241.30

10.9733(6)13.2252(4)3.81906(5)554.24(4)9.029

Crystals 2019, 9, 210

0.10329(6)0.90590(3)0.25
0.0182(2)0.0193(7)0.0230(4)0.0122(2)000.00020(18)0.24848(7)0.65233(4)0.25
0.0229(3)0.0274(8)0.0221(5)0.0192(2)000.0053(2)0.3343(4)0.0167(3)0.25
0.0161(9)0.010(4)0.021(2)0.0178(10)000.0001(11)

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

Bi1

Bi2

Pb3

S1

0.50

0.06702(6)0.39075(3)0.25
0.0150(2)0.0144(7)0.0168(4)0.0139(2)000.00162(16)

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

GPa

80

0.3330(3)0.0164(2)0.25
0.0148(8)0.010(4)0.0182(19)0.0162(10)000.0025(10)0.25147(5)0.65209(3)0.25
0.0198(2)0.0247(7)0.0185(4)0.0163(2)000.00440(17)0.10545(5)0.90648(3)0.25
0.0158(2)0.0174(6)0.0190(3)0.0110(2)000.00009(14)0.06582(4)0.39092(3)0.25
0.01358(19)0.0138(6)0.0149(3)0.0121(2)000.00131(13)1.66

0.3335(5)0.0169(2)0.25
0.0150(16)0.015(6)0.0141(19)0.0163(11)000.0014(14)0.25280(7)0.65203(3)0.25
0.0180(3)0.0219(9)0.0172(3)0.0150(2)000.0040(2)0.10621(7)0.90666(3)0.25
0.0140(3)0.0141(9)0.0178(3)0.0103(2)000.00001(19)0.06536(7)0.39102(3)0.25
0.0122(2)0.0117(8)0.0135(3)0.0115(2)000.00121(18)2.13 *

0.3313(4)0.0175(3)0.25
0.0124(10)0.011(4)0.012(2)0.0137(11)000.0008(11)0.25793(6)0.65201(4)0.25
0.0153(2)0.0199(7)0.0141(4)0.0119(2)000.00297(18)0.10974(6)0.90753(4)0.25
0.0123(2)0.0130(6)0.0146(3)0.0092(2)000.00000(16)0.06325(6)0.39170(3)0.25
0.0112(2)0.0109(6)0.0128(4)0.0097(2)000.00082(16)4.940.3306(4)0.0182(2)0.25
0.0127(9)0.016(4)0.0079(19)0.0147(10)00−0.0003(11)0.25963(6)0.65203(3)0.25
0.0141(2)0.0164(7)0.0146(4)0.0111(2)000.00267(17)0.11084(6)0.90789(3)0.25
0.0112(2)0.0100(6)0.0147(3)0.0090(2)000.00005(14)0.06267(6)0.39196(3)0.25
0.0106(2)0.0090(6)0.0136(4)0.0093(2)000.00074(14)

6.20

8.80
0.06172(6)0.39268(3)0.250.0103(2)0.0095(6)0.0123(4)0.0091(2)000.00052(14)11.4
0.06115(6)0.39333(3)0.25
0.0102(2)0.0093(6)0.0127(4)0.0085(2)000.00024(14)15.10
0.06082(6)0.39446(3)0.25
0.01034(18)0.0108(6)0.0117(3)0.00849(18)000.00030(14)16.43 *
0.06073(6)0.39490(3)0.25
0.01120(17)0.0123(5)0.0120(3)0.00937(17)000.00024(13)17.10
0.06076(6)0.39508(3)0.25
0.01005(17)0.0113(5)0.0106(3)0.00829(17)000.00023(13)20.900.06071(6)0.39644(3)0.25
0.01091(16)0.0125(5)0.0115(3)0.00868(16)000.00013(13)0.3309(5)0.0189(2)0.25
0.0108(16)0.006(6)0.0134(16)0.0133(10)000.0002(13)0.26198(7)0.65214(3)0.25
0.0132(3)0.0167(9)0.0131(3)0.0099(2)000.00241(18)0.3303(4)0.0187(2)0.25
0.0122(10)0.012(4)0.013(2)0.0122(10)000.0017(12)0.26265(6)0.65219(4)0.25
0.0126(2)0.0160(6)0.0122(4)0.0096(2)000.00217(16)0.3295(4)0.0200(2)0.25
0.0113(10)0.010(4)0.0115(19)0.0122(10)000.0014(11)0.26497(6)0.65241(3)0.25
0.0118(2)0.0145(6)0.0125(3)0.0085(2)000.00197(15)0.26874(6)0.65274(3)0.25
0.01206(18)0.0166(6)0.0109(3)0.00876(18)000.00157(14)0.26905(5)0.65279(3)0.25
0.01068(17)0.0149(5)0.0096(3)0.00746(17)000.00158(14)0.3285(4)0.3280(4)0.3281(4)0.0210(2)0.0214(2)0.0217(2)
0.25
0.25
0.25
0.0108(9)0.0117(8)0.0119(9)0.009(4)0.010(4)0.016(4)0.010(2)0.0106(18)0.0066(18)0.0134(10)0.0145(10)0.0128(10)
0
0
0
0
0
0
−0.0006(11) −0.0002(10) −0.0012(10)0.26766(6)0.65264(3)0.25
0.01114(19)0.0142(6)0.0113(3)0.00793(19)000.00169(15)0.3267(4)0.0228(2)0.25
0.0110(8)0.008(3)0.0117(17)0.0131(10)000.0000(10)0.27136(5)0.65308(3)0.25
0.01094(17)0.0145(5)0.0104(3)0.00790(16)000.00153(13)0.11234(7)0.11270(6)0.11413(6)0.11562(6)0.11623(6)0.11627(5)0.11716(5)0.90843(3)0.90869(3)0.90935(3)0.91036(3)0.91074(3)0.91097(3)0.91199(3)
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.0106(3)0.0107(2)0.0101(2)0.00983(18) 0.01075(17) 0.00945(16) 0.01001(16)0.0101(9)0.0111(6)0.0095(6)0.0104(6)0.0125(5)0.0113(5)0.0115(5)0.0132(3)0.0126(3)0.0126(3)0.0109(3)0.0107(3)0.0093(3)0.0099(2)0.00821(19) 0.00908(18) 0.00768(17) 0.00863(16)0.0085(2)0.0086(2)0.0081(2)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
−0.00003(17) −0.00007(15) −0.00010(15) −0.00006(14) −0.00009(13) −0.00008(13) −0.00030(13)0.06199(7)0.39247(3)0.25
0.0108(3)0.0110(9)0.0123(3)0.0091(2)000.00024(18)8.10 *

Table 2. Final atomic coordinates and isotropic displacement factors at diﬀerent pressures.

Crystals 2019, 9, 210

0.0197(4)0.0197(4)0.71258(19)0.7124(2)0.25
0.25
0.0143(9)0.0130(7)0.018(4)0.015(3)0.0123(19)0.0118(17)0.0127(9)0.0122(8)0000−0.0031(11) −0.0034(9)

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

S3

S4

0.0543(3)0.0948(2)0.25
0.0139(9)0.013(4)0.0139(17)0.0152(9)00−0.0013(9)0.0547(4)0.0940(2)0.25
0.0149(10)0.011(4)0.017(2)0.0174(11)000.0007(10)

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

1.66

S2

0.50

0.2595(4)0.2591(3)0.29633(19)0.2961(2)0.25
0.25
0.0135(10)0.0123(8)0.011(4)0.008(4)0.017(2)0.0150(19)0.0127(10)0.0142(9)0000−0.0001(11) 0.0011(9)

x
y
z
Uiso
U11
U22
U33
U23
U13
U12

GPa
4.94

6.20

8.10 *

8.80

11.4

15.10

81

0.0535(5)0.0966(2)0.25
0.0112(10)0.009(4)0.013(2)0.0116(11)000.0003(11)0.0534(4)0.0530(5)0.0975(2)0.0984(2)0.25
0.25
0.0109(9)0.0121(17)0.008(4)0.014(6)0.015(2)0.0107(18)0.0104(10)0.0117(9)0000−0.0003(10) −0.0028(12)0.0525(4)0.0988(2)0.25
0.0101(9)0.007(4)0.012(2)0.0110(9)000.0000(10)0.0518(4)0.1004(2)0.25
0.0096(9)0.007(4)0.012(2)0.0091(9)000.0001(10)0.0510(4)0.1020(2)0.25
0.0096(9)0.009(4)0.0096(19)0.0097(10)000.0002(9)16.43 *

17.10
0.2620(4)0.2973(2)0.25
0.0096(8)0.010(3)0.0115(19)0.0078(9)000.0000(10)0.0510(4)0.0504(4)0.1029(2)0.1032(2)0.25
0.25
0.0116(9)0.0087(8)0.013(4)0.008(3)0.0122(19)0.0094(18)0.0099(9)0.0084(9)0000−0.0011(10) −0.0007(9)0.2618(4)0.2973(2)0.25
0.0098(8)0.008(3)0.0107(18)0.0107(9)000.0002(9)20.900.0499(4)0.1052(2)0.25
0.0099(8)0.012(3)0.0088(17)0.0093(9)00−0.0005(9)0.2627(4)0.2979(2)0.25
0.0098(8)0.010(3)0.0103(18)0.0092(9)00−0.0009(9)* Data collected during decompression.

0.0191(5)0.0194(4)0.0190(4)0.0194(5)0.0194(4)0.0185(4)0.0183(4)0.0188(4)0.0184(4)0.0185(4)0.7129(2)0.7142(2)0.7146(2)0.7151(2)0.7154(2)0.7156(2)0.7157(2)0.7154(2)0.7157(2)0.7151(2)
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.0109(13)0.0108(10)0.0106(9)0.0103(15)0.0110(9)0.0108(9)0.0098(9)0.0113(9)0.0105(8)0.0101(8)0.011(5)0.011(4)0.010(4)0.009(5)0.010(4)0.010(4)0.007(4)0.013(3)0.012(3)0.010(3)0.0100(16)0.013(2)0.0131(19)0.0118(16)0.014(2)0.0136(19)0.0135(19)0.0101(17)0.0101(17)0.0102(16)0.0115(9)0.0081(10)0.0091(9)0.0101(9)0.0085(9)0.0091(9)0.0093(9)0.0108(9)0.0092(9)0.0101(9)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
−0.0023(12) −0.0036(11) −0.0040(10) −0.0035(12) −0.0040(11) −0.0051(11) −0.0037(11) −0.0039(10) −0.0040(10) −0.0019(9)0.0543(5)0.0947(2)0.25
0.0116(14)0.007(5)0.0148(17)0.0132(10)00−0.0011(12)0.2601(5)0.2602(4)0.2597(4)0.2602(5)0.2605(4)0.2614(4)0.2626(4)0.2965(2)0.2964(2)0.2967(2)0.2969(2)0.2969(2)0.2971(2)0.2972(2)
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.0118(14)0.0091(9)0.0104(9)0.0105(14)0.0098(10)0.0085(8)0.0097(9)0.010(5)0.002(4)0.008(4)0.009(5)0.007(4)0.003(4)0.007(4)0.0140(17)0.014(2)0.013(2)0.0123(17)0.013(2)0.013(2)0.013(2)0.0113(10)0.0115(11)0.0106(9)0.0100(9)0.0092(10)0.0094(9)0.0091(9)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
−0.0008(12) −0.0010(11) −0.0011(10) −0.0002(12) −0.0005(10) −0.0005(11) −0.0004(11)2.13 *

Table 2. Cont.

Crystals 2019, 9, 210

Crystals 2019, 9, 210

Table 3. Polyhedral evolution with Pressure. (a) M1, (b) M2, (c) M3 polyhedra. For each polyhedra,
single bond lengths (in A), average bond lengths (in Å) and polyhedral volumes (in Å3 ) are reported.
The standard errors are on the last digit. Measurements at 0.0001 GPa come from Olsen et al. [21].
P (GPa)0.0001
0.50
1.66
4.93
6.20
8.80
11.40
15.10
17.10
20.90
16.43
8.10
2.13

M1-S2

M1-S4(X2) ’

M1-S1(X2)

M1-S1’

Vol

Mean M1-S

2.6890
2.643
2.632
2.624
2.614
2.608
2.604
2.596
2.580
2.571
2.582
2.609
2.638

2.1788
2.719
2.699
2.661
2.650
2.633
2.613
2.592
2.581
2.565
2.584
2.639
2.690

2.9646
2.966
2.953
2.931
2.922
2.893
2.885
2.857
2.843
2.822
2.848
2.905
2.949

3.0623
3.043
3.023
2.953
2.931
2.887
2.855
2.819
2.801
2.781
2.810
2.893
3.000

30.44
30.43
29.90
28.84
28.47
27.76
27.24
26.50
26.07
25.52
26.11
27.95
29.68

2.843
2.843
2.826
2.783
2.782
2.759
2.742
2.719
2.705
2.688
2.710
2.765
2.819

P (GPa)M2-S3 (x2)

M2-S3’

M2-S4

M2-S2(x2)

M2-S1

Vol

Mean M2-S

0.0001
0.50
1.66
4.93
6.20
8.80
11.40
15.10
17.10.
20.90
16.43
8.10.
2.13

2.73942.749
2.745
2.7319
2.725
2.707
2.695
2.677
2.667
2.656
2.676
2.716
2.744

2.7962
2.788
2.776
2.747
2.744
2.725
2.716
2.691
2.679
2.660
2.683
2.731
2.766

2.9913
2.973
2.965
2.924
2.911
2.882
2.869
2.847
2.833
2.820
2.840
2.892
2.960

3.0559
3.041
3.012
2.935
2.921
2.881
2.849
2.810
2.797
2.768
2.802
2.892
2.994

3.1960
3.151
3.087
2.974
2.945
2.891
2.852
2.795
2.771
2.727
2.775
2.914
3.078

36.195
35.68
34.85
32.89
32.40
31.25
30.38
29.26
28.76
27.96
28.98
31.67
34.52

2.939
2.927
2.906
2.854
2.842
2.811
2.789
2.758
2.744
2.722
2.751
2.822
2.897

P (GPa)

M3-S4

M3-S2(x2)

M2-S1(x2)

M3-S3(x2)

M2-S4’

Vol

Mean M3-S

0.0001
0.50
1.66
4.93
6.20
8.80
11.40
15.10
17.10
20.90
16.43
8.10
2.13

2.8400
2.822
2.837
2.869
2.880
2.882
2.895
2.894
2.893
2.893
2.890
2.883
2.852

2.9320
2.912
2.898
2.860
2.845
2.819
2.798
2.768
2.751
2.730
2.757
2.828
2.894

3.0083
2.989
2.976
2.931
2.917
2.894
2.867
2.835
2.817
2.788
2.823
2.902
2.970

3.2375
3.189
3.142
3.037
3.007
2.957
2.918
2.870
2.848
2.810
2.851
2.969
3.122

3.7572
3.739
3.683
3.543
3.500
3.431
3.370
3.301
3.270
3.224
3.286
3.451
3.600

38.91
37.78
37.01
35.08
34.47
33.39
32.50
31.30
30.70
29.78
30.86
33.70
36.72

3.028
3.000
2.981
2.933
2.917
2.889
2.866
2.834
2.818
2.793
2.822
2.897
2.975

3. Results
3.1. Compressibility
The evolution of the unit-cell of galenobismutite with pressure is reported in Figure 2 and in
Table 1. The behavior of the cell parameters shows no discontinuities in the investigated pressure
range and indicates that no phase transition occurs in galenobismutite structure up to 20.9 GPa.
The volume-pressure data were ﬁtted with a third-order Birch-Murnaghan equations-of-state, using
the EOSFIT7-GUI software [29], as suggested by fE -FE , namely the “Eulerian ﬁnite strain” versus“normalized stress” plot [30], (Figure 3). The third order Birch-Murnaghan Equation of State (EoS) ﬁt
yields V0 = 697.4(8) Å3 , K0 = 51(1) GPa and K’ = 5.0(2). The bulk modulus and the ﬁrst derivative values

82Crystals 2019, 9, 210

were in good agreement with the values obtained from the fE -FE plot [30]. The intercept value and theslope obtained by a linear regression give FE0 and K’ values equal to 51(1) GPa and 4.8(8), respectively.

Figure 2. Evolution of the unit cell volume and a, b, c lattice parameters normalized to the values
at room conditions as a function of pressure (GPa), ﬁtted by a third-order Birch-Murnaghan EoS.
Olsen et al. [21] data are shown by stippled lines and triangles for comparison.

Figure 3. Evolution of the Eulerian ﬁnite strain fE versus the “normalized stress” FE. The solid line is
the weighted linear ﬁt of the data for V, a, b and c lattice parameters.

The lattice parameter moduli, calculated using a third-order Birch-Murnaghan equation of state,
were for the axis M0a = 115(7) GPa and Ma ’ = 28(2), for the b axis M0b = 162(3) GPa and Mb ’ = 8(3),for the c axis M0c = 142(8) GPa and Mc ’ = 26(2), with reﬁned values of a0 11.791(7) Å, b0 = 14.540(6)Å, c0 = 4.076(3) Å, respectively. Since the results gave large diﬀerences in M’ parameters, the lattice
parameter moduli were calculated using the second order Birch-Murnaghan equation of state, ﬁxing
M’ to 12 in order to evaluate the anisotropic behavior. The results of this ﬁtting give M0a 191 (9) GPa
with a0 equal to 11.74 (2) Å, M0b = 123(5) GPa with b0 equal to 14.96 (2) Å and M0c = 226 (10) GPa withc0 equal to 4.058(6) Å. The compressional anisotropy of crystallographic axes, showed that b and c

83

Crystals 2019, 9, 210

were the most and the least compressible lattice parameters, respectively, with the anisotropic ratio
M0a :M0b :M0c = 1.55:1:1.84.
Density of galenobismutite changed from 7.243 g/cm3 at 0.5 GPa to 9.029 g/cm3 at 20.9 GPa, withan increase of about 22% in the investigated pressure range.
To compare the present data with those of other sulﬁdes of metalloids from literature (galena [31],bismuthinite [32], stibnite [33], chalcostibite [34], lillianite [35], heyrovskyite [36], berthierite [37]) a K’
vs K0 plot was elaborated (Figure 4). In the plot, the conﬁdence ellipses at 90 and 68 % of conﬁdence
level for the present data and those reported by Olsen et al. [21] are shown. In order to allow a more
direct comparison of K0 and K’ calculated with the two data sets and to evaluate if the observed
diﬀerences were due to the diﬀerent pressure range, we also calculated K0 and K’ restricting our data
to the same pressure range investigated by Olsen et al [21]. We observed a strong negative correlation
between K’ and K0 in agreement with the data presented by Olsen et al. [21]. However, the ellipsoides
for the two data sets did not overlap, even if they are quite close. The reason might be that the results ofOlsen et al. [21] were biased by an unequal distribution of pressures at which the data were measured.

Figure 4. Bulk Moulus (K0 ) vs its pressure derivative (K’) for diﬀerent sulﬁdes. Conﬁdence ellipses
at 90% of conﬁdence level are reported for K0 and K’ calculated with the present data collected up to
20.9 GPa (solid black line) as well as with data limited at 8.8 GPa (stippled black line). Conﬁdence
ellipse at 90% for Olsen et al. [21] data is also shown (stippled blue line).

K0 and K’ values for galena (Figure 4), PbS, before the phase transition, were very close to
those observed for galenobismutite. On the other hand, K0 for bismuthinite, Bi2 S3 , were signiﬁcantly
lower (Figure 4). Olsen et al. [21] suggested an empirical relation between the bulk modulus
of galenobismutite and those of PbS and Bi2 S3 corresponding to the proportion of Bi and Pb in
galenobismutite: KPbBi2S4 = (2KBi2 S3 + KPbS)/3. Although this relation holds approximately for thedata from the previous study, the present corrected data for galenobismutite does not support thisobservation. We can conclude that a simple relation between a bulk modulus for a complex composition
cannot be derived straightforwardly from the bulk moduli of its simpler constituents [38] even if they
contain the same general structural modules (like in sulfosalts). Obviously, a more complex cooperative
mechanism between the structural modules should be involved [39]. For sulfosalts it is important totake into account that they contain cations with active lone electron pairs (LEPs), which can strongly
aﬀect the polyhedral distortion, and the overall structural compressibility to diﬀerent extents. Sb3+

84

Crystals 2019, 9, 210

LEP’s stereochemical activity is generally higher than that of Bi3+ , evaluated from the measurementsof the eccentricity of Sb and Bi polyhedra at room pressure conditions, which show larger diﬀerence ininteratomic Sb-S distances compared to Bi-S ones. Under high pressure, the polyhedra become moreregular and the eccentricity reduces more rapidly for Sb3+ polyhedra with respect to those of Bi3+ ,because the longest interatomic contacts in atomic coordinations generally compress faster than the
shortest ones. As a consequence, Sb sulfosalts have bulk moduli lower than the corresponding Bi
sulfosalts, as illustrated by the isomorphic chalcostibite-emplectite series [34].
Pb2+ also contains a LEP, but it is generally less expressed than that of Bi3+ . LEP of Pb2+ is evenfully suppressed in several structures, like in galena or the earlier mentioned PbSc2 S4 . To the best of ourknowledge the only observed regular coordination of Bi3+ is the octahedral coordination in the mineral
kupcikite, Cu4 Bi5 S10 [40,41]. It is interesting that the pressure can force coordinations with suppressed
LEP to a structure with highly expressed stereochemical activity through phase transition [35,36,42].
Very few theoretical calculations provide an analysis of the relation between electronic structure,lone electron pairs and structural geometrical parameters. Olsen et al 2011 [43], by using SIESTA DFT
code considered the eﬀect of pressure in Bi2 S3 and compared the theoretical with experimental data.
Their data on the eﬀective Bi s-p hydridization support the origin of the stereochemically active lonepair and its evolution with pressure increases.
A comparison of the bulk modulus of galenobismutite to those of CF type structures shows muchlarger diﬀerences. Dubrovinsky et al. [20] reported the CF type NaAlSiO4 bulk modulus measured
up 40 GPa. Their data gave a very high bulk modulus of 220 GPa and its pressure derivative was
equal to 4.1(1), similar to the values measured for other compounds with a calcium ferrite structure.
For example, the value of K0 , with K’ ﬁxed to 4, of MgAl2 O4 measured by Yutani et al. [44] was241(1) GPa, whereas K0 measured for Fe3 O4 by Haavik et al. [45] was 202(7) GPa, with K’ equal to4. The general rule, suggested by Anderson et al. [46], KV = constant, where V represents the molar
volume (36.58cm3 /mol for NaAlSiO4 , 36.13 cm3 /mol for MgAl2 O4 and 41.89 cm3 /mol for Fe3 O4 ), seems
to be followed by this group of calcium ferrite structures [20]. In comparison, galenobismutite has ahigher molar volume (105.0 cm3 /mol) but, at the same time, a much lower bulk modulus, resulting ina violation of the Anderson’s relation. This is most probably due to a large diﬀerence in chemistry,inﬂuenced by both cation and anion electronic conﬁgurations and especially by the presence of cation
LEPs in galenobismutite.
3.2. Structural Evolution with Pressure
The M1, M2 and M3 polyhedral evolution with pressure was analyzed through changes in bondlengths and polyhedral volumes reported in Table 3.
Figures 5 and 6 show the changes of bond distances and volumes with pressure. The bulk
moduli of M1, M2 and M3 polyhedra, calculated as the reciprocals of linear compressibilities are
114 (3) GPa, 86(2) GPa and 84(2) GPa, respectively. The values agree with the general relationshipsuggested by Finger and Hazen [14], which relates the polyhedral bulk moduli to inverse of the meancation-anion distances for several oxides, silicates as well as sulﬁdes and selenides and several othertypes of compounds.

85Crystals 2019, 9, 210

D

E

F
Figure 5. Evolution of the bond distances with pressure for M1 (a), M2 (b), M3(c) polyhedra.

86Crystals 2019, 9, 210

Figure 6. Variation of polyhedral volume for M1 (a), M2 (b), M3 (c) polyhedra with pressure.

The distortion parameters of the coordination polyhedra can give an additional insight in thecompressibility behavior of atomic coordinations. Figure 7 shows the development of the eccentricities,
asphericities and shape distortions (or volume distortions [47]). For M3 we calculated the parameters
for both CN7 and CN8, because of its speciﬁc character. The eccentricities of all coordinations decreased
continuously with pressure but much faster for M2 and M3 than for M1. After 4 GPa M1 reachedthe most eccentric coordination in spite of its smallest CN. It is interesting that the eccentricity of M3
related to only the closest seven S atoms levels oﬀ after 12 GPa and does not show further changes
with pressure. However, for CN8 it continued to decrease, due to a continuous approach of the eight S
atom. The asphericities showed much smaller changes with pressure. Note that M1 from the start had
negligible asphericity, meaning that all six S atoms ﬁt practically perfectly to a common sphere. It isinteresting that the asphericity of the M3 coordination for CN7 actually increased with pressure, in spiteof a constant decrease in asphericity calculated for CN8. It must, however, be noted that the asphericity
for CN8 was signiﬁcantly higher. The shape distortion, which shows the departure of the arrangementof ligands compared to an ideal polyhedron, shows an increase with pressure for all coordination
polyhedra. The parameters are in all cases calculated compared to the ideal polyhedron which shows
the smallest VS /VP ratio for a given CN, where VS and VP are the volumes of the circumscribed
sphere and the polyhedron, respectively. For CN6 this is the regular octahedron, for CN7 the regularpentagonal bipyramid and for CN8 the “maximum volume” bisdisphenoid. Compared to the latter
two, an ideal monocapped trigonal prism would have a “shape distortion” of 0.159, and an ideal
bicapped trigonal prism would have a “shape distortion” of 0.073. In this respect, the values calculated
for M2 and M3 (both for CN7 and for CN8) are actually a sign of approaching the shapes closer toideal monocapped, respectively bicapped trigonal prism. M1, however, departed more from an ideal
octahedron shape with increasing pressure.
The orientation and expression of a LEP can be calculated from the relative positions of thecentral atom in a coordination and the centroid of the ligand arrangement [48]. The black spheres in
Figure 8 have their centers in centroids of coordinations, thus, they illustrate the orientations and theexpressions of the LEPs of cations.

87Crystals 2019, 9, 210

(a)

(b)

(c)
Figure 7. Asphericity (a), Eccentricity (b) and shape distortion (c) evolution with pressure for M1, M2,
and M3 polyhedra.

88Crystals 2019, 9, 210

(a)

(b)

Figure 8. Galenobismutite at room pressure (a) and at 20.9 GPa (b). Black spheres are centered on
centroids of coordinations and indicate the orientations of M1, M2 and M3 lone electron pairs.

Taking into account the changes in bond distances and distortion parameters plus the global
aspects of the crystal structure, the changes that occur in galenobismutite under increasing pressure
can be summarized as follows: The main change is that both M2 and M3 atoms move towards thecenters of the bodies of respective trigonal prisms. It can be visually veriﬁed by comparing the crystal
structures at 1 bar and 20.9 GPa, as represented in Figure 8, and by checking the development of thebond lengths, as in Figure 5. Here, the atoms making the body of the trigonal prism were two S3 atoms,
two S2 atoms plus S3 and S1 for M2. Note that bond distances to these six S atoms showed a merging
tendency with increasing pressure. The distance to the capping S4 atom decreased with a much lower
gradient than the ones to two S2 plus S1 atom (that are longer at 1 bar) and actually became the longest
one from 12 GPa on. In the case of M3, two S2, two S1 and two S3 atoms formed the prism bodyand one can observe the same tendency of merging the bond distances up to approximately 10 GPa;above this pressure they became the shortest bond distances in the coordination polyhedron. It is truethat the longest distance to one of the capping S4 atoms had a signiﬁcant decrease during the wholemeasurement range, but with a gradient that was similar to the one of the two S3 atoms belonging
to prism body. On the contrary, the distance to the other S4 capping atom actually slightly increased
under compression. This all testiﬁes also in this case that M3 moves inside the body of the trigonal
prism with a consequence that it also moves away from the closest capping S4 atom. As the distance tothe other one largely decreases due to its approach to the prism body, the two distances to the capping
S atoms show a merging tendency and we can assume that the coordination’s character changes fromthe 7+1 type towards the real CN8, becoming a more regular bicapped trigonal prism (also conﬁrmedby the values of the shape distortion in Figure 7c).
The changes in the M1 coordination were very small compared to M2 and M3. The eccentricity ofthis site changed very little (Figure 7a) as the diﬀerence between the three shortest and three longest
bonds remained almost the same (Figure 5a). There was actually a slight but constant increase in
the distortion of the octahedral shape (Figure 7c). The main change in this coordination is due tothe polyhedral accommodation to the contraction of the b axis that had the largest compressibility(Figures 2 and 8a). The expression of the LEP of M1 slightly changed, but its orientation, seen from theatomic nucleus, changed more signiﬁcantly from the diagonal one, oriented towards the space between
the two neighboring M1 coordinations, to a direction along the b axis (Figure 8b). The changes in theexpression of LEPs of M2 and M3 were more signiﬁcant and their orientations changed to directions
closer to the M2-capping S and M3-most distant capping S, in accordance to the movement of M2 andM3 towards the centers of their respective trigonal prisms.

89

Crystals 2019, 9, 210

4. Discussion and Conclusion
The comparison of data collected at diﬀerent pressures on galenobismutite allows the following
conclusions:
(a)

(b)

(c)

(d)

The structural evolution is completely reversible with pressure increase up to 20.9 GPa. The samevalues were measured increasing and decreasing the pressure and the same equation of state ismeasured by using values collected increasing or decreasing pressure. No evidence of hysteresis
in the changes were observed, meaning that the changes are completely elastic.
The change in atomic coordinations bring the M3 coordination polyhedron closer to the shapeobserved in other members of the CF structural family (from CN7+1 to CN8). However, unlike
other CF crystal structures, M2 keeps and even equalizes its seven-fold coordination withincreasing pressure. This emphasizes the speciﬁc character of galenobismutite in this structural
family. We suggest that the main reason is a comparatively large size of the M2 cation, comparable
to that of the M3, unlike the other examples of CF structures, where M2 is signiﬁcantly smaller
than M3.
The structure remains stable at very high pressures (up to 20 GPa) notwithstanding the moderatebulk modulus, at least under the structural point of view, since there are no incompatible distances
up to 20.9 GPa. All sulfur-sulfur distances, which could indicate instability of the structure,remained quite large with the shortest S3-S4 distance equal to 3.140 Å.
Calcium ferrite structure type reveals enough ﬂexibility in incorporating various element
combinations through the example of galenobismutite. Thus, not only Al and Na, incompatible in
the periclase or perovskite crystal structures under the lower mantle conditions, can be considered
to prefer this structure type, but it might incorporate also some other important or less abundantelements or combinations of elements.

Supplementary Materials: The following is available online at http://www.mdpi.com/2073-4352/9/4/210/s1,
Table S1: HKL at diﬀerent pressure of galenobismutite.
Author Contributions: Conceptualization, P.C. and T.B.-Z.; methodology, M.H. and I.C.; software, M.H., I.C.
and A.Z.; formal analysis, A.Z., data curation, A.Z.; writing of original draft preparation, P.C. and T.B.-Z.;
writing—review and editing, P.C., T.B.-Z., A.Z.
Funding: This research received no external funding.
Acknowledgments: The European Synchrotron Facility is acknowledged for allocating beam-time for theexperiment ES-723 (main proposer P.C.).
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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crystals
ArticleCrystal Structures and High-Temperature Vibrational
Spectra for Synthetic Boron and Aluminum Doped
Hydrous Coesite
Yunfan Miao 1 , Youwei Pang 1 , Yu Ye 1, *, Joseph R. Smyth 2 , Junfeng Zhang 1,3 , Dan Liu 1 ,Xiang Wang 1 and Xi Zhu 1
1

2
3

*

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,
Wuhan 430074, China; miaoyfa@163.com (Y.M.); youweipang@126.com (Y.P.); jfzhang@cug.edu.cn (J.Z.);
ldddcug@126.com (D.L.); xiangwang@cug.edu.cn (X.W.); zhuxi@cug.edu.cn (X.Z.)
Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA; smyth@colorado.edu
School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Correspondence: yeyu@cug.edu.cn

Received: 5 November 2019; Accepted: 3 December 2019; Published: 5 December 2019

Abstract: Coesite, a high-pressure SiO2 polymorph, has drawn extensive interest from themineralogical community for a long time. In this study, we synthesized hydrous coesite samples
with diﬀerent B and Al concentrations at 5 and 7.5 GPa (1273 K). The B concentration could be
more than 400 B/106 Si with about 300 ppmw H2 O, while the Al content can be as much as 1200 to
1300 Al/106 Si with CH2O restrained to be less than 10 ppmw. Hence, B-substitution may prefer themechanism of Si4+ = B3+ + H+ , whereas Al-substitution could be dominated by 2Si4+ = 2Al3+ + OV .
The doped B3+ and Al3+ cations may be concentrated in the Si1 and Si2 tetrahedra, respectively,
and make noticeable changes in the Si–O4 and Si–O5 bond lengths. In-situ high-temperature Raman
and Fourier Transformation Infrared (FTIR) spectra were collected at ambient pressure. The single
crystals of coesite were observed to be stable up to 1500 K. The isobaric Grüneisen parameters (γiP ) of
the external modes (<350 cm−1 ) are systematically smaller in the Al-doped samples, as comparedwith those for the Al-free ones, while most of the OH-stretching bands shift to higher frequencies in
the high temperature range up to ~1100 K
Keywords: coesite; high-temperature Raman; FTIR spectrum; single crystal structure; isobaric
Grüneisen parameters; OH-stretching modes

1. Introduction
Coesite, a high-pressure polymorph of SiO2 , was ﬁrstly synthesized at 3.5 GPa (773–1073 K) in
1953 [1], and subsequently discovered in many locations, such as in the shocked sandstone ejecta
samples from craters [2,3] as well as in eclogite [4–6]. Coesite is a very important index silica mineral for
ultrahigh-pressure metamorphism [7,8], which provides key clue for the continental dynamics such aslithospheric subduction, exhumation, and reentry in extreme depths of more than 100 km. Furthermore,
the physical and chemical properties of coesite at high-pressure and high-temperature conditions
also attract a lot of interest from the community of mineral physics, like thermo-elasticity [9–12],
phase transitions [13–15], and vibrational spectra under high-pressure (P) and high-temperature (T)conditions [16–20].
Water (OH− ) incorporation into coesite has a signiﬁcant impact on the stability of coesite at
high-P/T conditions [21,22], which is important for exploring preservation of coesite in the deep mantle.
There could be up to 200–300 ppm ppmw H2 O in synthetic coesite samples [19,23–26] resulted fromhydro-garnet substitution (Si4+ + 4O2− = V + 4OH− ) as well as electrostatically coupled substitution

Crystals 2019, 9, 642; doi:10.3390/cryst9120642

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Crystals 2019, 9, 642

with M3+ incorporation (Si4+ = M3+ + H+ ; M = B, Al), although natural coesite has been found to benearly dry so far [27,28].
In this study, we synthesized hydrous coesite samples with various compositions (Si-pure, B-doped,
Al-doped, as well as B plus Al-doped), and explored the eﬀects of B and Al on the hydration mechanism
and internal structure of coesite. Taking advantage of in-situ high-temperature Raman and FTIR
vibrational spectra, we have also studied thermal response of lattice vibration with the contributions
from the trace elements of H, Al, and B, which may provide important constraints on thermodynamic
properties of coesite (such as heat capacities and entropy) under deep mantle conditions.
2. Materials and Methods
2.1. Sample Synthesis and Characterization
A total of ﬁve coesite samples were synthesized at the P-T conditions of 5 and 7.5 GPa and1273 K with heating durations of 9–12 hours (Table 1), using welded Pt capsules in sintered MgOoctahedron assemblies in the 1000-ton multi-anvil press at China University of Geosciences (Wuhan).
The corner truncation of the 25.4-mm tungsten carbide cubes was 12 mm for synthetic experiments
at 5 GPa and 8 mm for the runs conducted at 7.5 GPa, respectively. Temperature was monitoredwith a W5Re95–W2Re74 (C-type) thermocouple, and a graphite furnace was used in our experiments.
Analytical reagent SiO2 , Al(OH)3 , B(OH)3 (purity of >99.99%) were adopted as the starting materials
to synthesize hydrous coesite samples with diﬀerent compositions: Si-pure (Run 503), B-doped (R663),
Al-doped (R749), and B,Al-doped (R694 and R712). Excess liquid water (1 μL) was added in each
capsule to guarantee the water fugacity. Single crystals up to 300 μm were recovered from thesesynthetic experiments, while no other crystallized phases were detected (by Raman spectra) in therun products.
In-situ analyses of the trace elements of B and Al in these synthetic coesite samples were conducted
on an Agilent 7900 inductively coupled plasma mass spectrometry (ICP-MS) (Agilent Technology,
Tokyo, Japan) combined with a Yb femtosecond laser ablation (fs-LA) system (GeoLas 2005, Lambda
Physik, Göttingen, Germany), without applying an internal standard [29]. The ICP-MS works at a
power of 1350 W with a plasma and an auxiliary gas ﬂow rate of 15.0 and 1.0 L/min, respectively;
while the fs-LA system (λ = 257 nm) is operated at a repetition rate of 8 Hz and a pulse length of 300 fs.
The spot size is 24 μm with an energy density of 2.8 J/cm2 , and a mixture of He and Ar is used as thecarrier gas. The element contents of B and Al were calibrated against multiple-reference materials(BCR-2G, BIR-1G, and BHVO-2G) using the 100% oxide normalization method [30] with the detection
limits of 0.1 ppmw for B and 0.8 ppmw for Al, and the determined average B and Al concentrationsin these reference materials show relative deviations of −5 to about −10% from the recommended
values [31]. The derived B and Al concentrations are listed in Table 1.
Because the Al concentration in R749 is as high as about 400 ppmw as indicated by fs-LA–ICP-MS,
we further checked the Al composition by a JEOL JXA-8100 electron probe micro analyzer (EPMA)(JEOL Ltd., Akishima, Japan), which is equipped with four wavelength-dispersive spectrometers(WDS). The EPMA system is operated at an accelerating voltage of 15 kV and a beam current of 5 nA,
while the spot size is reduced to 10 nm to minimize the ﬂuctuations of X-ray intensity as well as sample
damage [32]. The certiﬁed mineral standards of pyrope garnet (for Al) and olivine (for Si) were adopted
for quantiﬁcation using ZAF wavelength-dispersive corrections. Totally, twelve points were selected
for measurements on the sample of R749, and the derived Al2 O3 content is 0.0742 ± 0.0096 wt.% with adetection limit of 100 ppmw (corresponding to 393 ± 51 ppmw for the Al element), which is consistent
with the result from fs-LA–ICP-MS within the experimental uncertainties.

94

5

SiO2 (96) + Al(OH)3 (4) + 1 μL H2 O

R749:

1273

1273
1273
1273
1273

Temperature
(K)

9

12
10
10
9

Time
(h)

—

—
74.6 ± 3.5
73.4 ± 9.0
39.7 ± 8.2

B(ppmw)——
445 ± 21437 ± 54237 ± 49

B/106 Si——
115.6 ± 52.0
139.8 ± 31.7
445.8 ± 86.5 a
392.7 ± 50.8 b

Al(ppmw)

a : measured by ICP-MS; b : determined by an electron probe micro analyzer (EPMA).

7.5
5
5
7.5

SiO2 + 1 μL H2 O
SiO2 (96) + H3 BO3 (4) + 1 μL H2 O
SiO2 (94) + H3 BO3 (3) + Al(OH)3 (3) + 1 μL H2 O
SiO2 (94) + H3 BO3 (3) + Al(OH)3 (3) + 1 μL H2 O

Pressure
(GPa)

R503:
R663:
R694:
R712:

Run No. Starting Materials(wt.%)——
343 ± 154415 ± 941323 ± 257 a
1166 ± 151 b

Al/106 Si

7.2 ± 2.9

32.3 ± 14.2
51.0 ± 19.8
22.8 ± 11.7
24.1 ± 10.9

H2 O
(ppmw)

48 ± 19215 ± 95
340 ± 132152 ± 78
161 ± 73

H/106 Si

Table 1. Starting composition and synthetic condition for each run. The B and Al element concentrations are mainly measured by femtosecond laser ablation(fs-LA)–ICP-MS, while the H contents are estimated by FTIR.

Crystals 2019, 9, 642

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Crystals 2019, 9, 642

2.2. Single-Crystal X-ray Diﬀraction (XRD)
A single grain (with a diameter of 100–120 μm) from each synthetic sample was selected for
XRD at ambient conditions. The unit-cell parameters (Table 2) were reﬁned on a Rigako XtalAB minidiﬀractometer (Rigaku, Akishima, Japan) with a 600-W rotating Mo-anode X-ray source, which isoperated at 50 kV and 20 mA. A Saturn 724 HG CCD detector (with a resolution of 1024 × 1024) was
mounted on this diﬀractometer. The average wavelength of Mo Kα1 –Kα2 was calibrated to 0.71073 Å,
and intensity data were collected in the 2θ scanning range of up to 52◦ . The reﬁnements of atomic
positions (Table 3) and anisotropic displacement parameters (Table S1) were conducted using thesoftware package of CrysAlisPro/Olex2 [33]. The data collection parameters are also listed in Table 2,
including the numbers of the measured equivalent and unique reﬂections, as well as the model ﬁtvalues for Goof, R1 , and Rint . For all these synthetic single crystals, the Goof parameters remain below
1.1, while the R1 and Rint values are lower than 2.9% and 1.5%, respectively. The Si4+ [34] and O2− [35]ionic scattering factors were adopted, and the Si1 and Si2 occupancies were ﬁxed at 1 (full) during thestructural reﬁnement procedures.
Table 2. Intensity data collection and unit-cell parameters for the synthetic coesite samples.

a (Å)
b (Å)
c (Å)
β (◦ )
V (Å3 )
No. total reﬂ.
No. unique total
No. unique I > 4σ
Goof
R1 for all (%)
R1 for I > 4σ (%)
Rint (%)

R503

R663

R694

R712

R749

7.1458 (5)12.3922 (10)7.1778 (8)120.293 (11)548.82 (10)
1028
366
348
1.084
2.45
2.33
1.33

7.1332 (9)12.3886 (5)7.1828 (16)120.31 (2)548.01 (18)
777
436
427
1.059
2.89
2.84
0.62

7.1355 (5)12.3678 (5)7.1763 (8)120.358 (10)546.47 (9)
2117
523
511
1.021
2.91
2.86
1.04

7.1426 (13)12.3698 (8)7.1788 (16)120.37 (2)547.24 (19)
911
467
446
1.051
2.67
2.57
1.23

7.1437(7)12.3964 (6)7.1858 (12)120.292 (15)549.46 (13)
1346
487
461
1.041
2.83
2.72
1.47

The unit-cell angles: α = γ = 90◦ .

2.3. Vibrational Spectra at Room and High Temperatures
Single grains of a diameter less than 150 μm were chosen for in-situ high-temperature Raman
measurement, using a Horiba LabRAM HR Evolution system (HORIBA JobinYvon S.A.S., Paris, France)with a Ar+ laser excitation source (λ = 532 nm) and a micro-confocal spectrometer. Each crystal piece
was loaded on a sapphire plate in a Linkam TS 1500 heating stage (Linkam Scientiﬁc Instruments
Ltd., Tadworth, Surrey, UK). High temperatures were generated by a resistance heater from 300
K up to 1500 K, with an increment of 50 K and a heating rate of 20 K/min. To further test thetemperature dependence of these lattice vibrational modes, we also chose another grain from R503
for low-temperature Raman measurement. The sample piece was loaded on a sapphire window
in a Linkam THMS 600 heating/cooling stage, and low temperatures were cooled down to 80 K byliquid nitrogen with a cooling rate of 15 K/min. The temperatures were automatically controlledwith uncertainties less than 5 K. Each target temperature was maintained at least 5 minutes before
measurement to guarantee thermal equilibrium.
To analyze the water contents in these synthetic coesite samples, 7–9 cleaned crystal pieces (ina diameter of 100–160 μm) were selected from each sample source for Mid-FTIR measurement at
ambient condition. All these crystals were double-side polished to a thickness of 60–80 μm before
measurements, and the water contents for each of these coesite samples are estimated as an averageof these measured pieces in the following discussion. The IR spectra were collected using a Nicolet
iS50 FTIR instrument (Thermoﬁsher, Madison, WI, USA) coupled with a Continum microscope, a KBr
beam-splitter, and a MCT-A detector cooled by liquid N2 . For in-situ high-T FTIR measurement, four96Crystals 2019, 9, 642

polished sample pieces (R503, R663, R694 and R749) was selected and loaded at the sapphire window ofa custom HS1300G-MK2000 external heating stage (INSTC, Boulder, CO, USA). The FTIR spectra wereobtained in the wavelength range above 3200 cm−1 , with a resolution of 4 cm−1 and an accumulationof 256 scans. Temperatures were measured from room temperature to about 1200 K with an intervalof 50 K and a heating rate of 15 K/min. Background was also obtained after the measurement on thesample for each step.
Table 3. Reﬁned atomic position coordinates.
R503

R663

R694

R712

R749

Si1

x
y
z

0.14027 (12)0.10826 (6)0.07226 (10)0.14048 (13)0.10833 (5)0.07240 (15)0.14044 (9)0.10835 (4)0.07242 (9)0.14049 (11)0.10832 (5)0.07232 (11)0.14050 (11)0.10829 (5)0.07242 (11)

Si2

x
y
z

0.50653 (12)0.15808 (6)0.54061 (10)0.50655 (13)0.15794 (5)0.54028 (15)0.50668 (9)0.15796 (8)0.54069 (8)0.50682 (11)0.15798 (5)0.54073 (11)0.50675 (11)0.15795 (5)0.54079 (10)

O2

x
y
z

0.5
0.1162 (2)
0.75

0.5
0.1164 (2)
0.75

0.5
0.1164 (15)
0.75

0.5
0.1163 (2)
0.75

0.5
0.11656 (17)
0.75

O3

x
y
z

0.7340 (3)0.12289 (16)0.5603 (3)0.7333 (3)0.12287 (15)0.5593 (4)0.7336 (2)0.12290 (12)0.5594 (3)0.7334 (3)0.12284 (15)0.5594 (3)0.7335 (3)0.12280 (13)0.5595 (3)

O4

x
y
z

0.3115 (3)0.10385 (17)0.3282 (3)0.3103 (4)0.10397 (15)0.3278 (4)0.3109 (2)0.10386 (11)0.3278 (2)0.3112 (3)0.10372 (15)0.3282 (3)0.3107 (3)0.10377 (12)0.3276 (3)

O5

x
y
z

0.0172 (3)0.21182 (17)0.4784 (3)0.0168 (3)0.21177 (15)0.4787 (4)0.0174 (2)0.21174 (11)0.4786 (2)0.0175 (3)0.21173 (15)0.4786 (3)0.0175 (3)0.21168 (13)0.4786 (3)The O1 site is at the inversion center with x = y = z = 0.

3. Results and Discussion
3.1. Hydration and B/Al Concentrations
The representative FTIR spectra for these synthetic coesite samples at ambient condition are shownin Figure 1. Four OH-stretching bands of v1 (3572 cm−1 ), v2a,b (3522 cm−1 ), v3 (3458 cm−1 ), and v4(3300 cm−1 ) are detected for all these synthetic samples, which are independent of presence of B or Al.
The mode v6 (3500 cm−1 ) is clearly observed with most absorbance for the B-doped samples (R663,
R694, R712), as compared to the B-free samples (R503 and R749), which is caused by B-substitution incoesite (Si4+ = B3+ + H+ ) [25]. It should be noted that the B concentrations in this study are much higher
than those synthesized in Koch-Müller et al. [25] (BR01-03), and the v6 absorbance is consequently
signiﬁcantly stronger than that for v2a,b .
The total H2 O content in coesite (CH2O , wt%) can be calculated on the basis of Lambert–Beerlaw [24]:
1.8 × Ai,
(1)
CH2O =
ρ × εi × dwhere ρ is density (2.93 g/cm3 ), d is the thickness of sample (cm−1 ), while εi is the integrated molar
absorption coeﬃcient for H2 O, which was calibrated to be 190000 ± 30000 L·mol−1 ·cm−2 for coesite [24].
The integrated absorbance Ai in the wavenumber range from v1 to v2 is expressed as
Ai =

 v2


log

v1

97


I0
· dv
I

(2)

Crystals 2019, 9, 642

where I0 and I are the intensities of incoming and transmitted radiation, respectively. For each coesite
sample, several unoriented crystal pieces were selected and polished for FTIR measurement at room
temperature, and similar Iv3 /Iv1 and Iv2 /Iv1 intensity ratios are observed among these FTIR spectra.
The averaged hydration concentrations are listed in Table 1 with statistical uncertainties.

Figure 1. Representative FTIR spectra obtained at ambient conditions with the OH-stretchingbands noted.

The hydration concentrations in these synthetic samples show a general trend:
R663 (B-doped) > R503 (B,Al-free) > R694/R712 (B,Al-doped) > R749 (Al-doped). This observation
can be satisfactorily interpreted as results of diﬀerent incorporation mechanisms between B and Al in
coesite. The predominant B-substitution mechanism in coesite should be an electrostatically coupled
substitution Si4+ = B3+ + H+ [25], which could increase hydration solubility, as compared with the
Si-pure sample R503. In contrast, most of Al cations were incorporated into the internal structureof coesite by causing oxygen vacancies (2Si4+ = 2Al3+ + OV ), which is similar to the Al-substitution
mechanism in stishovite [36,37]. Such Al-corporation may have an eﬀect of reducing water solubility
in coesite, according to the estimated water content in the sample R749. In the case of R749, the atomic
concentration ratio of Al:H reaches more than 24:1, while in the B,Al-doped samples (R694 andR712), the sums of B and Al atomic concentrations are still four or ﬁve times of that for hydrogen.
In addition, we also tried to collect Raman spectra on these samples in the similar frequency range of3200–3700 cm−1 , but no OH-stretching modes were detected due to the low water concentrations (nomore than 60 ppmw).
The magnitudes of the B/106 Si and Al/106 Si concentrations in this study are about one order ofmagnitude higher than those (BR01, BR02, BR03, and BRcal2) from Koch-Müller et al. [25], whereas themagnitudes of the measured water contents from both studies are in the same range (H/106 Si in a few
hundred atomic ppm). The synthetic conditions (including pressure, temperature, heating duration,98

Crystals 2019, 9, 642

as well as excessive B and Al in the starting materials) are similar or comparable for both studies, while
the main diﬀerence is that the Ni:NiO buﬀer was adopted in Koch-Müller et al. [25] to control water(oxygen) fugacity. However, it should be also noted that Deon et al. [26] also synthesized a coesite
sample with 1600 atomic ppm B (B/106 Si) and 900 atomic ppm H (H/106 Si), both of which are evenhigher than those in our sample R663, at a P-T condition of 9.1 GPa and 1673 K.
Hence, the B and Al solubilities in coesite at high P-T conditions still need to be carefully examined,
and the eﬀect of oxygen fugacity should also be taken into consideration. What is more important,Koch-Müller et al. [25] measured B and Al concentrations by ion microprobe [38], whereas we used
fs-LA–ICP-MS in this study, as well as EPMA to cross check the Al content in the sample R749. Hence,
discrepancies between diﬀerent analytical methods in diﬀerent laboratories should also be considered.
3.2. Crystal Structures
The space group of coesite is C2/c, and SiO4 tetrahedra form an inﬁnite three-dimensionalframework of a (b-unique) monoclinic structure (Figure 2). There are a total of two Si sites (Si1and Si2) and ﬁve O sites (special O1 and O2 sites, as well as general O3, O4, and O5 ones) in thelattice. The reﬁned crystal structures in this study are consistent with the previous studies [10,39–43].
The measured unit-cell volumes of the B-doped (R663), B,Al-doped (R694 and R712), and Al-doped(R749) samples diﬀer −0.15%, −0.3 to about −0.4%, and +0.1%, away from that for the Si-pure one(R503), respectively, while such diﬀerences are signiﬁcantly larger than the experimental uncertainty
from single-crystal XRD. Hence, even a few hundred ppm concentrations of B and Al trace elements
could have noticeable impact on the volume of coesite, and a similar phenomenon was also noted by
Koch-Müller et al. [25].

Figure 2. Crystal structure of coesite sketched on the basis of the structure reﬁnement for the
sample R503 in this study. The smaller (at centers of tetrahedra) and larger balls represent Si and O
atoms, respectively.
99

Crystals 2019, 9, 642

To further investigate any B and Al eﬀects on the internal structure of coesite, we conductedstructure reﬁnements on these ﬁve synthetic samples. The calculated bond lengths and angles arelisted in Table 4, using the software package XTALDRAW [44]. As compared with the Si-pure sample(R503), the B-doped (R663) and B,Al-doped (R694 and R712) ones exhibit signiﬁcantly shorter Si1–O4
and Si1–O5 bond lengths, while the Al-doped one (R749) shows noticeably longer Si2–O4 and Si2–O5
bond lengths, which are generally consistent with the order of cation sizes of B3+ < Si4+ < Al3+ .
The Si2–O3 bond length for R503 is longer than those for other samples. In addition, there are no
signiﬁcant diﬀerences for the O–Si–O bond angles among these samples. Hence, we proposed thatthe B3+ cations may concentrate in the smaller Si1 tetrahedra, while the Al3+ cations would prefer thelarger Si2 tetrahedra. The B,Al-coupled substitution seems to collapse the lattice structure even more,as compared with the case with only B substitution in coesite.
Table 4. Bond lengths (Å), bond angles (◦ ), and polyhedral volumes (Å3 ).
R503

R663

R694

R712

R749

Si1–O1
Si1–O3
Si1–O4
Si1–O5<Si1–O>
Poly. Vol.
Si2–O2
Si2–O3
Si2–O4
Si2–O5<Si2–O>
Poly. Vol.

1.5966 (8)1.6134 (16)1.6146 (19)1.6230 (20)
1.6119 (22)2.1485 (25)
1.6130 (12)1.6194 (18)1.6050 (20)1.6200 (20)
1.6144 (23)2.1579 (24)1.5968 (7)1.6130 (20)
1.6090 (30)1.6184 (19)1.6093 (19)2.1382 (30)1.6147 (12)1.6125 (19)1.6060 (30)1.6213 (19)1.6136 (22)2.1563 (30)1.5949 (5)1.6105 (15)1.6088 (16)1.6191(14)1.6083 (13)2.1344 (24)1.6109 (7)1.6150 (14)1.6049 (15)1.6191 (14)1.6125 (14)2.1515 (18)1.5953 (7)1.6120 (20)
1.6120 (20)1.6205 (19)1.6100 (16)2.1415 (21)1.6121 (11)1.6150 (20)
1.6046 (19)1.6194 (19)1.6128 (12)2.1522 (22)1.5980 (6)1.6134 (17)1.6106 (19)1.6229 (16)1.6112 (16)2.1457 (20)
1.6121 (9)1.6160 (16)1.6111 (18)1.6283 (17)1.6169 (6)2.1645 (20)

O1–Si1–O3
O1–Si1–O4
O1–Si1–O5
O4–Si1–O3
O4–Si1–O5
O3–Si1–O5
O2–Si2–O4
O2–Si2–O5
O3–Si2–O2O3–Si2–O4
O3–Si2–O5
O5–Si2–O4

110.37 (9)109.41 (8)109.94 (8)110.26 (11)108.86 (10)107.97 (10)109.30 (11)110.35 (11)109.42 (8)108.80 (10)109.47 (10)109.45 (11)110.28 (9)109.34 (8)110.03 (16)110.51 (12)108.81 (12)107.84 (11)109.41 (10)110.22 (12)109.56 (10)108.93 (12)109.35 (10)109.35 (11)110.35 (6)109.33 (5)109.83 (6)110.34 (9)108.91 (8)108.05 (7)109.37 (7)110.21 (8)109.75 (7)108.71 (8)109.45 (7)109.25 (7)110.41 (8)109.28 (8)109.75 (8)110.30 (11)108.92 (10)108.14 (10)109.30 (10)110.22 (12)109.75 (9)108.69 (11)109.54 (10)109.31 (10)110.32 (7)109.32 (6)109.80 (7)110.39 (10)108.92 (9)108.05 (9)109.48 (8)110.17 (10)109.55 (8)108.72 (10)109.55 (8)109.23 (9)3.3. Lattice Vibrations and Grüneisen Parameters γiP
The Raman spectra measured at ambient condition (in the frequency range up to 1200 cm−1 )are shown in Figure 3 for these synthetic samples. The ﬁtted peak positions are listed in Table S2,
and the vibrational bands at 521 cm−1 are always detected with most intensity. The Raman spectra areessentially the same among these coesite samples, while the most noticeable diﬀerence is that for the
Al-doped samples (R694, R712, and R749). The intensities of the Raman modes at 151 and 178 cm−1 are
relatively stronger and even comparable to the one at 119 cm−1 , as compared with those for the Al-freesamples (R503 and R663). There are a total of 33 Raman-active modes (16 Ag (R) + 17 Bg (R)) as well as
36 IR-active modes (18 Au (IR) + 18 Bu (IR)) predicted for coesite [16,17], while fewer peaks are detectedin this Raman measurement.

100

Crystals 2019, 9, 642

Figure 3. Raman spectra obtained at ambient condition for these synthetic coesite samples.

Next, we carried out in-situ high-temperature Raman experiments on the Si-pure (R503), B-doped(R663), Al-doped (R749), and B,Al-doped (R712) samples, as well as low-temperature measurement on
R503. The representative Raman spectra for R503 at various temperatures are shown in Figure 4A as an
example, while the high-T spectra for other coesite samples are deposited in the supplementary Figure
S1 (See Supplementary Materials). The R503 sample was heated up to the temperature of 1500 K, andno phase transition was detected throughout the heating procedure. Although the signals got weaker
and the background radiation became stronger especially at high temperatures above 1300 K, most of
the Raman peaks could still be distinguished and ﬁtted at the high temperatures. Another spectrum
was recorded when the temperature was quenched to room temperature, and no clear shifts wereobserved among these Raman bands compared with those before heating (Figure 4B). Meanwhile,
Bourova et al. [11] superheated a coesite sample to the temperature of 1776 K (at ambient pressure),
which was 900 K higher than the predicted metastable melting point [45], and the coesite sample
remained stable without any signiﬁcant phase transition, melting, or amorphization. On the other
hand, Liu et al. [19] reported amorphization of a hydrous coesite sample at a relatively low temperatureof 1473 K. (See Supplementary Materials).

101

Crystals 2019, 9, 642

Figure 4. (A) Selected Raman spectra for the sample of R503 at various temperatures; (B) comparisonof the Raman spectra taken before and after heating.

Variation of these Raman-active modes for R503 is plotted as a function of temperature in
Figure 5A–C, and the data points at low temperatures are in consistence with those at high temperatures(Figures S2–S4 for R663, R712, and R749, individually). All these bands systematically shift to a lower
frequency at elevated temperature, and linear regression was ﬁtted to each mode with the negative
slopes (δνi /δT) (at P = 0 GPa) (Table S2). The values of (δνi /δT)P are typically in the range of −0.01
to about −0.03 (cm−1 ·K−1 ) for the modes below 350 cm−1 or above 700 cm−1 , while −0.002 to about−0.007 (cm−1 ·K−1 ) for the ones in the range from 350 to 700 cm−1 . Our result is essentially in agreementwith the previous high-temperature Raman studies on SiO2 -pure coesite [17,46].

102

Crystals 2019, 9, 642

Figure 5. Variation of the frequencies for the Raman-active modes (R503) with temperature, in thefrequency ranges of (A) 0–400 cm−1 , (B) 400–800 cm−1 , and (C) 800–1200 cm−1 . Linear regression is
ﬁtted for each dataset.

The isobaric mode Grüneisen parameter (γiP ) is deﬁned as
γiP = −



1
∂vi
·
,
α · vi ∂T P

(3)

where α is the averaged volumetric thermal expansion coeﬃcient (α = 8.4 × 10−6 K−1 for coesite [11]).
The calculated γiP parameters are shown in Figure 6A for the samples of R503, R663, R712, and R749.
The Raman-active modes above 400 cm−1 are mostly associated with the internal bending andstretching vibrations of SiO4 tetrahedra linked in a three-dimensional framework for coesite [17,19,47].
The corresponding γiP parameters (1.4–3.2) are systematically larger than those internal modes (0–1.4)
for isolated SiO4 units as in forsterite (Mg-pure olivine) [48,49] and pyrope garnet [50], as well asa one-dimensional Si2 O6 chain as in enstatite (MgSiO3 -orthorpyroxene) [51], which are the mostabundant minerals in the upper mantle above 410-km seismic discontinuity. Although the magnitudesof the (δνi /δT)P slopes are similar among these studies, the thermal expansion coeﬃcient for coesite [11]is much smaller as compared with these silicate minerals [52–54]. On the other hand, for the bands
below 350 cm−1 , which are typically attributed to the external vibrations of SiO4 tetrahedra in coesite,
the values of γiP Grüneisen parameters are distributed in a much wider value range from −5 to 20.
Next, the diﬀerences of the γiP parameters among the samples of R663, R712, R749, and R503
(reference) are plotted in Figure 6B. The most signiﬁcant diﬀerence is that in the frequency range below
350 cm−1 , the γiP parameters for the Al-doped samples (R712 and R749) are systematically lower thanthose for the Al-free ones (R503 and R663), while no such diﬀerences are observed above 400 cm−1 .
When the Al3+ cations take the place of Si4+ in the tetrahedra, the thermal response of the enlarged
tetrahedra units could get hindered to some extent at high temperature, while the smaller B3+ cations
do not show such an eﬀect on the external vibrations of the tetrahedra units in coesite.

103

Crystals 2019, 9, 642

Figure 6. (A) The isobaric mode Grüneisen parameters (γiP ) for the synthetic samples of R503, R663,
R712, and R749; (B) comparison among the γiP parameters with the ones for the sample R503 set
as reference.

3.4. OH-Stretching Modes at High Temperature
The selected high-temperature FTIR spectra for R503 are shown in Figure 7A in the temperature
range up to 1150 K (Figure S5 for other coesite samples). The IR signal became weaker and broader at ahigher temperature, which might be caused by rapid proton hopping between adjacent O atoms [55,56],
as well as a black body radiation eﬀect. Above 700 K, the broadened OH-stretching modes of v1 ,
v2a,b , and v3 (in the frequency range of 3450–3600 cm−1 ) merge to be a broad hump and could not
be distinguished from each other. The weak and discrete v4 band (around 3300 cm−1 ) vanishes very
quickly and cannot be detected above 500 K. Another FTIR spectrum was collected when quenched
to room temperature, and all these four OH-stretching bands could be clearly identiﬁed at the same
positions as before heating (Figure 7B). The integrated absorbance for all these OH modes is about
80% of that before heating, and then 20% of the OH groups in the sample could be dehydrated during
the heating procedure up to 1150 K. On the other hand, 30–40% dehydration was also observed for104

Crystals 2019, 9, 642

other samples at temperatures of up to 1000–1100 K, by comparing the integrated IR absorbance of
the OH-stretching modes before and after heating. Meanwhile, Liu et al. [19] also conducted high-T
FTIR measurement on hydrous coesite, and they observed noticeable dehydration above 870 K as wellas completed dehydration at the temperature of 1473 K. In addition, we also conducted reﬂectance
FTIR measurements [56,57] on these coesite samples. Nevertheless, due to the low water contents,
the signals are signiﬁcantly weaker as compared with those collected in the transmission method atambient condition, and the OH bands could not be observed in the reﬂectance IR spectra above 500 K.

Figure 7. (A) Representative spectra for the sample of R503 at elevated temperatures; (B) comparisonof the OH-stretching bands measured before and after heating.

Variations of the OH bands with temperature are plotted in Figure 8A–D for samples R503, R663,
R694, and R749. Throughout the high-T measurments, the modes of v2a,b , v3 , as well as v6 (for B-doped
samples of R663 and R712) were observed to show a ‘blue-shift’ with a slope (δνi /δT)P of +0.01 to about+0.20 cm−1 ·K−1 , whereas a slight ‘red shift’ is detected for the v1 mode (at a high frequency of around105

Crystals 2019, 9, 642

3600 cm−1 ) with a temperature derivative of –0.08 to about –0.20 cm−1 ·K−1 . Above 600–700 K, theseOH vibrations cannot be distinguished from each other, and the broad hump is observed to gradually
move to a higher frequency at a higher temperature, with a temperature dependence of +0.05 to about+0.09 cm−1 ·K−1 . In addition, the v4 mode also shows a ‘blue shift’ with temperature increasing to below
500 K.

Figure 8. The frequencies of the OH-stretching modes as a function of temperature for the samples of(A) R503, (B) R663, (C) R694, and (D) R749. Linear regression lines are ﬁtted (Table S3), and the verticalerror bars represent the full-width of half maximum for each OH-stretching mode.

The OH-stretching modes observed in this study (in the frequency range above 3300 cm−1 with
O...O distance of >2.74 cm−1 ) should be attributed to protonation outside SiO4 tetrahedra with the
OH bonds pointing away from the centers of the tetrahedra [24–26]. The oxygen anion, that belong
to diﬀerent SiO4 tetrahedra, may try to get away from each other during the thermal expansion andrelaxation procedure at high temperature (i.e., the O...O distance between tetrahedra becomes larger).
Consequently, we observe a ‘blue shift’ for most of the OH-stretching bands in the high-temperature
FTIR measurements. On the other hand, Koch-Müller et al. [25] reported that some other OH-stretchingmodes (v7 , v8 , v9 , and v10 in the wavenumber range of 3370–3470 cm−1 ) shift to higher frequencies at ahigh pressure of up to 10 GPa.

106

Crystals 2019, 9, 642

4. Conclusions
(1)

(2)

(3)

(4)

We synthesized several hydrous coesite samples with diﬀerent B and Al compositions at pressuresof about 5–7.5 GPa (1273 K) in the multi-anvil press. The concentrations of the B/Al traceelements were measured by fs-LA–ICP-MS, while H2 O contents were estimated by FTIR. The B
concentrations are more than 400 atomic ppm (B/106 Si) with ~350 ppmw H2 O, while the Al3+
contents are about 1100–1300 atomic ppm, which were cross checked by both ICP-MS and
EPMA. Al-substitution signiﬁcantly reduces the hydrogen concentration in coesite. Hence,
the mechanism controlled by oxygen vacancies (2Si4+ = 2Al3+ + OV ) may be dominant for the Al
incorporation, which is similar to that in aluminous stishovite, while the B incorporation may
prefer the electrostatically coupled substitution (Si4+ = B3+ + H+ );The doped B3+ and Al3+ cations would prefer the Si1 and Si2 tetrahedra, respectively, and thesingle-crystal structure reﬁnements reveal that B3+ signiﬁcantly shortens the Si1–O4 and Si1–O5
bond lengths, whereas Al3+ noticeably elongates the Si2–O4 and Si2–O5 distances;In-situ high-temperature Raman spectra were collected on these synthetic samples of up to 1500 K(at ambient condition), and no amorphization of phase transition was observed throughout theheating procedures. The derived isobaric mode Grüneisen parameters (γiP ) for the external
vibrations of SiO4 units (below 350 cm−1 ) are signiﬁcantly reduced for the Al-doped samples,
as compared with the Al-free ones. Hence, the relaxation of the SiO4 units might be hindered
to some extent due to the enlarged tetrahedra units by Al-substitution. On the other hand,the γiP parameters for the internal bending and stretching modes of SiO4 tetrahedra in coesite(above 400 cm−1 ) are signiﬁcantly larger than those of most silicate minerals, due to the abnormallysmall thermal expansion coeﬃcient for coesite;
The OH-stretching modes v1 , v2a,b , v3 , and v4 are observed for all these hydrous samples with thevarious compositions, and another strong band v6 is also observed for the B-doped ones. Most ofthese OH vibrational modes shift to higher frequencies at elevated temperatures (except the weak
v1 mode around 3600 cm−1 ), implying that the O...O distances between diﬀerent SiO4 gets longer
during the thermal relaxation of the lattice framework at a high temperature. On the other hand,about 20–40% dehydration of OH groups were observed for these hydrous coesite samples athigh temperatures above 1000 K at ambient pressure.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/9/12/642/s1,
Figure S1. Selected high-temperature Raman spectra as well as the pattern taken when quenched to room
temperature for the samples of (A) R663, (B) R712 and (C) R749. Figure S2. The frequencies for the Raman-activemodes as a function of temperature for the sample of R712: (A) 0–400 cm−1 , (B) 400–800 cm−1 and (C) 800–1200 cm−1 .
Linear regression is ﬁtted for each dataset. Figure S3. The frequencies for the Raman-active modes as a function oftemperature for the sample of R663: (A) 0–400 cm−1 , (B) 400–800 cm−1 and (C) 800–1200 cm−1 . Linear regression
is ﬁtted for each dataset. Figure S4. The frequencies for the Raman-active modes as a function of temperature forthe sample of R749: (A) 0–400 cm−1 , (B) 400–800 cm−1 and (C) 800–1200 cm−1 . Linear regression is ﬁtted for eachdataset. Figure S5. Representative FTIR spectra obtained at high temperatures as well as when quenched to room
temperature for the samples of (A) R663, (B) R694 and (C) R749. Table S1. Anisotropic displacement parameters(Å2) for the synthetic coesite samples in this study. Table S2. The frequencies of the Raman-active modes atambient condition, as well as the temperature dependence and γiP parameters. Table S3. The frequencies of the OH
bands by FTIR measurement at ambient temperature if not noted, as well as their temperature dependence. The cif
ﬁles are for the single-crystal structure reﬁnements for the ﬁve coesite samples (R503, R663, R694, R712 and R749).
Author Contributions: Conceptualization, Y.Y.; methodology and investigation, Y.M., Y.P., D.L., X.W., and X.Z.;
writing—original draft preparation, Y.M.; writing—review and edition, Y.Y., J.R.S., and J.Z.; visualization, Y.M.
and Y.Y.; supervision, Y.Y.; project administration, Y.Y.; funding acquisition, Y.Y. All authors discussed the results
and commented on the manuscript.
Funding: This research was funded by the National Key Research and Development Program of China (Grant No.
2016YF0600204), the National Natural Science Foundation of China (Grant Nos. 41590621 and 41672041).

107Crystals 2019, 9, 642

Acknowledgments: The multi-anvil press synthesis, fs-LA–ICP-MS, EPMA, and high-temperature Raman
measurements were conducted at China University of Geosciences (CUG) at Wuhan; the single-crystal X-ray
diﬀraction experiments were carried out at Huazhong University of Science and Technology (HUST); while thehigh-temperature FTIR spectra were collected at Zhejiang University. Many thanks to Tao Luo (CUG), Yan Qin(HUST), and Xiaoyan Gu (Zhejiang University) for their experimental assistance.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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110crystals
ArticleBacterial Eﬀect on the Crystallization of Mineral
Phases in a Solution Simulating Human Urine
Alina R. Izatulina 1, *, Anton M. Nikolaev 1,2 , Mariya A. Kuz’mina 1 ,Olga V. Frank-Kamenetskaya 1 and Vladimir V. Malyshev 3
1
2
3

*

Crystallography Department, Institute of Earth Sciences, St. Petersburg State University, St. Petersburg
199034, Russia; ﬂoijan@gmail.com (A.M.N.); m.kuzmina@spbu.ru (M.A.K.); ofrank-kam@mail.ru (O.V.F.-K.)
Grebenshchikov Institute of Silicate Chemistry of the Russian Academy of Sciences,
St. Petersburg 199034, Russia
Microbiology Department, S.M. Kirov Military Medical Academy, St. Petersburg 194044, Russia;
vladmal_spb@list.ru
Correspondence: alina.izatulina@mail.ru; Tel.: +7-911-770-3824

Received: 24 April 2019; Accepted: 16 May 2019; Published: 18 May 2019

Abstract: The eﬀect of bacteria that present in the human urine (Escherichia coli, Pseudomonas aeruginosa,
Klebsiella pneumoniae, and Staphylococcus aureus) was studied under the conditions of biomimetic
synthesis. It was shown that the addition of bacteria signiﬁcantly aﬀects both the phase compositionof the synthesized material and the position of crystallization boundaries of the resulting phosphate
phases, which can shift toward more acidic (struvite, apatite) or toward more alkaline (brushite)
conditions. Under conditions of oxalate mineralization, bacteria accelerate the nucleation of calcium
oxalates by almost two times and also increase the amount of oxalate precipitates along withphosphates and stabilize the calcium oxalate dihydrate (weddellite). The multidirectional changes in
the pH values of the solutions, which are the result of the interaction of all system components and the
crystallization process, were analyzed. The obtained results are the scientiﬁc basis for understanding
the mechanisms of bacterial involvement in stone formation within the human body and the creationof biotechnological methods that inhibit this process.
Keywords: pathogen crystallization; biomimetic synthesis; renal stone; calcium oxalate; apatite;brushite; struvite; octocalcium phosphate; whitlockite; Escherichia coli; Klebsiella pneumoniae;
Pseudomonas aeruginosa; Staphylococcus aureus

1. Introduction
Urolithiasis is an example of pathogenic mineral formation in the human body. Various exogenousand endogenous factors are considered among the reasons for the development of urolithiasis [1,2].
The more factors act simultaneously, the more diﬃcult the pathogenesis of urolithiasis and the worse
its prognosis, which is due to frequent recurrence of the disease and the rapid growth of stones.
Currently, there are many theories explaining the causes and mechanisms of pathogenic stone
formation in the human urinary system [3–10]. All theories are based on the complex interaction ofbiogenic and abiogenic substances, but none of them are exhaustive. The least studied is the bacterial
theory [4].
It is well known that the presence of a variety of bacteria in the urine is very likely and bacterial
inﬂammation often accompanies stone formation [11]. Assumptions about the signiﬁcant eﬀect of
microorganisms on the processes of lithiasis in the human urinary system have been made in a
number of works [2–13]. The crystallization system (urine) contains about a dozen bacteria species.
Microbiological examination of removed urinary stones’ microﬂora shows that more than half ofurinary stones are infected, in most cases by several types of bacteria [2,14]. Infectious diseases of
Crystals 2019, 9, 259; doi:10.3390/cryst9050259

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Crystals 2019, 9, 259

the urinary tract are direct or indirect provocateurs of stone formation in the human urinary system.
According to the observations of practicing urologists, infectious sequelae after lithotripsy are rather
frequent, even against the background of sanitized urine, which indicates that the stones are infectedby bacteria during the formation [2]. The results of urine stone sowing showed that Enterococcus faecalis,Enterococcus faecium, Staphylococcus epidermidis, Staphylococcus haemolyticus, Pseudomonas aeruginosa,
Klebsiella pneumoniae, Proteus mirabilis, and Escherichia coli, as well as Streptococcus spp, Staphylococcus
aureus, Acinetobacter baumanii, Candida albicans, and Morganella morganii were among the most frequently
excreted microorganisms [14]. The presence of pathogens in the urine aﬀects the parameters andcomposition of urine, which in turn should aﬀect the crystallization of urinary stones’ mineral phases.
A number of studies have shown that bacteria can form bioﬁlms on the surface of a stone, which leads
to the formation of chronic infection during diseases of the urinary system [2,15,16].
A substantial portion of papers on the eﬀect of bacteria on the stone formation in the humanurinary system is devoted to the so-called infectious renal stones, consisting mainly of struvite((NH4 )MgPO4 ·6H2 O), and sometimes containing hydroxylapatite (Ca5 (PO4 )3 (OH)) and brushite(Ca(HРО4 )·2Н2 О) [12]. The bacteria that cause the secondary phosphate stone formation belong to
the urease-forming microﬂora [17]. Infectious stones are formed as a result of urea hydrolysis toammonium ions and bicarbonate, increasing the urine pH to normal or alkaline values and binding toavailable cations to produce magnesium ammonium phosphate (struvite) and carbonate apatite [12].
Struvite stones are found only in a small number of patients susceptible to urinary tract infections. Thus,in our collection of renal stones of St. Petersburg and the Leningrad region residents, which consists ofmore than 2000 samples, only 27 belong to this "infectious" type (Figure 1). It is assumed that oxalatestones may also have an infectious origin [2,3,17]. The data on the initiation of the crystallization andaggregation of calcium oxalates in the presence of E. coli [18], as well as the work on the crystallizationof weddellite (CaC2 O4 ·2H2 O) in the presence of E. coli [19], favor of this assumption. In addition,
a number of papers suggest that bacteria can serve as centers of crystallization and the subsequent
growth of renal stones, forming a phosphate shell around itself [20].

Figure 1. Infectious renal stones: (a) Apatite–struvite–brushite, (b) struvite, and (c) struvite–brushite.

The results of model experiments on the crystallization of pathogenic phase analogs in the presenceof bacteria have shown that bacteria change the pH of solutions and can increase the amount and112

Crystals 2019, 9, 259

alter the morphology of the resulting oxalate and phosphate crystals [3,6,7,13,21]. Unfortunately,
the currently available data are insuﬃcient to characterize the eﬀect of the bacterial presence in theurine on the phase composition of the resulting renal stones.
In order to advance in this direction, we conducted a synthesis experiment using solutions thatsimulate the composition of human urine, including containing bacteria common for human urine,
and revealed their role in the crystallization of urinary phosphate and oxalate stones.
2. Materials and Methods
Biomimetic syntheses in the presence of bacteria were carried out by precipitation at 37 ◦ C fromsolutions that simulated the composition of human urine and its inorganic components, where thecontent corresponded to their minimum or maximum values (Table 1). The volume of the solution
after mixing of initial components in accordance with Figure 2 was 500 mL. The content of calcium
cations in solutions ranged from 5 to 7.7 mmol/L, which is due to the fact that in small volumesof solution (0.2 L) and with a limited time to carry out the synthesis (1–2 days) the formation of acrystalline precipitate at lower calcium concentrations does not occur. To accelerate the crystallizationof calcium and magnesium phosphates, oxalate ions (in the form of ammonium oxalate) were also
added to the initial solution in a low concentration (0.1 mmol/L). Also, experiments in the so-called“oxalate system” containing only calcium ions and oxalate ions were conducted (calcium oxalate
supersaturation is equal to 7, which corresponds to the physiological values of urine), since calcium
oxalate does not crystallize in the system simulating the composition of urine. Ovalbumin was added
to the experiments at a concentration of 10 mmol/L [22]. Syntheses were carried out by precipitationin an aqueous solution or in solutions of protein-containing nutrient media, the Müller Hinton
Broth (MHB) nutrient medium or the Meat-Peptone Broth (MPB), which were prepared according
to standard techniques [23,24]. In addition, bacteria associated with inﬂammatory processes andpresent in signiﬁcant quantities both in the environment and in the human body were added toeach of the protein media and to the model media in an amount of 106 particles per liter: Escherichia
coli («e»), Klebsiella pneumoniae («kl»), Pseudomonas aeruginosa («ps»), and Staphylococcus aureus («s»).
The following bacterial American Type Culture Collection (ATCC) strains were used in the experiments:
25922 («e»), 70060325922 («kl»), 27853 («ps»), and 29213 («s»). The pH of the solutions varied between
5.77–7.26 (minimum concentrations of inorganic components) and 6.10–8.07 (maximum concentrationsof inorganic components). The acidity of the initial solutions was adjusted using aqueous solutions of
HCl and NaOH. The crystallization start time (clouding of the solution) and phase composition of
the obtained precipitates were recorded during experiments. Clouding of the solution was recorded
visually. The precipitate obtained a day later was ﬁltered, washed with distilled water, and dried atroom temperature; at least three iterations were performed for each experiment.
Table 1. Elemental composition (mmol/L) of model solutions and urine.
Component
Na+
K+
Ca2+
Mg2+
NH4 +
ClCO3 2PO4 3SO4 2-

Model Solution
Min Concentration
60
21.7
5–7.7
5.3
20.8
67
0
13
21.7

Max Concentration
73
102
5–7.7
11
49.4
80
33
33
69

113

Human Urine [22,25]
67–13333–47
1.7–5
5.3–11
20–50
67–167
0–33
13–33
27–80

Crystals 2019, 9, 259

Figure 2. The scheme of the synthesis experiment in system which simulated the composition of urine
by inorganic components.

The phase composition of precipitate products was determined by means of powder X-ray
diﬀraction method (PXRD). The measurements were performed using a Rigaku «MiniFlex II» powder
diﬀractometer (CuKα radiation, λ = 1.54178 Å; 30 kV/15 mA; Bragg–Brentano geometry; PSD D-Tex
Ultra detector). X-ray diﬀraction patterns were collected at room temperature in the range of
3–60 ◦ 2θ with a step of 0.02 ◦ 2θ. Phase identiﬁcation was carried out using the ICDD PDF-2
Database (release 2016). The unit cell parameters were reﬁned by the Pawley method using TOPAS
4.2 software [26]. The background was modeled using a Chebychev polynomial of 12th order. The peak
proﬁle was described using the fundamental parameters approach.
3. Results
3.1. pH Changes of the Medium
The pH of the solutions in the crystallization process of the phosphate phases always decreased inexperiments without organic additives and with the addition of nutrient media and bacteria, it either
increased or decreased (Table 2). As can be seen from Table 2, the nutrient media and bacteria aﬀected
the pH values, which can be explained both by the inﬂuence of crystallization processes and bacterial
activity. For instance, interaction of the solution with MHB media slightly reduced the pH value ofthe solution in the case of the minimum concentrations of inorganic components and in the case of
maximum concentrations the pH of the solution increased. Addition of Pseudomonas aeruginosa bacteria
to the MHB medium slightly increased the pH value of the solution (by 0.4), while addition of thesame bacteria to the MPB medium increased the pH value of the solution by much more (by 0.6).

114

Crystals 2019, 9, 259

Table 2. The change in pH of the solutions during the experiment in nutrient media with the additionof bacteria.
Minimum Concentration

Maximum Concentration

Nutrient Medium

Additives
Bacteria

Initial pH

Final pH

Initial pH

none

none

5.95–7.54

5.94–6.71

5.81–7.73

5.75–7.50

none

5.81–7.15

5.84–6.25

6.10–8.07

6.27–7.39

Müller–Hinton BrothMeat–Peptone BrothFinal pH

Escherichia coli (“e”)

5.81–7.15

5.27–6.39

6.10–8.07

5.85–7.35

Klebsiella pneumoniae (“kl”)

5.81–7.15

5.51–6.40

6.10–8.07

5.84–7.50

Pseudomonas aeruginosa (“ps”)

5.81–7.15

6.21–6.51

6.10–8.07

6.16–7.90

Staphylococcus aureus (“s”)

5.81–7.15

5.06–6.45

6.10–8.07

5.66–7.54

none

5.77–7.26

5.78–7.08

6.10–8.03

6.18–7.90

Escherichia coli (“e”)

5. 77–7.26

5.94–7.02

6.10–8.03

6.07–7.67
6.03–7.80

Klebsiella pneumoniae (“kl”)

5.77–7.26

5.90–7.00

6.10–8.03

Pseudomonas aeruginosa (“ps”)

5.77–7.26

6.39–7.40

6.10–8.03

6.18–7.90

Staphylococcus aureus (“s”)

5.77–7.26

6.25–7.23

6.10–8.03

6.14–8.05

3.2. Model Solutions with Minimum Concentrations of Additional Ions Characteristic of a Healthy Person’s
Urine Composition
In syntheses of phosphates with minimum concentrations of inorganic impurities without
additives, formation of the following crystalline phases was observed: Brushite (Ca(HРО4 )·2Н2 О),octacalcium phosphate (Ca8 (HPO4 )2 (PO4 )4 ·5H2 O), and whitlockite (Ca9 Mg(HPO4 )(PO4 )6 ) [27,28].
Brushite formed in synthetic experiments when the initial pH of the solution ranged from 6.46 to 6.86.
Octacalcium phosphate was usually observed together with brushite (less often with whitlockite) in
the pH range of 6.46 to 6.95. Whitlockite was obtained in the pH range of 6.95 to 7.54.
Addition of MHB medium to the model solution changed the phase composition of the sediment(Figure 3). In the pH range 6.75–7.3, the brushite phase was detected. Brushite also formed after
addition of various bacteria to the solution. Moreover, the whitlockite phase was detected in thesyntheses that were carried out in the presence of “kl” at a pH of 7.15. In addition, in the experimentswith “e” and “ps” bacteria at pH 7.05–7.15, formation of struvite was identiﬁed (together with brushite).
Addition of the MPB medium to the model solution also led to changes in the phase compositionof the sediment (Figure 3). In this case, brushite was detected at pH ~7.06. Brushite did not crystallize
at such a high pH in the experiments without additives. Another diﬀerence in the phase compositionof the precipitate was the formation of struvite at a pH of 7.26, which is absent in the products ofsyntheses without additives. The brushite phase was detected in the sediments of all syntheses, whichwere carried out in the presence of bacteria. Whitlockite was formed only in the synthesis in which
the E. coli bacteria were present at a pH of 7.07. Struvite was formed in the syntheses with bacteria,
except those experiments with the addition of Staphylococcus aureus, at a pH of 7.0 or higher. In all thesyntheses with bacteria, the formation of apatite was observed at a pH of 6.72 or higher.

115

Crystals 2019, 9, 259

Figure 3. Phase composition of synthesized products from model solutions with minimum
concentrations of additional ions characteristic of a healthy person’s urine composition. Legend:♦—brushite, +—struvite, Δ—whitlockite, •—apatite, —octacalcium phosphate, ×—no precipitation;
Escherichia coli —«e», Klebsiella pneumoniae—«kl», Pseudomonas aeruginosa—«ps», and Staphylococcus
aureus—«s».

3.3. Model Solutions with Maximum Concentrations of Additional Ions Characteristic of a Healthy Person’s
Urine Composition
In the phosphate syntheses with maximum concentrations of inorganic impurities without
additives, the formation of brushite and struvite was observed (Figure 4). Brushite was formed in awide range of pH values of the initial solution from 5.81 to 7.63. Struvite growth occurred at higher pH
values (from 7.23 to 7.73) and usually along with brushite.
When MHB medium was added to the model solution, hydroxylapatite was clearly observedin the precipitate composition, in addition to common brushite and struvite. Brushite and struvite
phases also formed when various bacteria were added to the solution, while apatite was detected only
in syntheses with E. coli and Pseudomonas aeruginosa. In all the systems, except for the synthesis in
the presence of Klebsiella pneumoniae, there was a signiﬁcant shift in the beginning of the precipitate
formation toward higher pH values. Thus, brushite was obtained in syntheses within the pH range
7.0–7.03 (in the system with “kl” at a pH of 6.10), apatite was observed only at a pH of 7.0, and struvite
at a pH of 7.0 or higher.
Addition of MPB medium to the model solution did not lead to changes in the phase compositionof the sediment. Brushite and struvite were formed when various bacteria were added to the solution(Figure 4). The brushite phase was found in all systems, but at diﬀerent pH values: Between 6.10 and116

Crystals 2019, 9, 259

7.06 (syntheses with “st”, “kl”, and “ps”) or 6.96–7.06 (syntheses with “e”). Struvite was formed in all
the systems at a pH of about 7 and higher.

Figure 4. Phase composition of synthesized products from model solutions with maximum
concentrations of additional ions characteristic of a healthy person’s urine composition. Legend
as in Figure 3.

3.4. Crystallization in the Oxalate System
As the result of the biomimetic syntheses, it was found that the presence of bacteria accelerates
nucleation within the oxalate system (Table 3). Perhaps, bacteria can act as nucleation centers.
The greatest eﬀect (more than twice) in accelerating the crystallization rates of the calcium oxalates
was observed in the presence of Pseudomonas aeruginosa.
Table 3. Nucleation of calcium oxalates in the presence of bacteria at various supersaturations (γ).
Bacteria
None
Staphylococcus aureus
Klebsiella pneumoniae
Escherichia coli
Pseudomonas aeruginosa

Nucleation Time, s
γ=3More than 2400 (>40 min)
1500
1290
1200
1140

γ=7
840
510
470
420
370

γ = 10
140
30–50
30–50
30–50
30–50

PXRD analyses of the precipitates obtained in the presence of bacteria within the oxalate systemshowed formation of calcium oxalate mono and dihydrate (whewellite and weddellite, respectively),while in the syntheses without bacteria only whewellite was formed. According to the PXRD data,117

Crystals 2019, 9, 259

the whewellite/weddellite ratio in precipitants was determined along with unit cell parameters, fromwhich the content of “zeolite” water, x, in the structure of weddellite (CaC2 O4 ·(2 + x)H2 O) was
calculated (Table 4) [29]. The presence of bacteria did not practically aﬀect the whewellite/weddellite
ratio, as well as the content of “zeolite” water (x). Moreover, the results obtained for the syntheses withbacteria were close to the eﬀect of proteins that stabilize calcium oxalate dihydrate crystallization [1,30].
Table 4. Characteristics of phases synthesized within the oxalate system in the presence of bacteria andprotein additives.
Additives

None
Ovalbumin
Escherichia coli
Pseudomonas aeruginosa
Staphylococcus aureus

Selected Crystallographic Data for the Weddellite Phase

Whewellite/Weddellite Ratio

whewellite
5:2
5:2
5:2
5:2

Unit Cell Parameter, ÅAmount of “Zeolite” Water(x), p.f.u. *–12.349(1)12.344(1)12.341(2)12.346(2)

–
0.26
0.23
0.21
0.24

* Per formula unit; calculations were made with regard to the a unit cell parameter, using the regression equation
reported in [29].

4. Discussion
Perhaps, the most important results of the study are that in systems with minimum and maximum
concentrations of inorganic ions, only analogs of the phosphate renal stone mineral phases wereobserved, while calcium oxalates were obtained under given conditions only with an increase in theconcentration of oxalate ions up to the oxalatouria values, both in experiments with bacteria and withoutthem. This result is in general agreement with the literature data on model crystallization experimentsin the human urinary system [21,27,31]. Thus, according to the thermodynamic calculations andexperiments in systems that simulate composition of the physiological liquid, calcium oxalates areformed in much smaller quantities than what is actually observed during pathogenic processes in thehuman body. Moreover, the weddellite phase (calcium oxalate dihydrate) does not form at all [31].
Introduction of bacteria and protein (ovalbumin) to the system leads to a similar result in all theexperiments, increasing the portion of weddellite and increasing the amount of calcium oxalates ingeneral. It should be also noted that, according to the unit cell parameters of weddellite crystals which
are formed in the presence of bacteria, the amount of “zeolite” H2 O molecules (x) falls into a rather
narrow range of values, whereas those in the structures of weddellite crystals from human renal stones
vary much more (from 0.13 to 0.37 p.f.u.).
According to our data, all bacteria initiate the nucleation of calcium oxalates and promote thecrystallization of metastable calcium oxalate dihydrate (weddellite) in the oxalate system (containing
only Ca2+ and [C2 O4 ]2– ions). The initiation of calcium oxalate nucleation in the presence of bacteria
is in agreement with the results of some recent studies, which describe an increase in the number ofcalcium oxalate crystals and their size in the presence of bacteria [32].
As it was shown by the results of phosphate crystallization experiments, the addition of bacteria
and nutrient media leads to a change in the phase composition of the precipitate and to the shift ofthe phosphate phase’s formation boundaries (Figures 3 and 4). The addition of the MHB medium tothe model solution with the minimum concentration of inorganic impurities led to the disappearanceof octacalcium phosphate and whitlockite, followed by the formation of brushite and rare struvite
occurrences. The same addition to the model solution with the maximum concentration of inorganic
impurities led to the crystallization of apatite, along with brushite and struvite, and to the signiﬁcant
shift of brushite and apatite formation areas toward higher pH values of the solution (~7.0).
The addition of the MPB medium to the model solution with a minimum concentration of
inorganic impurities led to the formation of brushite and whitlockite and, in addition, crystallizationof struvite was detected at a pH of 7.26, so the shift in the struvite phase formation boundary in this118

Crystals 2019, 9, 259

system moved toward being signiﬁcantly more acidic. The brushite phase was observed in this system
in a narrower pH range around 7.06. Since brushite did not form in experiments without organic
additives at such a high pH, this suggests that the boundary of its formation expanded to the morealkaline side. The same addition to the model solution with the maximum concentration of inorganic
impurities did not lead to any change in the phase composition of the synthesized products. At thesame time, it can be stated that the boundary of the brushite formation area has shifted to the morealkaline region of solutions and the boundary of the struvite formation area shifted to the more acidic
region (pH of 6.96).
The addition of bacteria to the appropriate media led to additional changes in the composition of
the precipitates (Figures 3 and 4). Thus, in the syntheses with minimal concentrations of inorganic
impurities, the appearance of Escherichia coli and Pseudomonas aeruginosa in an MHB medium led to theformation of struvite and shifted its starting crystallization boundary to the more acidic region (pH of
7.05). Although struvite was initially present in the synthetic products, the appearance of bacteria in
the MHB medium contributed to the displacement of its crystallization area to the more acidic region.
The eﬀect of Escherichia coli and Staphylococcus aureus bacteria on the crystallization of brushite wasalso well demonstrated in systems containing MHB; the shift of the brushite initial crystallization area
occurred toward the more alkaline region. The eﬀect of the bacteria addition on the crystallization of
apatite was clearly visible in the MPB medium; the appearance of bacteria promoted crystallization of
apatite and shifted its formation boundary to the more acidic region (pH of 6.72).
The change in the pH values of the solution during the biomimetic syntheses process occurred in
diﬀerent directions, due to both the crystallization process of various phases and the eﬀect of a certain
protein medium type and all types of bacteria addition. The decrease in pH in systems that modeledurine using inorganic components can be explained by the result of phosphate phase crystallization,
while an increase in systems with bacteria can be explained by the inﬂuence of metabolic products.
The presence of urease-producing bacteria such as Pseudomonas aeruginosa, Klebsiella pneumoniae andStaphylococcus aureus in urea led to an increase in pH [11,12].
The displacement of struvite crystallization boundaries obtained in the experiments, which led
to its intensive formation, once again underlines the involvement of bacteria in the formation of"infectious" renal stones, described in a number of works [3,12]. At the same time, the expansion of thebrushite crystallization area boundaries and the crystallization of apatite, as well as the formation of
weddellite in the oxalate system, shows that the inﬂuence of the presence and function of bacteria inthe crystallization medium was not only limited to the alkalization of the urine and the formation ofammonium ions, but signiﬁcantly aﬀected the types of growing mineral phases and the size of theircrystallization areas with natural variations in urine pH.
5. Conclusions
Under the conditions of model experiments, the eﬀect of bacteria that are present in humanurine (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus) on
the formation of the renal stone mineral phases, such as brushite, struvite, vitlocite, octacalcium
phosphate, apatite, whewellite, and weddellite, was studied in systems simulating the composition ofhuman urine and using two types of nutrient media (Muller–Hinton Broth and Meat–Peptone Broth).
Multidirectional changes in the pH values of the solutions were analyzed, which are the result of all
system components’ interactions with the crystallization process.
It was shown that the presence of bacteria has a diﬀerent eﬀect on the phosphate and oxalate
phases’ formation. The presence of pathogens and nutrient media signiﬁcantly aﬀect the precipitantphase composition and the position of the resulting phosphate phase’s crystallization boundaries,
which can shift both to more acidic (struvite, apatite) and more alkaline (brushite) areas. Underconditions of oxalate mineralization, bacteria accelerate the nucleation of calcium oxalates by almost
two times and also increase the amount of oxalate precipitates along with phosphates and stabilize thecalcium oxalate dihydrate to weddellite.

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Crystals 2019, 9, 259

As it can be seen from the reported results and the available literature data, the bacterial eﬀect on
oxalate and phosphate phase formation is diﬀerent. Thus, in the case of oxalate mineralization, primarily(most likely), the inﬂammatory process will contribute to the decrease of oxalate supersaturation in
urine due to calcium oxalate crystallization. In the case of phosphate mineralization, the change inurine pH and the products of bacterial metabolism will be of major importance. Studies aimed at
identifying the speciﬁc action of certain microorganisms on the crystallization of certain mineral phases
should serve to develop individual methods of treatment and prevention of urolithiasis.
The obtained results could be regarded as the scientiﬁc basis for understanding the mechanisms ofbacterial participation stone formation in the human urinary system and the creation of biotechnologicalmethods for the prevention of this disease.
Author Contributions: Conceptualization, A.R.I., O.V.F.-K., and V.V.M.; methodology, A.R.I., M.A.K., and O.V.F.-K.;
investigation, A.R.I., A.M.N., M.A.K., and V.V.M.; writing—original draft preparation, A.R.I., M.A.K., and A.M.N.;
writing—review and editing, A.R.I., O.V.F.-K., and V.V.M.; visualization, A.R.I. and A.M.N.
Funding: This research was funded by the Russian Science Foundation (grant 18-77-00026 to A.R.I.).
Acknowledgments: The XRD have been performed at the X-ray Diﬀraction Centre of the St. Petersburg State
University. We are grateful to reviewers for useful comments.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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ArticleSynthesis and Characterization of (Ca,Sr)[C2O4]·nH2O
Solid Solutions: Variations of Phase Composition,
Crystal Morphologies and in Ionic Substitutions
Aleksei V. Rusakov *, Mariya A. Kuzmina, Alina R. Izatulina and Olga V. Frank-Kamenetskaya *Crystallography Dept., Institute of Earth Sciences, St. Petersburg State University, 7/9, University emb.,199034 St. Petersburg, Russia; m.kuzmina@spbu.ru (M.A.K.); alina.izatulina@spbu.ru (A.R.I.)* Correspondence: alex.v.rusakov@gmail.com (A.V.R.); o.frank-kamenetskaia@spbu.ru (O.V.F.-K.);
Tel.: +7-921-924-2229 (A.V.R.)
Received: 14 November 2019; Accepted: 6 December 2019; Published: 8 December 2019

Abstract: To study strontium (Sr) incorporation into calcium oxalates (weddellite and whewellite),
calcium-strontium oxalate solid solutions (Ca,Sr)[C2 O4 ]·nH2 O (n = 1, 2) are synthesized andstudied by a complex of methods: powder X-ray diﬀraction (PXRD), scanning electron
microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Two series of solid solutions,
isomorphous (Ca,Sr)[C2 O4 ]·(2.5 − x)H2 O) (space group I4/m) and isodimorphous Ca[C2 O4 ]·H2 O(sp.gr.
P21 /c)–Sr[C2 O4 ]·H2 O(sp.gr. P 1), are experimentally detected. The morphogenetic regularities of theircrystallization are revealed. The factors controlling this process are discussed.
Keywords: calcium oxalate; strontium oxalate; solid solutions; ionic substitutions; weddellite;
whewellite; X-ray powder diﬀraction; scanning electron microscopy; EDX spectroscopy

1. Introduction
Natural calcium oxalates dihydrous weddellite Ca[C2 O4 ]·(2.5 − x)H2 O and monohydrous
whewellite Ca[C2 O4 ]·H2 O are widespread biominerals. They often form in the human body, for instance,a major part of the stones in the human urinary system consist of both [1,2]. These minerals also
are found in salivary stones and other human pathogenic formations [3–5]. Besides, weddellite andwhewellite often occur in crustose and foliose lichen thalli on the surface of Ca-bearing rocks andminerals [6–9]. Oxalate crystallization is a result of the interaction between metabolites of lichens andassociated microscopic fungi with an underlying stone substrate in this case [10–14].
The crystal structure of monoclinic whewellite (space group P21 /c) and tetragonal weddellite (sp.grI4/m) of renal stones are well known [15–21]. Variations of weddellite unit cell parameters are well
explained by the variable water content [18–20]. There is no evidence of ionic substitutions occurringat calcium sites of calcium oxalate crystal structures of renal stones. The complex multicomponent
composition of the crystallization medium (of urea and of other human physiological liquids), however,allows for the probability of such substitutions [22]. The probability of Cd–Ca substitutions in synthetic
whewellite is proposed by McBride et al., 2017 [23].
We found strontium impurities in weddellite and whewellite crystals in lichen thalli on Sr-bearing
apatite rock via energy-dispersive X-ray (EDX) spectroscopy [9]. It allows us to suggest that Sr2+ ionsleach from ﬂuorapatite and substitute Ca2+ ions in oxalates. Synthetic strontium oxalates, monohydrate
and dihydrate, have been reported by Baran, Sterling and Christensen, Hazen [7,15,24]. The tetragonal
Sr [C2 O4 ]·(2.5 − x)H2 O is isotypic to weddellite and, similarly, contains a variable number of watermolecules. The monohydrate, Sr[C2 O4 ]·H2 O, belongs to the triclinic crystal system (sp gr. P1).
This suggests the presence of two series of solid solutions, isomorphous (Ca,Sr)[C2 O4 ]·(2.5 − x)H2 O(sp.gr. I4/m) and isodimorphous Ca[C2 O4 ]·H2 O(sp.gr. P21 /c)-Sr[C2 O4 ]·H2 O (sp.gr. P1).
Crystals 2019, 9, 654; doi:10.3390/cryst9120654

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Crystals 2019, 9, 654

To clarify the patterns of Sr2+ ion incorporation into calcium oxalates (whewellite and weddellite),
we synthesize (Ca,Sr)[C2 O4 ]·nH2 O (n = 1, 2) solid solutions and study the variations of their phasecomposition and crystal morphologies as well as in ion substitutions.
2. Materials and Methods
2.1. Synthesis
Ca–Sr oxalates were crystallized by precipitation from aqueous solutions (0.5 L volume) containing
calcium chloride (CaCl2 , 99% purity, Vekton), strontium nitrate (Sr(NO3 )2 , 99% purity, Vekton),sodium oxalate (1.5 mmol, Na2 C2 O4 , 98% purity, Vekton) and citric acid (6.5 mmol, C6 H8 O7 ·H2 O, 99%
purity, Vekton). The atomic ratios of Sr/(Sr + Ca) cations ranged from 0 to 100%. The total amount of
Ca2+ and Sr2+ ions was 5 times higher than the content of oxalate ions in each synthesis.
A non-stoichiometric composition of the solution and the presence of citric acid were used tostabilize weddellite formation [25]. The pH range of the solutions was 4.6–6.1 due to the small additionsof solutions of NaOH (NaOH, 99%, Vekton) or HCl (HCl, 35–38 wt.% aqueous solution, 99.9% purity,
Vekton). The syntheses were carried out at room temperature (22–25 ◦ C), with the exposure of thesolution for ﬁve days until complete precipitation. The resulting precipitate was ﬁltered oﬀ, washedwith distilled water and dried in air at room temperature. The precipitate consisted of ﬁne-grained
white crystalline powder, each 0.5 L solution volume provided around 200 mg of it.
2.2. Methods
2.2.1. X-Ray Powder Diﬀraction (PXRD)
The method was used to determine the phase composition of the precipitates. The measurements
were performed using a Bruker « D2 Phaser » powder diﬀractometer (CuKα radiation of wavelength
λ = 1.54178 Å). X-ray diﬀraction patterns were collected at room temperature in the range of 2θ = 3–60◦
with a step of 0.02◦ . Phase identiﬁcation was carried out using the ICDD PDF-2 Database (release 2016).
The unit cell parameters and coherently scattering domain (CSD) size were reﬁned by using TOPAS 4.2
software [26].
2.2.2. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray (EDX) Spectroscopy
Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used for theidentiﬁcation of calcium oxalates and for estimating calcium and strontium content in their crystals.
Tetragonal weddellite and monoclinic whewellite crystals and their intergrowths were identiﬁed on
SEM images by their previously described morphological features [12,27].
The study was carried out by means of a Zeiss Supra 40MP electron microscope, equipped witha variable-pressure secondary electron (VPSE) detector and Hitachi S-3400N with energy dispersive
attachment AzTec Energy 350, at an accelerating voltage of 2 or 5 kV (depending on the image
resolution). Magniﬁcation range varied from 100x to 1000x. Two SE detectors (secondary electron
Everhart–Thornley), as well as a BSE detector (scintillation detector based on the highly sensitive YAG
crystal with the resolution of 0.1Z of the atomic number) were used. The specimens were applied on
two sided conducting tape and were coated with carbon (~15 nm). EDX analysis was performed by astandardless method that is generally reliable for elements with Z > 10. The mineral standards usedwere diopside (Ca) and celestine (Sr).
3. Results
3.1. X-Ray Powder Diﬀraction
The results of PXRD (Table 1, Figure 1) showed that, in the absence of strontium in the solution,
the obtained precipitate was represented almost exclusively by weddellite (Figure 1a).

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Crystals 2019, 9, 654

Figure 1. Typical XRD patterns of precipitates (1—weddellite, 2—whewellite), obtained from solutions
with diﬀerent atomic ratio Sr/(Sr + Ca), %: a—0; b—5; c—35; d—70; e—100.

Accompanying the addition of a small amount of strontium to the solution (Sr/(Sr + Ca) ≤ 30%),
the precipitate became dominated by calcium oxalate monohydrate whewellite (Figure 1b).
Weddellite content ranged from 23 to 34%. Concurrently, at Sr/(Sr + Ca) = 25–30%, traces of calcium
oxalate trihydrate caoxite (Ca[C2 O4 ]·3H2 O) were recorded in the precipitate (Table 1). Along with anincrease of strontium content in the solution, the amount of whewellite in the precipitate decreased,
while weddellite content increased, and at a ratio of Sr/(Ca + Sr) ~40%, these oxalates were present in the
precipitate in almost equal amounts (Figure 1c). A further increase of strontium content in the solution(40% < Sr/(Sr + Ca) ≤ 80%) saw weddellite gradually start to prevail (Figure 1d). Accompanying ahigher strontium content in the solution, the precipitate became close to monophasic—it is almost
solely represented by weddellite (Figure 1e).
Along with the increase of strontium in the solution (and, consequently, in the formed crystals),
the parameters of the weddellite tetragonal unit cell increased: a from 12.341 to 12.770 Å, c from 7.356
to 7.529 Å (Table 2). Linear parameters of monoclinic whewellite also increased: a from 6.289 to 6.396
Å, b from 14.576 to 14.860 Å, c from 10.120 to 10.367 Å (Table 2). The irregular ﬂuctuations of the angle
β value increased after Sr/(Sr + Ca) ≥ 40% (Table 2).
The average CSD size in Sr-containing weddellites varied from 162 to 71 nm and was smaller than
269 Å in Ca-weddellite. The minimum values of average CSD size (smaller than 100 Å) were observed
at intermediate Sr/(Sr + Ca) ratios in solutions of 50–70%. Regarding Sr-containing whewellite, theaverage CSD sizes were from 57 to 28 Å, with an increase of strontium in the solution (Sr/(Sr + Ca)from 15 to 75), i.e., no less than two-times less than in weddellite (Table 2). Generally, the CSD size of
the whewellite crystals gradually decreased with an increase in Sr content in the solution (up to theratio Sr/(Sr + Ca) = 75%).

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Crystals 2019, 9, 654

Table 1. Weddellite content in oxalate precipitate and strontium concentration in solution and insynthesized oxalates (via EDX).
Sr/(Ca + Sr) in
Solution, %Sample

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Wd Content in Oxalate
Precipitate, wt%

0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0

Sr Content in Crystal Phase,
wt%

99.9
28.0
23.0
24.0
30.0
34.0 *
32.0 *
57.0
51.0
66.0
81.0
90.0
93.0
78.0
86.0
96.0
99.8
99.8
99.8
99.5

Sr/(Sr + Ca) in Crystal Phase, %

Wd

Wh

Wd

Wh

0.00
8.71
14.45
18.47
26.75
26.39
29.40
35.50
35.32
42.70
48.03
56.77
57.03
64.29
72.76
77.70
83.72
87.09
92.81
100.00

0.00
4.44
7.84
11.17
15.58
17.31
22.63
21.25
24.07
25.41
31.55
42.14
45.78
49.58
54.31
No data
No data
No data
No data
No data

0.0
4.2
7.2
9.4
12.6
13.8
16.0
20.1
20.0
29.7
34.2
37.5
37.8
45.2
55.0
61.4
70.2
75.5
85.5
100.0

0.0
2.1
3.8
5.4
7.8
8.7
11.8
11.0
12.7
17.4
24.0
25.0
27.9
31.0
35.5
No data
No data
No data
No data
No data* Caoxite (2 wt%) is present in the precipitate.

Table 2. Oxalate unit cell parameters and average coherent scattered domain (CSD) size.

Sample

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Sr/(Sr + Ca) in
Crystal Phase, %X-Ray Powder Diﬀraction Data
Whewellite (sp gr P21 /c)

Weddellite (sp gr I4/m)

Wd

Wh

a, Å

c, Å

CSD, nm

0.00
4.2
7.2
9.4
12.6
13.8
16.0
20.1
20.0
29.7
34.2
37.5
37.8
45.2
55.0
61.4
70.2
75.5
85.5
100.0

0.00
2.1
3.8
5.4
7.8
8.7
11.8
11.0
12.7
17.4
24.0
25.0
27.9
31.0
35.5
No data
No data
No data
No data
No data

12.341(1)12.3568(10)12.3772(14)12.3968(8)12.4027(9)12.4211(5)12.4360(7)12.4629(8)12.4784(7)12.5112(5)12.5301(5)12.5571(9)12.5902(7)12.5907(8)12.6315(6)12.6540(5)12.7070(15)12.7384(5)12.7698(6)12.8247(4)7.356(1)7.3624(6)7.3708(8)7.3810(4)7.3873(5)7.3927(3)7.4009(3)7.4033(4)7.4090(3)7.4255(2)7.4303(3)7.4382(4)7.4529(3)7.4583(4)7.4762(3)7.4839(3)7.4979(9)7.5133(3)7.5286(3)7.5377(2)269(5)
121(3)
93(3)
162(5)
110(2)
155(4)
140(4)
103(3)
110(3)
107(1)
95(1)
60(1)
91(2)
71(1)
102(2)
106(2)
184(7)
136(3)
127(3)
194(5)

a, Å

b, Å

c, Å

β, degWhewellite not detected
6.2889(6) 14.5762(14) 10.1200(12) 109.568(8)6.2942(8) 14.5936(18) 10.1312(12) 109.533(6)6.2999(5) 14.6134(12) 10.1415(10) 109.563(6)6.3026(5) 14.6166(12) 10.1482(10) 109.544(6)6.3072(4) 14.6313(1)10.1615(10) 109.546(7)6.3122(5) 14.6491(11) 10.1716(11) 109.527(1)6.3122(7) 14.6538(15) 10.1761(15) 109.545(1)6.3169(7) 14.6666(15) 10.1836(16) 109.535(1)6.3362(8) 14.7075(17) 10.2130(2)109.579(2)6.344(2)14.725(3)10.243(4)109.65(3)6.333(4)14.769(7)10.258(9)109.39(8)6.339(3)14.817(6)10.229(9)109.11(6)6.351(2)14.790(5)10.301(6)109.56(4)6.396(3)14.860(8)10.367(6)110.07(5)CSD, nm
43(1)
47(1)
47(1)
46(1)
43(1)
43(1)
37(1)
40(1)
35(1)
32(1)
37(3)
57(7)
28(1)
30(2)

No data

3.2. Scanning Electron Microscopy (SEM) and Energy-Dispersive X Ray (EDX) Spectroscopy
SEM data on the phase composition of the synthesized calcium oxalates support the XRD data
(Table 1, Figure 2).

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Figure 2. SEM images of formed (Ca,Sr)[C2 O4 ]nH2 O (n = 1, 2) crystals, synthesized from solutions
with diﬀerent Sr/(Sr + Ca), % ratio: a: 0, b: 5, c: 20, d: 35, e: 70, f: 80.

SEM images clearly show that weddellite dipyramidal crystals represent calcium oxalates
synthesized from strontium-free solutions, the dipyramidal base edge (Dp) of which does not exceed
5–6 microns (Figure 2a).
Found in the precipitate obtained from a solution with a ratio of Sr/(Sr + Ca) = 5%, small spherical
spherulites of calcium oxalate monohydrate whewellite (diameter ~4–5 μm) prevail, among which there
are individual weddellite crystals and their intergrowths (Figure 2b). According to EDX spectroscopy,
strontium content in weddellite (Sr/(Sr + Ca) = 4.2%) is close to that in the solution and is two times
greater than in whewellite (Sr/(Sr + Ca) = 2.1%). Weddellite crystals are deﬁned by dipyramidal
habit (Dp ~10–25 μm). Not very well-developed prism faces appear on smaller crystals (rib length
between prism faces Pr = 3–4 μm, Figure 2b inset 1). The value of the average Pr/Dp ratio (which
describes the degree of prism face development compared to dipyramid) is only 0.1. There also aretwin intergrowths which form “quadruplets” consisting of two intergrown tetragonal dipyramidswith a common fourth-order symmetry axis, with each dipyramid being rotated relative to each otheraround this axis of symmetry by 45◦ (Figure 2b inset 2).
Accompanying an increase in—ratio in the solution up to 30%, whewellite continues to prevailover weddellite (Figure 2c). According to EDX, Sr content increases in both phases (up to ratio
Sr/(Sr + Ca) = 16.0% in weddellite and up to 11.8% in whewellite), but it decreases relative to thesolution. The size of whewellite spherulites increases (average diameter ~6 μm, maximum 15 μm).
Weddellite average crystal size also increases (Dp ~20–25 μm). While the number of weddellite
crystals of dipyramidal-prismatic pattern increases, the prism face continues to develop: Pr ~10 μm,
the Pr/Dp ~0.35.

127Crystals 2019, 9, 654

When the Sr/(Sr + Ca) ratio in the solution reached 35–40%, the number of crystals of whewellite
and weddellite became comparable (Figure 2d). According to EDX, Sr content continued to increase inboth phases (up to Sr/(Sr + Ca) = 20% in weddellite and up to 11–13% in whewellite) and decreased
relative to the solution. The size of whewellite spherulites was relatively small (5–6 microns in diameter,
but reached 20 microns). Weddellite crystals also increased in size (Dp ~15–30 μm). All weddellite
crystals had prism faces and the prism faces continued to develop (Pr ~10–20, Pr/Dp ratio ~0.40–0. 45).
When the ratio in the solution Sr/(Sr + Ca) ≤ 40%, weddellite began to prevail over whewellite.
Two generations of weddellite crystals were observed in the precipitate: large dipyramidal (Dp up
to 80 μm) and smaller dipyramidal-prismatic (Dp ~25 μm, Pr ~15–20 μm) (Figure 2e). The Pr/Dp
ratio in weddellite crystals continued to increase and reached 0.56 at Sr/(Sr + Ca) = 50% in solution,and then began to decrease to 0.33 at Sr/(Sr + Ca) = 70%. The diameter of spherulites of whewellite
decreased to 3–5 microns. According to EDX, the Sr/(Sr + Ca) ratio reached 55.0% in weddellite and35.5% in whewellite.
Reaching a Sr/(Sr + Ca) ratio of 80% and more in the solution, weddellite crystallized almost
solely, and was represented by large dipyramidal (200–250 μm) and smaller dipyramidal-prismatic(30–50 μm) crystals (Figure 2f). Attaining Sr/(Sr + Ca) = 85–95%, the development of the prism faceslowed (Pr remained ~20 μm, the Pr/Dp ratio decreased to ~0.1). The Sr/(Sr + Ca) ratio in weddellite
via EDX increased from 78 to 100%, approaching the value of this ratio in solution.
According to the EDX data, the Sr/(Sr + Ca) ratios of 29.7–45.2 in weddellite and of 17.4–31.0 in
whewellite correspond to the minimum CSD size for both phases.
4. Discussion
The results of the study show that in all syntheses, solid solutions (Ca,Sr)[C2 O4 ]·nH2 O (n = 1,
2) with a variable ratio of dihydrate and monohydrate phases are obtained. Phase and elemental
composition of synthesized solid solutions, as well as the morphology of their crystals, is strongly
relevant to strontium concentration in the solution.
4.1. The Eﬀect of Strontium Concentration in Solution on Phase Composition of the Precipitate
The strontium content in the solution signiﬁcantly aﬀects the phase composition of the precipitate.
Considering the absence of Sr2+ ions in the solution of non-stoichiometric composition (Ca/C2 O4 = 5)
containing citric acid, tetragonal calcium oxalate dihydrate weddellite (Figures 1a and 2a) is
obtained, which is unstable in the crystallization ﬁeld of monoclinic calcium oxalate monohydrate
whewellite [17,21]. The addition of a small amount of Sr2+ ions (Sr/(Sr + Ca) = 5%) to this solution
violates the conditions favorable for weddellite crystallization and leads to intensive crystallizationof whewellite, the content of which in the precipitate exceeds the amount of weddellite by 2.6 times(Figures 1b and 2b, Table 1). Accompanying an increase in the strontium content in the solution, theamount of weddellite in the precipitate gradually increases and, at a ratio of Sr/(Sr + Ca) = 35–45%,
matches the amount of whewellite (Figures 1c and 2d). A further increase in strontium content again
creates conditions favorable for the crystallization of weddellite, the content of which in the precipitate
begins to prevail (Figures 1d and 2e). Reaching an Sr/(Sr + Ca) ≥ 80% ratio in solution, the precipitate
consists almost exclusively of weddellite. Thus, the presence of strontium in the crystallization
medium can be favorable for the crystallization of both weddellite and whewellite, depending on
the concentration.
4.2. Sr-Ca Ionic Substitutions in Solid Solutions
According to EDX, strontium is present in crystals of both calcium oxalate monohydrate anddihydrate (Table 1). The ratio Sr/(Sr + Ca) varies from 0 to 100% in weddellite and from 2.1 to 35.5% in
whewellite. A narrower range of observed strontium concentrations in whewellite is explained by thesynthesis conditions under which whewellite, unlike weddellite, precipitates only in solutions with
an Sr/(Sr + Ca) ratio varying from 5 to 75% and always alongside weddellite. Altogether, strontium
128Crystals 2019, 9, 654

content in weddellite is always higher than in whewellite by 1.4–2 times (Table 1), which indicates thatthe entry of Sr into whewellite is diﬃcult compared to weddellite.
It is known that, despite the signiﬁcant diﬀerence in ionic radii (rCa VIII = 1.12 Å, rSr VIII = 1.26
Å, [28]), Sr2+ ions easily replace Ca2+ ions which occupy large cavities coordinated by 7–9 cations
in various crystal structures, like in apatite [29,30]. This suggests that all (or almost all) strontium
recorded via EDX isomorphically substitutes calcium in its positions in weddellite and whewellite.
A simultaneous increase of the unit cell parameters of weddellite and whewellite with the increasing
Sr content (Figures 3 and 4) conﬁrms this assumption.
An increase in the unit cell parameters of weddellite, primarily parameter a, also occurs with
the increase of zeolite water (Wz ) [18,19]. An increase of zeolite water amount in weddellite (from0.13 to 0.37 apfu), however, leads to an increase in the a parameter by only 0.049 Å [18]. Conversely,
the contribution of strontium to the increase of the weddellite parameter a is an order of magnitude
greater; the change in parameter a reaches 0.469 Å (Figure 3a). The dependence of unit cell parametersof weddellite on strontium content is not linear and follows a second degree polynomial function(Figure 3). Violation of the linear correlation between the strontium content and the parameter values
can be due, ﬁrst, to the fact that when the Sr/(Sr + Ca) ratio reaches ~40% in weddellite (~60% in
solution) Sr2+ ion entry into weddellite slows. Second, it is possible that at this strontium content, theamount of zeolite water in weddellite ceases to increase and a further slower increase in parameters
occurs only due to an increase in strontium.

(a)

(b)

Figure 3. The increase of weddellite unit cell parameters: a unit cell parameter versus strontium(present study) and zeolite water [18] content, % (a); c unit cell parameter versus strontium content,% (b).

Seen at low strontium concentrations in whewellite (with Sr/(Sr + Ca) ratios in crystals varying
from 2.1 to 12.7%; and in solution from 5 to 40%), there is a well-deﬁned linear relationship between
the values of linear parameters (a, b, c) and the content of strontium, while the angle β does not change
signiﬁcantly (Figure 4). The ﬂuctuations of both linear parameters and angle β at higher strontium
concentrations can be explained by desymmetrization (lowering the symmetry from monoclinic to
triclinic), since the crystal structure of strontium oxalate monohydrate (Sr[C2 O4 ]·H2 O) is triclinic [7,24].
Desymmetrization may be associated with reciprocal turns of calcium polyhedra and oxalate groups,
associated with the partial ordering of calcium and strontium atoms at the crystallographic sites. To thequestion of why desymmetrization is not at maximum in the middle of the series, further studies
should be made.

129

Crystals 2019, 9, 654

(a)

(b)

(c)

(d)

Figure 4. Variations of whewellite unit cell parameters versus strontium content, %: a (a); b (b); c (c); β (d).

Thus, assumption of the presence of two series of solid solutions, isomorphous (Ca,Sr)[C2 O4 ]·(2.5 − x)H2 O) (sp.gr. I4/m) and isodimorphous Ca[C2 O4 ]·H2 O(sp.gr. P21 /c)–Sr[C2 O4 ]·H2 O(sp.gr.
P 1), received experimental conﬁrmation. A detailed study of the desymmetrization pattern of themonoclinic whewellite structure with strontium incorporation via single-crystal X-ray diﬀraction
analysis is now in progress.
4.3. Morphogenetic Patterns of the Formation of Solid Solutions (Ca,Sr)[C2 O4 ]·nH2 O (n = 1, 2)
4.3.1. The Eﬀect of Sr Concentration in Solutions on Crystal Morphology
Regarding all variations of strontium content in solution (0 ≤ Sr/(Sr + Ca) ≤ 100%), weddellite
is represented by dipyramidal and/or dipyramidal-prismatic crystals, and whewellite by roundspherulites (Figure 2).
As strontium content in the solution increases, the size of the weddellite dipyramidal crystals
grows as well: Dp varies from ~5–6 μm in pure calcium crystals to 30 μm at Sr/(Sr + Ca) = 35%(Figure 2a–d). Accompanying a further increase of strontium concentration in the crystallization
medium, the formation of two weddellite crystal generations is clearly observed. Larger ones aremainly of dipyramidal pattern and smaller ones of dipyramidal-prismatic conﬁguration (Figure 2e).
The size of larger weddellite crystals increases with the increase of strontium concentration in thesolution, from Dp = 30 μm at Sr/(Sr + Ca) = 35% to Dp = 200 μm at Sr/(Sr + Ca) = 80–95% (Figure 2f).
130

Crystals 2019, 9, 654

The size of smaller weddellite crystals also increases with the increase of strontium content, from
Dp = 15 μm at Sr/(Sr + Ca) = 35% in solution to Dp = 50–60 μm at Sr/(Sr + Ca) = 80–95% in solution.
Tetragonal prism faces of smaller weddellite crystals are well developed and the size of these faces
along the prism edge also gradually increase with the increase of strontium content in the solution,from Pr = 3–4 μm at Sr/(Sr + Ca) = 5% (Figure 2b) to Pr = 20 μm at Sr/(Sr + Ca) = 70–80% (Figure 2e,f).
The aspect ratio between the prism and dipyramid faces with the increase of strontium content in thesolution increases from 0.1 (at Sr/(Sr + Ca) = 5%) to 0.56 (at Sr/(Sr + Ca) = 50%) and then it starts todecrease to 0.1 (at Sr/(Sr + Ca) = 95%).
Shown above, the ﬁrst and last syntheses of the studied series (purely calcium and purely
strontium solutions) are characterized by the formation of weddellite crystals in the form of a tetragonal
dipyramid, which indicates optimal conditions for the precipitation of calcium and strontium oxalate
dihydrate. The middle members in the weddellite series exhibit tetragonal prism faces in their pattern.
The ratio between linear sizes of the prism and dipyramidal faces increases with the increase instrontium content in solution from 0.1 (at Sr/(Sr + Ca) = 5%) to 0.56 (at Sr/(Sr + Ca) = 50%), and thendecreases to 0.1 (at Sr/(Sr + Ca) = 95%). Accordingly, with an increase in strontium in the crystallization
medium from 5 to 50%, the initial prism growth rate increases signiﬁcantly and then (at a ratio Sr/(Sr+ Ca) ≥ 65%) begins to decrease. The slowdown in the growth rate of weddellite prism faces in
the middle of the calcium-strontium series is most likely due to the deviation of the crystallization
conditions from optimal.
Kuzmina et al [25] demonstrates that the crystallization of whewellite in the form of spherulites iscommon for solutions containing citrate ions and indicates a more rapid growth of whewellite (comparedwith weddellite) under the conditions of a higher supersaturation of the solution. The average diameterof whewellite spherulites is dependent on the content of Sr2+ cations in the solution. It ﬁrst increases
from 4 μm (at Sr/(Sr + Ca) = 5%, Figure 2b) to 6 μm (at Sr/(Sr + Ca) = 35–50%, Figure 2d), then it
decreases to 1–2 μm (at Sr/(Sr + Ca) = 70–80%, Figure 2e,f) amid the decrease in the total amountof whewellite.
4.3.2. Sr-distribution in Oxalate System «Solution–Crystal»
The strontium concentration in oxalate monohydrate and oxalate dihydrate (0 ≤ Sr/(Ca + Sr)≤ 100%) depends on strontium concentration in the solution following a third degree polynomial
function (Figure 5):
Sr/(Sr + Ca)Solution = 0.0006[Sr/(Sr + Ca)wh ]3 − 0.0815[Sr/(Sr + Ca)wh ]2 + 4.3286[Sr/(Sr + Ca)wh ] −5.4929,Sr/(Sr + Ca)Solution = 0.0001[Sr/(Sr + Ca)Wd ]3 − 0.0266[Sr/(Sr + Ca)Wd ]2 + 2,67[Sr/(Sr + Ca)Wd ] −7.1757.
Fitting with a third degree polynomial function is necessary due to the fact that the diﬀerence
between strontium content in the crystallization medium and in synthesized solid solutions increases
in the middle of the corresponding series.
The described distribution of strontium in the oxalate “crystal-solution” system indicates that theincorporation of strontium ions into calcium oxalates from solutions with close to equal amounts of
Ca and Sr is diﬃcult, which can explain the decrease of weddellite prism face growth rate and thelowering of CSD sizes for both weddellite and whewellite.
The diﬀerence in strontium content between the solution and whewellite is greater than between
the solution and weddellite (Figure 5), which supports a proposal for a more diﬃcult incorporationof Sr2+ cations into whewellite than into weddellite. This eﬀect can most likely be explained by the
speciﬁc features of weddellite and whewellite crystal structures. This result also agrees well with theabove conclusion that whewellite crystals grow more rapidly than weddellite crystals.

131Crystals 2019, 9, 654

Figure 5. Sr/(Ca + Sr) ratio in the solution versus Sr/(Ca + Sr) ratio in the weddellite (•) and whewellite (•).

5. Conclusions
To clarify the patterns of Sr incorporation into calcium oxalates (whewellite and weddellite) the(Ca,Sr)[C2 O4 ]·nH2 O (n = 1, 2) solid solutions were synthesized and studied by complex methods(PXRD, SEM and EDX). It was shown that phase and elemental composition of synthesized solid
solutions, as well as the morphology of their crystals, is strongly relevant to Sr concentration in thesolution. The presence of two series of solid solutions, isomorphous (Ca,Sr)[C2 O4 ]·(2.5 − x)H2 O) (sp.gr.
I4/m) and isodimorphous Ca[C2 O4 ]·H2 O(sp.gr. P21 /c)–Sr[C2 O4 ]·H2 O(sp.gr. P 1) was experimentally
proven for the ﬁrst time. The causes of a diﬃcult incorporation of strontium ions into calcium
oxalates (especially into whewellite) were discussed. Morphogenetic regularities of their formation
were revealed.
The results of this work show the possibility of diverse ionic substitutions occurring at calcium
sites of calcium oxalate crystal structures, which opens a new page in the crystal chemistry of oxalic
acid salts. Among the oxalates found in nature, bioﬁlm biominerals formed as a result of the interactionof metabolism products of lithobiotic community with bedrock are highly favorable to exhibit ionicsubstitutions at a global scale. The regularities of these ionic substitutions are determined by boththe mineral and elemental composition of the underlying rock and the species composition of the
microorganisms inhabiting them. The applied value of the obtained patterns lies in the ﬁeld of
the development of biotechnologies which use the oxalate microbial crystallization as a source ofbioremediation for environments contaminated with toxic elements.
Author Contributions: Conceptualization (A.V.R. and O.V.F.-K.); Investigation (A.V.R., M.A.K. and A.R.I.);
Methodology (A.V.R., O.V.F.-K. and M.A.K.); Visualization (A.V.R. and A.R.I.); Writing—original draft (A.V.R.,
M.A.K. and A.R.I.); Writing—review & editing (A.V.R. and O.V.F.-K.).
Funding: This research was funded by the Russian Science Foundation (grant 19-17-00141 to A.V.R., O.V.F.-K.,
M.A.K., A.R.I.).
Acknowledgments: The laboratory researches were carried out in Research Resource Centers of Saint Petersburg
State University: XRD measurements—in the X-ray Diﬀraction Centre; SEM investigations—in the Interdisciplinary
Resource Center for Nanotechnology and Centre for Geo-Environmental Research and Modelling (Geomodel).

132

Crystals 2019, 9, 654

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

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134crystals
ArticleCharacterization of Biominerals in Cacteae Species
by FTIR
Alejandro De la Rosa-Tilapa 1 , Agustín Maceda 2 and Teresa Terrazas 1, *1
2*

Instituto de Biología, Universidad Nacional Autónoma de México, México City 04510, Mexico;aledrosa17@gmail.com
Programa de Botánica, Colegio de Postgraduados en Ciencias Agrícolas, Texcoco,
Estado de México 56230, Mexico; biologoagustin@hotmail.com
Correspondence: tterrazas@ib.unam.mx

Received: 7 May 2020; Accepted: 27 May 2020; Published: 29 May 2020

Abstract: A biomineral is a crystalline or amorphous mineral product of the biochemical activity ofan organism and the local accumulation of elements available in the environment. The cactus family
has been characterized by accumulating calcium oxalates, although other biominerals have been
detected. Five species of Cacteae were studied to ﬁnd biominerals. For this, anatomical sections andFourier transform infrared, ﬁeld emission scanning electron microscopy and energy dispersive X-ray
spectrometry analyses were used. In the studied regions of the ﬁve species, they presented prismaticor spherulite dihydrate calcium oxalate crystals, as the predominant biomineral. Anatomical sectionsof Astrophytum asterias showed prismatic crystals and Echinocactus texensis amorphous silica bodies in
the hypodermis. New ﬁndings were for Ariocarpus retusus subsp. trigonus peaks assigned to calcium
carbonate and for Mammillaria sphaerica peaks belonging to silicates.
Keywords: anatomy; Cactaceae; calcium carbonate; oxalate; silica; stem; weddellite

1. Introduction
A biomineral is a crystalline or amorphous mineral product of the biochemical activity of an organism
and the local accumulation of elements available in the environment [1]. In plants, the most common
biominerals are amorphous silica, calcium oxalate and calcium carbonate salts [2–7]. Some species of the
Cactaceae family accumulate up to 85% of their dry weight in calcium oxalate crystals [8,9]. These calcium
oxalate crystals may have one of two states of hydration: monohydrate (CaC2 O4 ·H2 O; whewellite) or
dihydrate (CaC2 O4 ·2H2 O; weddellite) [10–17]. In addition, other biominerals, such as magnesium oxalate(MgC2 O4 ·2H2 O; Glushinkite) [17], amorphous silica bodies (SiO2 ·nH2 O; opal) [18] and silica in crystalline
form (SiO2 ; α-quartz) [19], have been identified in cacti.
Like other plants, biominerals in Cactaceae develop mainly in the cellular vacuole of diﬀerent
epidermal, fundamental or vascular stem tissues. The accumulation of biominerals in a given tissue isusually highly speciﬁc in some species or genera [10,18,20–23]. Thirty-four species from the Cacteae
tribe have been studied with techniques such as X-ray diﬀraction to detect calcium oxalates [11],by Raman spectroscopy in Ferocactus latispinus and Coryphantha clavata [24] and by Fourier transform
infrared (FTIR) only in one species, Mammillaria uncinata [25]. In these techniques, the tissues wereblended or macerated together, so they have not been analyzed individually in the Cacteae species.
For this reason, the FTIR technique can be a good option to study biominerals due to theminimum amount of sample and the speed of data acquisition. Therefore, the aims of this study wereto characterize the biominerals and determine their hydration state in the diﬀerent tissues of ﬁve
Cacteae species. With this, it will be possible to identify other biominerals that are not calcium oxalate,magnesium oxalate or amorphous silica, in addition to the state of hydration of biominerals in thediﬀerent tissues.
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2. Material and Methods
Two adult and healthy plants of ﬁve species classiﬁed within the Cacteae tribe were collectedin their natural habitats (Table 1). For each species, a portion of the plant was prepared as a voucher,which was deposited in the National Herbarium (MEXU). In one of the two plants per species,
spines were removed. Then, using a dissecting microscope at diﬀerent magniﬁcations, the stem of
each sample was dissected into the epidermis plus hypodermis (EH), cortex (C), vascular cylinder(VC) and pith (P) (Figure 1A,B). Each tissue was blended with distilled water and ﬁltered with a meshof approximately 300 μm pore diameter to separate biominerals from cell debris. The biominerals
were precipitated, washed with distilled water several times until no residue was observed under astereomicroscope, and ﬁnally, dried at room temperature. Small samples from pith to epidermis wereprepared for FE-SEM-EDS (see 2.3, JEOL Ltd., Akishima, Tokyo, Japan).
Table 1. Species analyzed and the state of Mexico where they were collected. Vouchers deposited
at MEXU.
SpeciesCollection Number

Location

Astrophytum asterias (Zucc.) Lem.

TT1020TT846
TT1005
TT879
TT1021
TT851
TT1050TT1051La Esperanza, Tamaulipas
San Carlos Tamaulipas
La Soledad, Tamaulipas
Moctezuma, San Luis Potosí
La Esperanza, Tamaulipas
Tula, Tamaulipas
Ejido Huizache, Tamaulipas
Ejido Huizache, Tamaulipas

Ariocarpus retusus subsp. trigonus (F.A.C.Weber) E.F.Anderson & W.A.Fitz Maur.
Echinocactus texensis Hopﬀer
Mammillaria melanocentra subsp. rubrograndis (Repp. & A.B. Lau) D.R. Hunt
Mammillaria sphaerica A. Dietr.

Figure 1. Stem and macro and microscopic tissues studied. (A) Echinocactus texensis (TT1021), showing theglobose depressed stem. (B) Echinocactus texensis, transverse section illustrating the stem tissues.
(C) Mammillaria sphaerica (TT1051), microscopic transverse section showing the tissues. A = apical,
M = medium, B= basal, EH = epidermis-hypodermis, C = cortex, VC = vascular cylinder, P = pith.
Bar is 300 μm in C.

The second individual of each species was divided into three parts (apical, median and basal) no
larger than 1.5 cm length that included epidermal, cortical, vascular and pith tissues (Figure 1A,C).

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Each part was immediately ﬁxed in a formalin-acetic acid-ethanol solution (10:5:85) [26] and processedfor paraﬃn embedding according to Loza-Cornejo and Terrazas [27]. Transverse and tangential sections
14 μm thick were made with a rotary microtome, stained with Safranin-fast green, and mounted withsynthetic resin.
2.1. Fourier Transform Infrared (FTIR) Spectroscopy Analysis
Approximately 0.1 g of biomineral dry sample of each species was used to obtain the infrared
spectra. The spectra were obtained with an Attenuated Total Reﬂectance Fourier Transform Infrared
Spectrometer (ATR-FTIR) (Agilent Cary 630 FTIR, Agilent Technologies, Santa Clara, CA, USA)equipped with an ATR diamond unit [28]. The samples were processed with two wavenumber rangesof 4000–400 cm−1 and 4000–650 cm−1 (30 scans with a resolution of 4 cm−1 , 15 seconds per sample
and three replicates per sample) with the program MicroLab PC (Agilent Technologies, Santa Clara,
CA, USA). The baseline correction, ATR correction (applied only in 4000 to 400 cm−1 ) and the spectra
average were performed with the Resolution Pro FTIR Software program (Agilent Technologies,
Santa Clara, CA, USA). With the spectra obtained, the peaks corresponding to the mineral components
were identiﬁed and compared with those reported in the literature [29,30].
2.2. Polarized and Brightﬁeld Microscope Analysis
For each species, permanent slides were observed with both types of lighting in an Olympus BX
51 microscope (Olympus, Tokyo, Japan) and photographs obtained with Image Pro Plus 7.1 software(Media Cybernetics, MD, USA) to characterize the morphology and distribution of biominerals inthe diﬀerent tissues of the three regions studied (Figure 1). Additionally, photographs of isolated
biominerals from each tissue were also obtained with the two types of lighting.
2.3. Field Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray Spectrometry(EDS) Analysis
For FE-SEM observations, samples from pith to epidermis of stems (<1 cm) per species wereplaced between to coverslips and dried in an oven (56 ◦ C) overnight. The dried samples were ﬁxed toaluminum specimen holders with double-sided tape and coated with gold in a Hitachi-S-2460N sputter
coater. The coated samples were then observed using a Field Emission Scanning Electron Microscope(FE-SEM; JEOL-JSM7800, JEOL Ltd., Akishima, Tokyo, Japan) (20 Kv) coupled to Energy Dispersive
X-ray Spectrometry (EDS; Oxford X-Maxn2 50 mm2 , Oxford Instruments, Tubney Woods, Abingdon,
UK) at the Physics Institute, UNAM. It was calibrated with copper and standards were: C (Cvit),O (SiO2 ), Mg (MgO), Al (Al2 O3 ), Si (SiO2 ), K (KBr) and Ca (Wollastonite). The relative concentration of
the element is given in percentages of weight.
3. Results
3.1. Analysis by FTIR
Figure 2 shows the spectra of the four tissues analyzed from the ﬁve species. In all spectra,
the characteristic calcium oxalate crystals peaks were present (Table S1) in all species and in thefour tissues. Furthermore, in all the tissues of the ﬁve species, a weak peak at 915 cm−1 and awide peak ranging from 3000 to 3600 cm−1 due to the OH stretching were present and reﬂected
the occurrence of dihydrate calcium oxalate (CaC2 O4 ·2H2 O). This dihydrate calcium oxalate was
detected in Astrophytum asterias, Echinocactus texensis and Mammillaria sphaerica with the peak 517 cm−1
(Figure 2A,E,I), although there is noise on the spectra. The peaks between 1014 and 1048 cm−1 in sometissues of the ﬁve species studied were assigned to opal and silicates (Figure 2B,D,F,H,J), whereas somepeaks in the pith of Ariocarpus retusus subsp. trigonus were assigned to calcium carbonate (Figure 2C,D).
The peaks at 555 and 560 cm−1 could represent some contaminants or unknown biomineral residue(Figure 2A–J). The occurrence of primary cell wall debris was assigned to hemicellulose residues in137

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the pith (4) of Ariocarpus retusus subsp. trigonus (Figure 2D) and to cellulose residues in the pith (4)and epidermis-hypodermis (1) of Astrophytum asterias (Figure 2A,B), Ariocarpus retusus subsp. trigonus(Figure 2C,D) and Mammillaria sphaerica (Figure 2I,J).

Figure 2. Spectra of biominerals in each tissue studied. The numbers below each spectra (line) indicate
the wavenumbers of the peaks. A, C, E, G, I 4000–400 cm−1 spectra. B, D, F, H, J 4000–650 cm−1
spectra. (A,B) Astrophytum asterias. (C,D) Ariocarpus retusus subsp. trigonus. (E,F) Echinocactus texensis.
(G,H) Mammillaria melanocentra subsp. rubrograndis. (I,J) Mammillaria sphaerica. Tissues: 1(black line) =Epidermis-hypodermis, 2(red) = Cortex, 3(purple) = Vascular cylinder, 4(green) = Pith, W = weddellite(calcium oxalate dihydrate), O = opal (amorphous silica), C = Calcium carbonate, S = silicates.

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3.2. Anatomy with Light and Polarized Microscopy
The ﬁve species studied have the typical stem anatomy for the Cacteae. There is an epidermis
with straight or convex outer cell wall and a hypodermis of one to ﬁve strata of parenchyma orcollenchyma (Figure 3A,E and Figure 4A). Both cortex and pith have exclusively parenchyma cells(Figure 4A) and in the vascular cylinder, nonﬁbrous wood with wide medullary unligniﬁed rays
(Figure 4F–H). The studied species do not form visible biominerals in the epidermis (Figure 3). Two of
the ﬁve species studied here had biominerals in the hypodermal cells. Astrophytum asterias presented
prismatic biominerals (Figure 3A,C) that were birefringent under polarized light (Figure 3B,D) and
Echinocactus texensis had amorphous silica bodies that lacked birefringence under polarized light(Figure 3E,F). Biominerals in the hypodermal cells of Ariocarpus retusus subsp. trigonus and both
Mammillaria species were not detected in the anatomical sections.

Figure 3. Biominerals in the hypodermis of stem in Cacteae, transverse sections (TS) and isolated(IS). (A, C, D, E, F) Bright ﬁeld. (B, D) Polarized light. (A,B) Astrophytum asterias (TT846), TS. (C,D)
Astrophytum asterias (TT1020), IS. (E,F) Echinocactus texensis (TT859), TS. Bar is 50 μm in A, B, E; 20 μm
in C, D, F. e = epidermis, co = cortex, h = hypodermis, white arrows = amorphous silica bodies.

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In the cortex (Figure 4A,B,D), vascular cylinder (Figure 4F–H) and pith (Figure 4I,J), all species
had spherulites or prisms, which were birefringent with polarized light (Figure 4C,E,G,L,N,P). In thevascular tissue, the biominerals were deposited in the ray cells (Figure 4F,H).

Figure 4. Biominerals in the stem tissues of transverse sections (TS) and isolated (IS) using bright ﬁeld (b)or polarized light (p). (A) Ariocarpus retusus subsp. trigonus (TT1005), TSbAR, epidermis, hypodermis(orange arrowhead) and cortical tissue. (B,C) Astrophytum asterias (TT846), TSbpMR, spherulites incortical cells. (D,E) Astrophytum asterias (TT1020), IS, spherulites of cortical tissue. (F,G) Echinocactus
texensis (TT859), TSbpBR, spherulites in vascular cylinder. (H) Mammillaria sphaerica (TT1051), TSbBR,
vascular cylinder. (I) Ariocarpus retusus subsp. trigonus (TT879), TSbAR, pith. (J) Echinocactus texensis(TT859), TSbBR, spherulites (black arrows) in pith. (K,L) Ariocarpus retusus subsp. trigonus (TT1005),
ISbp, spherulites of cortical tissue. (M,N) Mammillaria melanocentra subsp. rubrograndis (TT1050), ISbp,
spherulites of pith. (O,P) Mammillaria sphaerica (TT1051), ISbp, spherulites of cortical tissue. Bar is
300 μm in A, B, C, F–J; 100 μm in M, N; 50 μm in D, E, K, L; 20 μm in O, P. AR = apical region;
MR = medium region; BR = basal region; X = tracheary cells; * = rays.

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3.3. FE-SEM-EDS Analysis
Figure 5 shows the morphology and elements detected by EDS analysis. Diﬀerences between
hypodermal and cortical element concentrations were observed. Astrophytum asterias has the highest
concentration Ca (49.53%) and Al (0.87%) in the prismatic crystals of the hypodermis, whereas in
Echinocactus texensis, hypodermis showed the lowest concentrations of Ca (5.08%) and Al (0.15%),
but the highest of Si (25.34%). No traces of Mg, K and Si were detected in Astrophytum asterias
spherulites (Figure 5F), but traces of Si (0.91), K (0.79%), Mg (0.17%) were detected in Ariocarpus retusus
subsp. trigonus (Figure 5H).

Figure 5. SEM images and EDS spectra for stem tissues of Cacteae species. (A,B) Astrophytum asterias(TT1020), prisms in hypodermis. (C,D) Echinocactus texensis (TT1021), amorphous silica bodies in
hypodermis. (E,F) Astrophytum asterias, spherulite in cortex. (G,H) Ariocarpus retusus subsp. trigonus(TT1005), spherulites in cortex. The white square on images was the area used to analyze the elements.
Bar is 10 μm in A, E, G; 1 μm in C.

4. Discussion
Calcium oxalates and amorphous silica bodies turned out to be the main biominerals in the
Cacteae species studied. Both are not exclusive to the cactus family, as calcium oxalates have been
described in at least 215 plant families [6] and amorphous silica bodies in 56 [31]. However, the presenceof these biominerals in Cactaceae has had great relevance in its systematics, because they are notdeposited randomly in the stem tissues [20,23]. Moreover, it has been suggested the possibility ofidentifying biominerals other than calcium oxalate [10,21,23] and diﬀerent crystalline structures havebeen described even within the stem of the same species [23] as we found in the studied species.
Calcium carbonate is poorly described for Cactaceae, possibly due to the ﬁxation technique of thesamples, so that unﬁxed samples combined with the FTIR analysis favor the recognition of calcium
carbonate as well as the presence of other compounds.
4.1. Biominerals Identiﬁcation
4.1.1. Calcium Oxalate
The diﬀerences between the monohydrate (whewellite, Wh) and dihydrate (weddellite, W) spectraof calcium oxalate have been documented by Conti et al. [32] and Petit et al. [33]. These authors
point out that in the infrared spectrum, the calcium oxalate monohydrate (Wh) is characterized in theelongation zone of water molecules (3000–3600 cm−1 ) with several peaks at 3058, 3258, 3336, 3429
and 3483 cm−1 . Furthermore, the pattern of calcium oxalate dihydrate has a more intense peak close141

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to 3469 cm−1 . In the studied species, there was an intense peak that varies from 3466 to 3327 cm−1
(Figure 1A,C,E,G,I). Other characteristic vibrations of calcium oxalate dihydrate are a weak signal at1475 and 915 cm−1 that belongs to the symmetric vibrations of CO and CO + H2 O, respectively [33],and were present from 1476 to 1466 and 913 to 910 cm−1 (Figure 1B,D,F,H,J) in most tissues of the species
studied. In the species studied, calcium dihydrate vibrations ranged from 774 to 762 cm−1 , very close to
the reported vibrations (770 cm−1 ), while the calcium oxalate monohydrate is assigned to 782 cm−1 [33].
Based on these results, we consider that calcium oxalate dihydrate is the dominant biomineral in thestudied species and is mainly distributed in the three tissues. Hartl et al. [12] documented the presenceof this state of hydration for other species of Ariocarpus (W), Astrophytum (W), Echinocactus (W) andMammillaria (Wh or W). Mammillaria is the largest genus of the Cacteae tribe with more than 100 species;
to date, only six have been studied, Wh (3) and W (3) (12, 25), plus the two here reported. The study ofother Mammillaria species is needed to understand the variability of calcium oxalate in terms of thestate of hydration as well as the identiﬁcation of other elements.
4.1.2. Silica–Amorphous Hydrated Silica
Zancajo et al. [34] and Corrales-Ureña et al. [35] identiﬁed the vibration of the Si-O-Si bonds
at 1090 and 1020 cm−1 , in amorphous silica bodies (phytoliths) from Sorghum and Ananas comosus.
In Echinocactus texensis, amorphous silica bodies were found in the EH spectrum region, with thevibration of the Si-O-Si bond at 1014 cm−1 (Figure 2D). These silica bodies were distinctive in the
anatomical sections of the hypodermis (Figure 3E,F). Anatomically, silica bodies were describedand characterized from the visualization of stem sections [20,21,36]. In their chemical composition,
just one previous study analyzed with Raman spectroscopy the presence of amorphous silica bodies inthree Opuntia species and Stenocereus thurberi (Engelm.) Buxb. [36] and identiﬁed diﬀerences in theirstructural composition, mainly in the SiH bonds. Therefore, with Raman spectroscopy, the structure of
the phytoliths could be analyzed in future studies. We consider that these amorphous silica bodies are
the product of controlled biomineralization [36,37] and should be considered as a taxonomic character.
The EDS analysis supports the presence of Si in the hypodermis of Echinocactus texensis with thehighest concentration, while the concentrations of Ca and Mg were lower. Although the precipitation
phases of hypodermic biominerals were not studied in Echinocactus texensis, Si and Mg may be the ﬁrst
phase of biomineral precipitation along with some form of calcium carbonate or amorphous calcium
oxalate (both have carbon, oxygen, and calcium), as in Ficus microcarpa L.f and Morus alba L. cystoliths.
In both Moraceae, Si and Mg precipitate in the ﬁrst phase of biomineral development, and amorphous
calcium carbonate is deposited later [38]. Biominerals of Echinocactus texensis precipitate in cells with asmall lumen (20 μm on average) compared to Ficus idioblasts that exceed 100 μm [38]. Density was
not quantiﬁed in Echinocactus texensis but they are quite abundant in hypodermis strata. For thesereasons, we consider that the silica bodies diﬀer from the cystoliths and the calcium carbonate does
not precipitate.
4.1.3. Calcium Carbonate
The pith of Ariocarpus retusus subsp. trigonus presented weak peaks belonging to calcium carbonate asreported by Palacio et al. [39] for some gypsophilic plants. Monje and Baran [40] identified on stems of anothercactus, Cylindropuntia kleiniae (DC.) F.M. Knuth, vibration in the infrared between 1415 and 1422 cm−1 ;they were assigned to the carbonate antisymmetric stretching mode of calcite. Anatomical techniques allow
the identification of biominerals in certain plant tissues, however, during fixation with substances such as
acetic acid, some calcium salts and phosphates could be lost [41–43]. This may be the case detected here in
the pith of Ariocarpus retusus subsp. trigonus, since no previous report of calcium carbonate for any memberof Cacteae is known. Probably calcium carbonate is rare in the group because they were not detected in theother species studied, even when the same method to separate biominerals were applied.

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4.1.4. Unknown Biominerals
In Astrophytum asterias, the spectra in the EH region showed a very broad peak in the 1027 cm−1
region, which unlike the biominerals of Echinocactus texensis, they showed birefringence and werenamed here as prismatic crystals (see 4.1.1). However, in Astrophytum asterias, there was a high intensity
peak in the vascular cylinder at 1028 cm−1 and an intense peak at 1026 cm−1 in the pith. These threepeaks in Astrophytum asterias suggest a diﬀerent biomineral composition, which according to Palacio
et al. [39], the spectrum of the regions that presents the strongest link patterns could be assigned
to some types of silicates (1100–950 cm−1 ) or phosphates (1100–1000 cm−1 ). We consider that thesevibrations belong to silicates, probably aluminosilicates due to the presence of silicon and aluminum inbiominerals detected by EDS in the hypodermis or aluminum oxides in the cortex (Figure 5). To support
this ﬁnding, new FTIR spectra at 400–100 cm−1 are needed [44]. It is important to mention that thesealuminosilicate-weddellite prismatic crystals appear to be conservative for the genus since they havebeen observed in the other four Astrophytum species studied by EDS [45]. However, they were notdetected by EDS in the other stem tissues as FTIR did here for Astrophytum asterias.
Here, we report for the first time the presence of silicates in Mammillaria biominerals.
In Mammillaria melanocentra subsp. rubrograndis, the peaks assigned to silicates occurred in the vascularcylinder and pith. It should be noted that these biominerals were also birefringent. Peaks assigned tosilicates were also present in three of the four Mammillaria sphaerica tissues (Figure 2). The surprising
results were the peaks in the epidermis-hypodermis of M. sphaerica because no vacuolar biominerals
were detected in this region by microscopy. Furthermore, no biominerals other than calcium oxalate
have been described for the genus [12,25]. Our results suggest that other Mammillaria species should bestudied as mentioned above to conﬁrm biomineral diversity.
Magnesium was detected by EDS spectra (Figure 5) but not with FTIR (Figure 2). The characteristic
peaks assigned to magnesium oxalate reported by Monje and Baran [17] for other cacti such as Opuntia
were not detected in our study by FTIR but Mg is present in traces.
4.2. Possible Functions of Biominerals in Diﬀerent Tissues
Different ecological functions are attributed to biominerals in plant families [37]. Pierantoni et al. [46]showed in Abelmoschus esculentus that the calcium oxalates can scatter light through photosynthetic
tissue and the amorphous silica bodies have a protective effect against UV radiation. The occurrence of
biominerals in these species that grow in the Chihuahuan desert could be a mechanism for protecting
the photosynthetic tissue and against excessive solar radiation in Astrophytum asterias and Echinocactus
texensis. The presence of calcium oxalates in the cortex, vascular cylinder and pith of the stem could be
explained as calcium deposits in the cellular vacuoles within the tissues, as suggested by Volk et al. [47]for Pistia stratiotes. The crystalline structure of calcium oxalate dihydrate has zeolitic channels that allow
the adsorption of large quantities of water molecules that can diffuse “freely” in the structure [33],suggesting that these biominerals function as small water reserves. This water would be available together
with the water stored in the vacuoles of cortical and pith cells, allowing plants to withstand the periods ofgreatest drought.
On the other hand, silicates in plants may have a role of improving the aluminum tolerance
capacity, so the presence of silicon transporters shows that the deposition of phytoliths is an active andregulated process by the plant [36]. Further to this, the presence of phytoliths in plant tissues works
as a defensive method against herbivores by abrasion of the teeth and reduction in the absorptionof nitrogen during digestion [48], so it is possible that in cacti, the presence of silicates works in thisway. These silicates were detected in experiments with Sorghum bicolor [49] and four cacti species [36].
In Astrophytum asterias and Mammillaria melanocentra subsp. rubograndis, these silicates could be present
in the soil; however, the metabolic uptake capacity of silicon in the soil depends on the species.
Silicon accumulation in diﬀerent species could be related to the presence of silicon transporters(Lsi proteins), belonging to the Nod26-like major protein (NIP) in the plasma membranes of root143

Crystals 2020, 10, 432

cells [4,48]. In Cacteae, the occurrence of these transporters has never been evaluated and futurestudies are needed.
5. Conclusions
There were vibrations associated with calcium carbonate in the P of Ariocarpus retusus subsp. trigonus.
The calcium carbonate reported here increases the diversity of biominerals in cactus. The hydration stateof calcium oxalate is conserved in the different tissues that were studied in the stems of the Cacteae
species. Both calcium oxalate and silica bodies were present in the same species but in different tissues
for Echinocactus texensis. The presence of silicate peaks belonging to species such as Astrophytum asteriasand Mammillaria sphaerica opens the opportunity to study the role of silicates in the physiology ofCacteae species.
Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4352/10/6/432/s1,
Table S1. Assignment of FTIR absorption bands of biominerals extracted from tissues of Cacteae species.
Author Contributions: T.T. and A.D.l.R.-T. designed the work, T.T. collected the plants, A.D.l.R.-T. and A.M.
performed the lab work and prepared the ﬁgures, A.D.l.R.-T., T.T. and A.M. analyzed the data, A.D.l.R.-T. wrote
the original draft. All authors reviewed and edited the manuscript. All authors have read and agreed to the
published version of the manuscript.
Funding: Funding for this research was provided by Programa de Apoyo a Proyectos de Investigación e Innovación
Tecnológica, Universidad Nacional Autónoma de México (PAPIIT-UNAM); grant no. IN205419 to TT and Consejo
Nacional de Ciencia y Tecnología (CONACyT), grant no. 703332 to ADR-T.
Acknowledgments: ADR-T thanks Posgrado en Ciencias Biológicas (UNAM) and AM & TT thank to Rubén San
Miguel-Chávez for helping us to use the FTIR.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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crystals
ReviewSynthesis Methods and Favorable Conditions forSpherical Vaterite Precipitation: A Review
Donata Konopacka-Łyskawa
Department of Process Engineering and Chemical Technology, Faculty of Chemistry,
Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland;
donata.konopacka-lyskawa@pg.edu.pl; Tel.: +48-58-347-2910
Received: 15 March 2019; Accepted: 18 April 2019; Published: 25 April 2019

Abstract: Vaterite is the least thermodynamically stable anhydrous calcium carbonate polymorph.
Its existence is very rare in nature, e.g., in some rock formations or as a component of biominerals
produced by some ﬁshes, crustaceans, or birds. Synthetic vaterite particles are proposed as carriers ofactive substances in medicines, additives in cosmetic preparations as well as adsorbents. Also, theirutilization as a pump for microﬂuidic ﬂow is also tested. In particular, vaterite particles produced
as polycrystalline spheres have large potential for application. Various methods are proposed
to precipitate vaterite particles, including the conventional solution-solution synthesis, gas-liquid
method as well as special routes. Precipitation conditions should be carefully selected to obtain ahigh concentration of vaterite in all these methods. In this review, classical and new methods usedfor vaterite precipitation are presented. Furthermore, the key parameters aﬀecting the formation of
spherical vaterite are discussed.
Keywords: vaterite; calcium carbonate; polymorph; precipitation; synthesis; carbonation

1. Introduction
Vaterite is the least thermodynamically stable anhydrous calcium carbonate polymorph and it
easily transforms into more stable calcite or aragonite in the presence of water. This form of calcium
carbonate mineral was named to honor the German chemist and mineralogist, Heinrich Vater, in 1903.
Because of its instability, the existence of vaterite is very rare in nature. It have been found insome sediments and rocks [1], e.g., as a major constituent of a carbonated calcium silicate hydrogel
complex formed from larnite in Ballycraigy, Ireland [2]. Vaterite can be precipitated in some mineralsprings when speciﬁc glacial conditions take place [3]. Also, vaterite crystals have been identiﬁed inmaterials produced by living organisms, e.g., otolith organs of ﬁshes [4–6], spicules of the ascidian
Herdmania momus [6,7], freshwater pearls, crustacean tissues, or bird eggs [6,8] as well as the chalky
crust on the surface of leaves of the alpine plant, Saxifraga scardica [9].
Synthetic vaterite particles have been used as a carrier of active compounds for medical
treatments [10–14]. They have been tested as a template for biodegradable polymer capsules, which
can be used for applications in nanomedicine [10,11,15]. Also, vaterite particles are added to personalcare products as abrasives, adsorbents, anticaking agents, buﬀers, or dyes [15]. Due to their unique
optical properties, vaterite microspheres has been useful in microrheology and microﬂuidics [16].
Spherical vaterite particles have been used to generate ﬂow within microﬂuidic channels that has
allowed the creation of an optical driven pump [17]. This polymorphic CaCO3 form has been proposed
as a coating pigment for ink jet paper [18]. The main advantages of vaterite particles are their easy andlow-cost preparation, ability to design particles with deﬁned characteristics, porous structure, mild
conditions for decomposition, non-toxicity, and biocompatibility [11,15].
Recently, this unstable mineral was widely investigated to identify the favorable conditions for its
production as well as to verify its usefulness for various applications. Therefore, the current opinions
Crystals 2019, 9, 223; doi:10.3390/cryst9040223

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Crystals 2019, 9, 223

on vaterite synthesis as well as a discussion on the variables aﬀecting its formation are overviewed.
The issues raised in this review are presented in the diagram in Figure 1.

Figure 1. Diagrammatic representation of raised issues.

2. Vaterite Properties
Synthetic vaterite particles are usually produced as polycrystalline spheres. The main advantagesof such particles’ morphology are the porous structure, large surface area, and greater hydrophilicity
in comparison to more stable calcite or aragonite [10,19,20]. Other morphological forms of vaterite canbe also obtained, i.e., plates [21], hexagonal crystals [22], lenses [23], lamellar aggregates [24], ﬂorets,
or rosettes [21,25] as well as microtablets [26]. Examples of vaterite particles are shown in Figure 2.

Figure 2. Vaterite particles: (a) and (b) typical spherical particles; (c) spherical and lens-like particles;(d) deformed lens-like and crossed lens-like particles.

Vaterite has a hexagonal crystal system, but the exact crystal structure of vaterite is still underdiscussion. The analysis of experimental data is consistent in that all vaterite structures belong tothe order-disorder (OD) family [27]. It means that the occurrence of multiple polytypes on the microto macroscopic scale, as well as considerable stacking disorder, are both to be expected. Recently
Burgess and Bryce [28] used the combined 43 Ca solid-state nuclear magnetic resonance spectroscopic
and computational method to indicate two crystal structures, i.e., the hexagonal lattice, P32 21, and148

Crystals 2019, 9, 223

monoclinic lattice, C2, which have the best agreement between the simulated spectra and diﬀractogramswith the experimental data.
Selected properties of the vaterite are summarized in Table 1.
Table 1. Selected properties of vaterite.
Properties
Density
Ksp 1 at 25◦ C
Ksp for t = 0–90 ◦ C
Optical properties
Eﬀective birefringence
Refractive index
αV 2 at 25 ◦ C
Surface energy

ValuesRef.

2.54 g/cm3

[2]
[27]
[29]
[29]
[17]
[17]
[27]
[30]
[31]

2.65 g/cm3
1.22·10−8
Ksp = −172.1495–0.077993T + 3074.688/T + 71.595logT
Semitransparent, colorless
Δn = 0.06–0.1
nω = 1.55, nε = 1.65
35.5·10−6 K−1
Calculated: 90 mJ/m2 ; experimental 34–73 mJ/m2

1 Solubility product; 2 Volumetric thermal expansion coeﬃcient.

The surface of the vaterite particles is usually hydrophilic. The hydrophobic vaterite can beobtained by the adsorption of amphiphilic molecules, e.g., oleic acid, at the interface of the produced
vaterite [32]. The charge of vaterite particles depends on the composition of the solution and its pH.
The values of the ζ-potential were negative when the vaterite particles were dispersed in saturated
CaCO3 solution at pH 9.0 (−4 mV) and at pH 10.6 (−26 mV) [33]. Another experiment showed thatwhen the solution was composed from 0.01 mol/dm3 CaCl2 , 0.002 mol/dm3 Na2 CO3 , and 0.5 mol/dm3
NaCl, the charge of the vaterite particles was positive in the range of pH 7.5 to 9.9 [34]. The addition oforganic compounds, like polypeptides or fulvic acid, can change the charge of the vaterite particles
due to its adsorption at the precipitated crystal interface [33,34].
The mechanical properties of synthetic vaterite particles were determined using nanoidentation
analysis [30]. The elastic modulus was found to be in the range of 16 to 61 GPa and the calculated
mean value of this parameter was 31 GPa. The determined hardness of vaterite was in the range of 4.2
to 0.3 GPa with a mean value of 0.9 GPa.
During the heating of vaterite particles, thermal transformation and decomposition occurs.
The exothermic transformation of vaterite into calcite takes place at temperatures between 395 and540 ◦ C [6,35,36]. The exact transformation temperature of vaterite into calcite depends on the particle
characteristics, the presence of additives, and the heating rate. The shift of the vaterite transformation
to a lower temperature may be observed when calcite is present in the sample [35,36] or in the caseof the incorporation of organic molecules or foreign ions into the vaterite particles [6,37]. Also, thecoexistence of pure vaterite and vaterite in contact with the calcite phase (e.g., vaterite particles coveredby a calcite layer) [36] can cause the appearance of a broad range of transformation temperatures.
Recently, a report on the pressure-induced phase transition of vaterite was presented [38].
With increasing pressure, vaterite transformed to high-pressure vaterite forms (vaterite II, vaterite III,and vaterite IV) or partially to calcite. All phase transitions related to vaterite were reversible, except
for vaterite II to calcite III.
The presence of vaterite in calcium carbonate samples can be determined using Fourier transformed
infrared spectroscopy (FTIR) [39], powder X-ray diﬀraction (PXRD), Raman spectroscopy [40], or 43 Ca
solid state nuclear magnetic resonance (43 Ca ssNMR) [41]. Characteristic peaks of calcite, aragonite, andvaterite, obtained in spectra or diﬀractograms that allow these polymorphic forms to be distinguished
by FTIR, XRD, Raman spectroscopy, and 43 Ca ssNMR, are listed in Table 2.

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Table 2. Characteristic peaks of CaCO3 polymorphs in FTIR, XRD, Raman spectroscopy, and 43 Ca
ssNMR analysis.
Analytical technique

Vaterite

Aragonite

CalciteRef.

FTIR, Wave number, cm−1

745
29.5
750
−3

710, 713
45.9
705
−34

713
25.0
711
6

[39]
[40]
[40]
[41]

XRD, 2Θ◦Raman, Wave number, cm−1
43 Ca ssNMR, δ1
iso , ppm

1 The isotropic chemical shift.

3. Mechanisms of Spherical Vaterite Formation
The crystallization of calcium carbonate crystalline polymorphs often occurs via amorphous
calcium carbonate (ACC), which is initially formed in the solution [42]. The structure of the ACC
precursor consists of a porous calcium-rich nanoscale framework containing water and carbonate ions.
ACC is transformed into vaterite due to the rapid dehydration and internal structure reorganization.
It is suggested that the possibility of crystallization of the vaterite particles occurs, as the initiallygenerated ACC exhibits proto-vaterite features. [43]. In the ﬁnal stage, the slow transformation ofvaterite into calcite takes place via a dissolution and recrystallization process [42].
There are two main concepts proposed for the explanation of spherical vaterite formation. The ﬁrst
one is based on the aggregation of nanoparticles and the second mechanism is a development of theclassical theory of crystal growth [44]. According to the ﬁrst concept, the production of polycrystalline
vaterite particles is a result of the assembly of nano-sized crystals by oriented or not-oriented
attachment [44,45]. The aggregation requires the production of many small particles at the beginning of
the reaction, which is supported by a high supersaturation. The aggregation mechanisms were applied
to interpret both the formation of core-shell microspheres [45] and hollow vaterite particles [46]. It wassuggested that the core-shell vaterite structure was a result of a successive aggregation and coverageof formed spheres with hexagonal plates [45] in the presence of poly(styrene sulfonic acid) sodium
salt, while the precipitation with the addition of ethylene glycol led to the formation of an initial shellstructure from the primary nanoparticles and then hollow vaterite particles [46].
However, the spherulitic growth mechanism is based on the classical theory of crystal growth [44].
Spherulites are produced when a new nucleus arises on the surface of the growing crystal. Based on
the experiments, two concepts of spherical growth are proposed: (i) Spherulites arise from a central
precursor via multidirectional growth of crystalline ﬁbers, and (ii) spherules grow from a precursor via
low angle branching starting on the edges [44,47]. The spherulitic growth mechanism was used to
explain the formation of vaterite particles with spherical and dumbbell morphology [47,48].
4. Methods of the Synthesis of Vaterite Particles
4.1. Solution Route (L-L)
In this method, a solution containing calcium salt is mixed with a solution of carbonate salt [49].
When calcium chloride and potassium carbonate solutions are used, the reaction is as follows:
CaCl2 + K2 CO3 → CaCO3 + 2KCl

(1)

The total volume of the ﬁrst solution used can be added immediately to the second solution [50,51]
or it can be injected to another one with a controlled rate [52]. Diﬀerent types of stirring are used to
produce a homogeneous reaction mixture. Stirrers applied in laboratories are usually mechanical andmagnetic, although other types are also proposed, e.g., ultrasound or microwave [13,53]. Recently, a“dropwise precipitation” has been adopted to calcium carbonate precipitation [54]. In this method, acalcium ion solution is added in very small portions to a carbonate solution.

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4.2. Carbonation Route (G-L)
Vaterite particles can be synthesized using carbon dioxide as a reagent. The reaction may be
carried out using calcium hydroxide or calcium salt as a source of Ca2+ ions. The reaction of calcium
hydroxide and carbon dioxide can be written as:
Ca(OH)2 + CO2 → CaCO3 + H2 O

(2)When gaseous CO2 is introduced into the aqueous solution of calcium salt (e.g., CaCl2 ), the overall
following reactions can be presented as:
CaCl2 + CO2 + 2OH− ↔ CaCO3 + H2 O + 2Cl−(3)

The detailed mechanisms of calcium carbonate precipitation are complex and include the transfer
of carbon dioxide from the gas phase into the liquid phase, the formation of a carbonic acid, and its
hydrolysis to produce carbonate ions that are the reagent for CaCO3 precipitation. The basic pH of thereactive mixture favors both CO2 absorption and carbonate ion formation. In case of calcium hydroxideslurry, the initial pH is about 12.4, and therefore the CO2 absorption is relatively easy. However, when
calcium salts (e.g., calcium chloride, CaCl2 , or calcium nitrate, Ca(NO3 )2 ) are used, the pH of solution
is more acidic and less advantageous to the formation of carbonate ions. Therefore, CO2 absorption
promoters, like ammonia [19,20,55,56] or amines [56,57], are added to the initial solution. Moreover,carbamate ions that form during carbon dioxide absorption in ammonia and primary or secondaryamine aqueous solutions may stabilize the vaterite phase [56,58]. A modiﬁcation of this approach is theuse of extracts obtained by the leaching of calcium to reach minerals or solid waste with ammonium
salt solutions [59]. The formed leachates contain calcium ions and ammonia and can be utilized in the
carbonation process.
Recently, an interesting synthesis method using solid CO2 (dry ice) was proposed [60]. The reaction
was carried out at a minimum temperature of −50◦ C in a water-methanol solution containing calciumoxide. The addition of dry ice pellets into a water-methanol solution kept the temperature low andthe use of methanol allowed the liquid state of the reaction system to be maintained. Such conditions
promoted the CO2 solubility and enabled the synthesis of vaterite nanoparticles.
4.3. Diﬀusion Method
Calcium carbonate in vaterite form can be produced by a diﬀusion method, in which ammonium
carbonate [52] or ammonium bicarbonate [61] is used. These compounds slowly decompose theformed ammonia gas and carbon dioxide. The precipitation is carried out in a closed vessel containing
a container with a calcium salt solution and a container with solid carbonates. The formed gases
diﬀuse into the calcium salt solution. After the absorption of ammonia and carbon dioxide, the calcium
carbonate is formed in the liquid phase according to the reaction described by Equation (3).
4.4. Synthesis in Emulsions and Microemulsions
The reaction between calcium ions and carbonate ions may take place in the emulsion system. A
water-in-oil emulsion containing a single reagent solution as the dispersed phase can be used for thereaction. A second reactant solution is added to the emulsion and precipitation occurs in the dispersedaqueous phase [62]. A stable double water-in-oil-in-water emulsion can be used for vaterite synthesis
as well. In this case, calcium ions dissolved in the external phase are extracted into the organic phase
containing oil-soluble extractant and stripped into the internal aqueous phase, where the reaction withcarbonate ions occurs [63]. Also, the use of microemulsions is proposed for the production of vaterite.
For this purpose, two microemulsions containing Ca2+ and CO3 2− ions are prepared. Then, thesemicroemulsions are mixed together to produce calcium carbonate [64].

151Crystals 2019, 9, 223

Another approach to calcium carbonate precipitation in a microemulsion system is thedecomposition of calcium bicarbonate:
Ca(HCO3 )2 ↔ CaCO3 + CO2 +H2 O(4)When a saturated solution of calcium bicarbonate is a water phase in a water-in-oil emulsion,vaterite is formed during the slow desorption of carbon dioxide from an aqueous phase. Sponge-likevaterite spheroids were produced using this method [65].
4.5. Synthesis Using a Precursor of Carbonate Ions
The synthesis of calcium carbonate can be carried out using a substance that forms carbonate ionsin the reaction environment. Such carbonate ion promoters may be ammonium carbamate [58] orurea [66,67]. Ammonium carbamate hydrolyzes in aqueous solutions to form ammonium carbonate:
NH2 CO2 NH4 + H2 O ↔ (NH4 )2 CO3(5)

Ammonium carbonate is also the product in the reaction of urea with water:
CO(NH2 )2 + 2H2 O ↔ (NH4 )2 CO3(6)

A high concentration of both soluble calcium salts and carbonate ions precursors is necessary
to precipitate vaterite polymorph [58,66]. Successful vaterite precipitation has been carried out attemperatures of 15, 25, and 50 ◦ C, when ammonium carbamate has been used [58]. While, the reactionin the solution containing soluble calcium salt and urea required a temperature of 90 ◦ C [66].
4.6. Hydrothermal and Solvothermal Methods
A hydrothermal or solvothermal process is a method used to create ceramic materials at elevatedtemperatures and pressure. Hydrothermal conditions were applied to precipitate vaterite-reachparticles using diethylenetriaminepentaacetic acid as an additive [68]. The ﬁrst step of this process
was CaCO3 precipitation using CaCl2 and Na2 CO3 solution at a temperature of 120 ◦ C. Then, thehydrothermal process was carried out using the ﬁltrate from the ﬁrst step as a reactive mixture.
The ﬁltrate was placed in an autoclave at temperatures between 130 and 230 ◦ C for 48 hours and thehighest concentration of vaterite (90%) was obtained at the highest tested temperature, i.e., 230 ◦ C.
The solvothermal method for vaterite synthesis was proposed by Li et al. [23]. Calcium chloride
and urea (a precursor of carbonate ions) were reagents dissolved in ethylene glycol, 1,2-propanediol,or glycerol. Carbonate ions were formed by decomposition of urea in solvothermal conditions attemperatures from 100 to 190 ◦ C for 12 hours. No additives or pH control were needed to precipitate
the pure vaterite phase. The pure vaterite particles were produced in ethylene glycol at 100 ◦ C, in
1,2-propanediol at 130 ◦ C, and in glycerol at 150 ◦ C.
5. Factors Inﬂuencing Vaterite Formation
Calcium carbonate can form several polymorphs, therefore, its crystallization requires careful
control of the process parameters to obtain the desired product. Factors aﬀecting the crystallization of
the preferred polymorphic form have been grouped by Kitamura in a set of primary and secondary
variables [69]. Primary factors include supersaturation, temperature, stirring rate, and seed crystals.
However, the solvent composition, additives, and pH are secondary factors. The main investigatedfactors inﬂuencing the precipitation of vaterite are presented below. The eﬀect of solvents and otherorganic substances is discussed together because organic solvents are treated as additives in manystudies on calcium carbonate precipitation.

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5.1. Supersaturation
Supersaturation is deﬁned as:


S =


aCa2+ aCO3 2− 1/2
Ksp

(7)

where aCa2 + and aCO32- are activities of a calcium and carbonate ions, respectively, and Ksp is asolubility product. In the liquid-liquid systems, vaterite particles precipitate at room temperature frommoderately supersaturated aqueous solutions, i.e., when S < 6.5 [21,33,69,70]. The supersaturation
can inﬂuence the size of the crystallite forming vaterite particles, and a smaller crystal subunit wasobserved when supersaturation increased [69,70]. When precipitation is carried out by a carbonationroute, the composition of the gas stream can impact on the supersaturation, i.e., an increase in the CO2
concentration in the gas phase resulted in the higher supersaturation. The high supersaturation in the
buﬀered pH range (from about 9.5 to 7.7) helps to trap metastable vaterite, preventing its transformationin calcite [19]. Also, more regular spherical particles are produced at higher concentrations of CO2 in
feed gas mixtures [71].
5.2. Temperature
Usually, vaterite particles can be formed in a broad range of temperatures using a solution methodof calcium carbonate precipitation [24,42,58,72]. The comparison of vaterite concentrations in calcium
carbonate samples precipitated at various conditions is shown in Figure 3. The favored range oftemperatures for vaterite precipitation in various experiments is up to 40 ◦ C. This range is also valid
when the carbonation route is applied to produce CaCO3 in the vaterite form [59,71].

Figure 3. Inﬂuence of temperature on vaterite concentration in CaCO3 samples; (a) L-L, S = 6.5,
pH = 9, based on data from [21]; (b) with carbamate, [Ca2+ ] = 1.5 M, based on data from [58]; (c) L-L,
[Ca2+ ] = 0.25 M, based on data from [24]; (d) L-L, [Ca2+ ] = 0.015 M [73]; (e) L-L, [Ca2+ ] = 0.015 M with
ethylene glycol based on data from [73]; (f) G-L, pH = 9–10, xCO2 = 1, VG = 50 dm3 /h (gray), VG = 100
dm3 /h (orange) based on data from [59].
153Crystals 2019, 9, 223

The application of the hydrothermal or solvothermal method allowed the precipitation of vateriteat temperatures above 100 ◦ C [23,68]. Some additives, e.g., ethylene glycol, can promote vaterite
formation at temperatures above 40 ◦ C [42] A higher concentration of vaterite was observed in the
CaCO3 product precipitated in an ethylene glycol-water solution at a temperature of 50 ◦ C comparedto the reaction carried out in an aqueous solution (see Figure 3d,e).
5.3. pHThe pH value of the reaction mixture is an important parameter during calcium carbonate
precipitation. It was reported that vaterite is a dominant polymorph in solutions with an initial basic
pH [21,42,74]. However, Han et al. [55] studied a carbonation system with ammonia as an absorption
promoter and found that pure vaterite is formed when the pH is 8, but an increase in the pH value ofthe reactive mixture resulted in a decrease in the vaterite concentration. Almost pure vaterite produced
in a gas-liquid system with the addition of ammonia and ammonium chloride was obtained when the
pH decreased from 9.7 to 7.7 during the carbonation process [19]. The inﬂuence of pH on the vaterite
content in selected processes is shown in Figure 4. Vaterite precipitation was promoted in the pH
range from 8 to 10 for all compared processes. An extension of this range was possible by conducting
precipitation in the presence of ethylene glycol [74] or ionic liquid surfactant [75].

Figure 4. Inﬂuence of pH on vaterite concentration in CaCO3 samples; (a) ) L-L, S = 6.5, t = 24 ◦ C, basedon data from [21]; (b) G-L, [Ca2+ ] = 0.1 M, t = 20 ◦ C, VG = 18 dm3 /h, xCO2 = 033, based on data from [55];(c) L-L, [Ca2+ ] = 0.5 M, ta , with ILS, based on data from [75]; (d); L-L, [Ca2+ ]:[CO3 2− ] = 1:3, t = 23 ◦ C,
with ethylene glycol, based on data from [74]; (e) L-L, [Ca2+ ]:[CO3 2− ] = 1:1, t = 23 ◦ C, with ethylene
glycol, based on data from [74]; (f) G-L, t = 25 ◦ C, xCO2 = 1, VG = 50 dm3 /h (gray), VG = 100 dm3 /h(orange), based on data from [59].

5.4. Time
Because vaterite is a metastable calcium carbonate polymorph, prolonging the reaction time
results in a reduction of the vaterite content when an aqueous solution without the presence of organic

154

Crystals 2019, 9, 223

additives is the medium of CaCO3 synthesis [13,49]. The remaining precipitated vaterite particles in
the aqueous solutions lead its recrystallization to the more stable calcium carbonate polymorph, i.e.,aragonite and calcite [58,69]. The time needed for the transformation of vaterite to calcite is usuallyin the range of a few minutes to several hours [13,58,69,76]. However, a longer time (up to 20 h) of
vaterite stability in aqueous solutions was also reported when calcium carbonate was synthesized at atemperature of 7 ◦ C [48]. Slight acceleration of the conversion of vaterite into calcite with an increaseof the ionic strength was observed [77]. Some organic compounds used as additives can prevent thetransformation of vaterite into calcite (see Section 5.6. Additives).
5.5. Mixing
The mixing rate of the reactant solutions is an important factor of the precipitation process [69].
The stirring intensity can aﬀect the activation energy of nucleation of calcite and vaterite in the aqueous
system [52]. Local non-homogeneities of the supersaturation can aﬀect the creation of conditions
conducive to the formation of one of the polymorphic forms. Homogeneous, high-shear, and constantagitation of reactant solutions enables precipitation of pure vaterite particles [49]. Also, ultrasonicagitation can be used for this purpose [13]. Ultrasounds provide a large amount of energy into
the reaction systems, which produces mechanical and thermal eﬀects and facilitates mass transfer.
When CaCO3 precipitation was carried out using ultrasounds, the vaterite concentration was higher
compared to the process using the same reagent mixed by a magnetic stirrer. In the carbonation method
using gaseous CO2 , mixing is generated by a gas ﬂow. Therefore, an increase in the gas ﬂow rate
resulted in an increase in the vaterite precipitation [19,59,78,79].
5.6. Additives
The presence of additives in the reaction mixture may aﬀect the precipitation by changing thesolubility of a forming substance, inﬂuencing the reactive crystallization rate, nucleation, and crystal
growth; the selective stabilization of a less stable polymorph; and the morphology of the forming
crystals. The selected compounds that were additives in the reaction mixture used for the precipitationof calcium carbonate and their eﬀect on the formation of vaterite are presented in Table 3.
The selection of the solvent for the precipitation of calcium carbonate has been intensively
investigated. The addition of an organic solvent may change supersaturation [80], because usually boththe solubility of each polymorphs and the activities of ions decreases. Therefore the promotion of vaterite
precipitation in aqueous solutions of organic solvents is frequently reported [10,31,73,74]. Moreover,organic solvent and water molecules may be inhomogeneously dispersed. Such phenomena areobserved, e.g., in ethanol-water mixtures, and are enhanced by an increase of the ethanol concentrationand the addition of an inorganic salt [81]. Another feature of an aqueous ethanol solution is the ability
to perform selective solvation as conﬁrmed by a molecular dynamic simulation [81]. In this case,carbonate ions were mostly solvated by water and hardly solvated by ethanol, while calcium ionswere solvated by both water ethanol molecules. These can lead to changes in the morphology ofthe precipitated particles of vaterite. As mentioned before, the addition of an organic solvent to thereaction mixture increases the supersaturation of the solution, hence the formation of vaterite particles
considerably smaller in size is observed [10,74]. This is a result of the higher nucleation rate thatprovides a large number of nucleation sites in these systems. Moreover, organic solvent molecules thathave negatively charged hydroxyl groups can adsorb at the forming vaterite surface, change the surface
energy of the vaterite, and, as a result, stabilize these phases, thus preventing its transformation into
more stable forms as has been reported for vaterite precipitation using the solvothermal method [23].
The formation of the adsorption layer at the produced crystals at the early stage of precipitation
by diﬀerent organic molecules added into the solution is often raised in the discussion of the role oforganic additives. The adsorbed layer can inhibit the dissolution step of vaterite that is attributed to
the remaining more unstable phases. If the adsorption energy of organic molecules at the solid surface
interface is not enough to overcome the hydration energy of the hydrophilic part of these molecules,

155

Crystals 2019, 9, 223

they are removed from the crystal surface. Then, the precipitated vaterite is re-dissolved into waterand a more stable calcite is formed by the recrystallization process [82].
Table 3. The inﬂuence of selective additives on vaterite precipitation.
Additive

Inﬂuence

Synthesis Method

Ref.

Rate

Stability

Morphology

Size

Ethanol

L-L, t = 25 ◦ C
L-L, t = 25–30 ◦ C

+

+

+
+

[31]
[81]

Iso-propanol

L-L, t = 25 ◦ C
L-L, t = 25–30 ◦ C

+

+

+
+

[31]
[81]

Diethylene glycol

L-L, t = 25 ◦ C

+

+

+

[31]

Ethylene glycol

L-L, t = 2–40 ◦ C
L-L, t = 25–80 ◦ C
L-L, t = 25–50 ◦ C
L-L, t = 40, 70 ◦ C
Solv., t = 100–150 ◦ C++

L-L, t = 2–40 ◦ C

+
+

[10]
[73]
[47]
[80]
[23]

+
+

+

[10]
[23]

+

+

+
+

GlycerolSolv., t = 100–150 ◦ C

Erythritol

L-L, t = 2–40 ◦ C+

1,2-propanediol

Solv., t = 100–150 ◦ C+

1,8-diaminooctane

G-S; t = 30 ◦ C

Glycine

L-L, Diﬀ., ta
G-S; t = 30 ◦ C

4-aminobutyric acid
6-aminohexanoic acid

G-S; t = 30 ◦ C++

Poly-glutamic acid
Poly-aspartic acid

L-L, t = 25 ◦ C++

Oleic acid

L-L, t = 30 ◦ C

EDTMPA

G-L, t = 30, 60 ◦ C+

Sucrose

L-L, t = 30 ◦ C
G-L, t = 22 ◦ C

+
+

[50]
[85]

SDSN
SDBS

CaCl2 +urea; t = 90 ◦ C

+
+

[66]

Tween 20
Tween 40
Tween 60
Tween 80

L-L, ta

+
+
+
+

[49]

Ionic liquid surfactant

L-L, t = 25 ◦ C

+

[47]

Guar gum

L-L, t = 0, 20, 40 ◦ C

+

[12]

[10]
[23]

+

[82]
+

+

[52]
[82]
[82]

+
+

[33]

+

+

+

[83]
+

[84]

Abbreviations: Solv.—a solvothermal method; G-S—a gas-slurry system, ta —an ambient temperature;
EDTMPA—ethylenediamine-tetrakis-N,N,N,N,-(methylenephosphonic acid); SDSN—sodium dodecylsulfonate;
SDBS—sodium dodecylbenzenesulfonate.

The stabilization of a vaterite polymorph was reported for alcohols, polyalcohols [10,23,31,81],
and aminoacids [82,86]. Vaterite particles were produced in the presence of aminoacids. Both polarinteractions from the hydrophilic groups of additives and the hydrophobic interactions due to thevan der Waals forces from the hydrophobic alkyl groups play an important role in stabilizing vaterite
particles [82]. The stabilizing eﬀect of oleic acid molecules was also demonstrated and the adsorptionof oleic acid at the vaterite surface was conﬁrmed by FTIR analysis [83]. Also, tetrazole [61] and fulvic
acid [34] were identiﬁed as compounds that were able to absorb at the vaterite surface and retard
vaterite dissolution. The stabilization of vaterite particles was also observed when polypeptides [33],156

Crystals 2019, 9, 223

bovine serum albumin, or soluble starch [87] was added into the reaction mixture. Hydrophilic andhydrophobic parts are present in surfactant molecules, therefore, these molecules can easily adsorb on
the hydrophilic surface of vaterite particles. In the same conditions, the vaterite polymorphic form was
precipitated in the solution containing surfactant while calcite formation was favored in a medium
without the addition of surfactant molecules [75,88]. Moreover, changes in the morphology of vaterite
particles precipitated in the presence of surfactant were observed [49,66].
Polymeric substances were tested as additives in vaterite precipitation, as well. Polypeptides
adsorbed at the interfaces of forming particles. They decreased in aggregation and changed theelectrokinetic and morphological properties of the precipitate [33]. However the addition of the guar
gum to the initial calcium chloride solution resulted in the production of hollow spherical vaterite
particles [12]. However, core-shell vaterite microspheres composed of nanoparticles in the core andhexagonal nanoplates at the outer layer were precipitated using solutions of calcium chloride andammonium carbonate with the addition of poly(styrene sulfonic acid) sodium salt [45]. When thesynthesis was performed using a sodium carbonate solution as a carbonate source, vaterite microspheres
covered by nanorods were precipitated [45].
The inﬂuence of additives on the rate of calcium carbonate precipitation was reported for somesystems. In the solution method, it was found that the addition of ethylene glycol [80] reduced
the precipitation rate. The retardation of crystal growth was observed when poly-aspartic acid andpoly-glutamic acid were present in the solution [33]. In these systems, high supersaturation waspreserved for a longer time and it resulted in a higher concentration of the vaterite phase in precipitatedcalcium carbonate. However, an increase in the vaterite growth was observed in the presence ofethanol, isopropanol, and diethylene glycol [31]. However, the stabilization of vaterite particles byorganic solvent molecules prevented its transformation to more stable calcite. The opposite eﬀect
was found when citric acid was added [34]. Then, the precipitation was also slower, but the creationof calcite was privileged. It seems that the addition of citric acid increased the solubility of calcium
carbonate and reduced the supersaturation in the system, which could result in the crystallizationof calcite. However, when precipitation was carried out by carbonation of the calcium hydroxide
suspension, the addition of amines, diamines, and amino acids resulted in a longer reaction time [82].
Vaterite was created when diaminooctane and amino acids were used. Although amino acids did notpromote the CO2 absorption and the formation of high supersaturation, their stabilizing eﬀect on thevaterite was prevalent. On the other hand, the addition of sucrose into the initial solution of calcium
chloride and ammonia reduced the reaction time [85]. Sucrose facilitated the absorption of CO2 andcaused the high supersaturation in the system, which promoted the creation of vaterite.
In summary, according to the Ostwald rule, the least stable vaterite precipitates ﬁrst andsubsequently transforms to the more stable one. In the absence of an additive, the kinetics is a dominantfactor inﬂuencing the vaterite concentration in the produced calcium carbonate. As additives can aﬀect
each stage of crystallization, i.e., nucleation, growth, and transformation, they can therefore change thecourse of the precipitation process. Comprehensive information on the inﬂuence of tested additives onvaterite precipitation is not available. As shown in Table 3, only one study presented the eﬀects of theadditive in three of the highlighted areas [31]. Little information is available for carbonation-based
precipitation, where additional substances can also aﬀect the rate of CO2 absorption and the generationof supersaturation in the system.
6. Summary
Vaterite is a polymorphic form of calcium carbonate, which is the subject of many studies due toits unique properties and related potential applications. Especially, spherical polycrystalline particles
of vaterite are indicated as the most promising ones for applications. Various utilizations requireparticles with deﬁned characteristics to be obtained. In this review, methods used for spherical vaterite
precipitation were presented. Classical routes and recently new proposed approaches for calcium

157

Crystals 2019, 9, 223

carbonate precipitation were summarized. Favorable conditions for vaterite particles precipitation
were also described.
A variety of proposed methods allows the selection of an approach due to the availability of
substrates and equipment, which may have an impact on the cost of the produced calcium carbonate.
The problem of separating the obtained particles, e.g., in the emulsion method, is not discussed, but it
may decide on the choice of a speciﬁc method. Also, the recovery or recirculation of the liquid residue
has not been investigated so far.
Spherical vaterite particles can be formed using the presented methods when appropriate processparameters are maintained. The most frequently indicated conditions conducive to the formation ofvaterite are the relatively high supersaturation, temperatures up to 40◦ C, and pH between 8 and 10.
However, these parameters can be shifted because there are many relationships between them thatare not fully understood. In particular, the presence of additives may aﬀect the range of conditions
favorable to the precipitation of vaterite. Therefore, the stabilizing role of additives seems to be a
promising research area. Especially, the selection of non-toxic compounds is very important whenvaterite particles are used in pharmaceutical and cosmetic preparations or for biomedical applications.
Funding: The research described in this paper was ﬁnancially supported by Faculty of Chemistry, Gdansk
University of Technology, grant number DS 033155.
Conﬂicts of Interest: The author declares no conﬂict of interest.

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crystals
ArticleA Complex Assemblage of Crystal Habits of Pyrite in
the Volcanic Hot Springs from Kamchatka, Russia:
Implications for the Mineral Signature of Life on MarsMin Tang 1 and Yi-Liang Li 2, *1
2*

Department of Geology, School of Earth Sciences, Yunnan University, Kunming 650500, China;mtang@ynu.edu.cn
Department of Earth Sciences, The University of Hong Kong, Hong Kong 999077, China
Correspondence: yiliang@hku.hk; Tel.: +852-25176912

Received: 30 April 2020; Accepted: 21 June 2020; Published: 23 June 2020

Abstract: In this study, the crystal habits of pyrite in the volcanic hot springs from Kamchatka,
Russia were surveyed using scanning electron microscopy. Pyrite crystals occur either as single
euhedral crystals or aggregates with a wide range of crystal sizes and morphological features.
Single euhedral crystals, with their sizes ranging from ~200 nm to ~40 μm, exhibit combinations of
cubic {100}, octahedral {111}, and pyritohedral {210} and {310} forms. Heterogeneous geochemical
microenvironments and the bacterial activities in the long-lived hot springs have mediated thedevelopment and good preservation of the complex pyrite crystal habits: irregular, spherulitic, cubic,
or octahedral crystals congregating with clay minerals, and nanocrystals attaching to the surface oflarger pyrite crystals and other minerals. Spherulitic pyrite crystals are commonly covered by organic
matter-rich thin ﬁlms. The coexistence of various sizes and morphological features of those pyrite
crystals indicates the results of secular interactions between the continuous supply of energy andnutritional elements by the hot springs and the microbial communities. We suggest that, insteadof a single mineral with unique crystal habits, the continuous deposition of the same mineral witha complex set of crystal habits results from the ever-changing physicochemical conditions with
contributions from microbial mediation.
Keywords: Kamchatka; hot springs; pyrite; complexity of crystal habits; Mars

1. Introduction
Numerous morphological, molecular, and geochemical biosignatures have been proposed overrecent decades in order to identify records of past life in the ‘sedimentary archives’ of the ancient
Earth or Mars [1–5]. Among these signatures, a variety of mineralogical biosignatures formed
directly or indirectly by bacterial activity provide records of biogenesis in certain environments [6–10].
Microbially-mediated mineral precipitation happens in hot springs because microbial activities
may change the concentration of ions in the micro-environments and provide nucleation sites formineralization. However, the characterization of a biosignature based on the morphology of a single
mineral often needs to be used cautiously because of the possibly confusing abiogenic imitatorsof biosignature [11]. As it is insuﬃcient to take only one single mineral as a biosignature, a suiteof parameters that may consistently indicate a biological origin must be considered [1]. It wasrecently proposed that the synthetic features of a mineral assemblage, including size, crystallinity, andmorphology, could be a reference for a speciﬁc environment with a certain microbial community. In a
study on the diversity of the crystal habits of gypsum, Tang et al. [12] described various morphologies
and sizes of gypsum that uniquely coexisted in a square-meter sized volcanic hot spring on the
Kamchatka Peninsula of Russia, and suggested that it was mainly due to the secular interactions
Crystals 2020, 10, 535; doi:10.3390/cryst10060535

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Crystals 2020, 10, 535

between microbial metabolism and geochemical environments. Pyrite is also one of the most common
biogenic minerals that are observed as euhedral or framboidal crystals in sediments or sedimentaryrocks [8,13]. Laboratory studies of the crystallization of pyrite take physicochemical parameters,such as temperature, pressure, and ion concentrations into consideration and have established therelationship between morphologies of pyrite crystals and their depositional environments [14–21].
Though the chemical pathway of pyrite formation is still in a debate [14,22,23], the direct and indirect
eﬀects of biological processes on pyrite crystallization is commonly accepted [20,24]. Microorganisms
obtain energy from the geochemical environment and release metabolic products that may aﬀect thechemical composition of their aqueous environments and initiate subsequent mineralization [25–28].
For instance, coupling to the oxidation of organic matter, sulfate-reducing bacteria enzymaticallyreduce sulfate to hydrogen sulﬁde, which further reacts with iron in euxinic environments and leads tothe precipitation of iron sulﬁdes [29]. Thiel et al. [24] identiﬁed a novel type of microbial metabolismthat favors energy conservation by oxidizing S2− in FeS to S− in FeS2 as a syntrophy coupling to thehydrogenotrophic methanogenesis (4FeS + 4H2 S + CO2 →4FeS2 + CH4 + 2H2 O). Microorganisms
not only reduce the activation energy barrier for mineral nucleation, but also oﬀer their cell walls as
substrates to facilitate the nucleation of crystals [25,30]. Thus, microorganisms are important agentsthat may induce pyrite mineralization. However, in most cases, it is known that microorganisms havelittle control over the speciﬁc crystal habit of pyrite [26,31]; nevertheless, the biological processes thatinduce the precipitation of pyrite crystals and/or their assemblages should still carry information aboutpast ecophysiological environments.
Framboid and euhedron are the two dominant morphologies of pyrite crystals in low-temperature
sedimentary environments [32,33]. Pyrite is also one of the dominant iron sulﬁdes in natural
high-temperature sedimentary environments, such as deep-sea hydrothermal vents and terrestrial
hot springs, which are analogs to the early environments for life on Earth [34–36], or possibly earlyMars [6]. Pyrite nanoparticles of irregular sizes and shapes in deep-sea hydrothermal vents are
considered important sources of iron for the deep ocean biosphere [37,38]. Framboidal and euhedral
pyrite crystals were assumed to be generated as a consequence of microbial sulfate-reduction in active
shallow submarine vents [39]. Microorganisms also thrive in hot springs that are usually characterized
by extreme conditions such as high temperature and low pH through deep geological time [40–43].
Combined with the active iron, the production rate of biogenic hydrogen sulﬁde inﬂuences the amountof iron monosulﬁde in the system, which may eventually transfer to pyrite [19,24]. Many studies on
pyrite in hot spring sediments have focused on the sulfur isotopic signatures (biogenic or abiogenic) [44].
Few data are available on the textures and crystal morphologies of pyrite and the relation to theirmicrobially-mediated environments [45].
Looking for mineral biosignatures in the sedimentary rocks of Earth and Mars has long
been an eﬀort because biogenic minerals have a much higher chance of surviving the changing
planetary environments during their multi-billion-year of evolution [6,7,46]. For instance, the single
domain magnetite of 35–120 nm produced by magnetotactic bacteria has clear protein-modulated
mineralization mechanisms and has very well-deﬁned ecophysiological signiﬁcance [47]. However, acompletely inorganic process can also produce single domain magnetite with the same morphology [11].
Textural structures [2,9,48] or the complexity represented by a set of crystal habits, including morphology
and size (e.g., gypsum, [12]), were suggested as a new type of biosignature of possible past Martianlife. In this paper, we provide a detailed description of the high diversity of mineralogical featuresof pyrite identiﬁed from the Kamchatka volcanic hot springs, which we suggest to be a signature of
microbiologically-mediated mineral deposition.
2. Materials and Methods
The Kamchatka Peninsula is located in the transition zone between the Eurasian, North American,
and Paciﬁc plates, and is one of the most tectonically active regions on Earth, featuring volcanoes andearthquakes [49]. There are 31 active volcanoes and hundreds of craters in Kamchatka, but hydrothermal

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activity is mostly located in the central and eastern volcanic zones [50,51]. As the largest living sulﬁdeore-forming hydrothermal system in Kamchatka, the Uzon caldera (54◦ 26 –54◦ 31 N, 159◦ 55 –160◦ 07 E)
is located in the center of the eastern volcanic zone with thick Paleogene-Neogene sedimentary rocks.
It formed after the collapse of the volcanic crater about 40,000 years ago and is underlain by Pliocene
volcanogenic sediments [52–54].
Of the hundreds of hot springs in Kamchatka, ﬁve were chosen for this study (Figure 1, [55–57]):
Burlyashii, Zavarzin, Thermophile, Jen’s Vent, and Oil Pool (Oil Pool lacks data on location andchemistry but is in the same area). Based on geochemical data listed in Table 1, Jen’s Vent and Burlyashii
hot springs have the highest temperatures, while Thermophile has the lowest among the hot springs
studied. All hot springs are predominantly in reduced geochemical conditions and with pH values
varying at large scales (pH = 4.4–7.5). Concentrations of soluble Fe and S2− species of the Burlyashii
hot spring are much higher than the other hot springs. The general geochemistry of these volcanic hot
springs can be found in Table 1 and Taran [55].

Figure 1. Locations of hot springs in Uzon Caldera, Kamchatka Peninsula (after 56,57).

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Table 1. The geochemistry of Kamchatka hot springs. All concentration values are in mmol/l unlessotherwise noted.
Parameters

Burlyashii

Zavarzin

ThermophileJen’s Vent 1

Jen’s Vent 2

Temperature (◦ C)

51–87
−90
6–6.5
1.18–1.23
3.75 × 10−3
0.23–2.3(6.3–43.8) × 10−3

54–74
−96
5.5–7.542–70

0.335–0.557

0.1–0.3(0.6–43.1) × 10−383
−240
5.3–5.9
2.2
0.18 × 10−3
1.35–1.96

85−240
5.3–5.9
0.16–0.18
0.54 × 10−3
1.29–3.125

0.063
0.41 × 10−3

0.011
0.54 × 10−3

[64,65][64,65]Eh (mV)pH
Alkalinity
Soluble Fe
SO4 2−
S2−
NO3 −
NO2 −
NH4 +
References

0.5

4.4–7

(0.1–0.3) × 10−3
1.1–1.5

0.84(0.2–0.6) × 10−3
0.2–4

[58–60][57–59,61–63][57,58,60]

Samples were collected with sterilized bottles by researchers from the University of Georgia [56],transported to the University of Hong Kong with dry ice, and stored at −21 ◦ C. For scanning electron
microscope (SEM) measurements, samples were dehydrated with anhydrous ethanol several times
and spread onto silicon chips. The silicon chips were sputtered with gold/palladium for 20 seconds
for electron microscopic observation. A Hitachi S4800 SEM in the Electron Microscope Center of
the University of Hong Kong was used for morphological and structural characterizations using thesecondary electron mode at low voltage (5 kV). Equipped energy-dispersive X-ray spectroscopy (EDS)
was used to measure the in-situ chemical composition of each sample to identify minerals based on the
primary results of SEM observations.
For Mössbauer spectroscopic measurements, samples were ground to a 200-mesh powder using
agate mortar after being freeze-dried. Each sample was mounted onto an acrylic holder (10 mm2 ) witha 5 mg Fe/cm2 thickness. The 57 Fe Mössbauer spectra were collected at room temperature (293 K) in
transmission mode with a 25mCi 57 Co/Pb source at the University of Hong Kong. The Mössbauer
spectroscopic hyperﬁne parameters were calibrated by the ﬁtted hyperﬁne parameters of the spectrumof a 25-μm α-Fe ﬁlm measured after every a few samples.
3. Results and Discussion
The Mössbauer spectra of Oil Pool and Jen’s Vent 1 hot spring sediments are shown in Figure 2
and the ﬁtting results are listed in Table 2. Samples from Jen’s Vent 1 showed two ﬁtted doublets withchemical isomer shifts (IS) of 0.30 mm/s and 1.06 mm/s, and quadrupole splitting (QS) of 0.60 mm/s
and 1.99 mm/s, respectively. The doublet with small IS and QS values was assigned to low-spin Fe2+ inpyrite [66], while that with large IS and QS values was assigned to high-spin Fe2+ on the lattice of clay
minerals. Although Fe2+ in pyrite has similar hyperﬁne parameters to those of Fe3+ on the lattice offerric iron oxides, detailed SEM observations and previous geochemical measurements (Table 1; [41,55])
conﬁrmed the existence of pyrite rather than ferric iron oxides. The parameters in agreement withthose of pyrite and Fe2+ in silicates [67,68] suggest that iron in Jen’s Vent1 mainly existed as Fe2+ inpyrite and Fe2+ -bearing silicates. For the Oil Pool, two doublets with similar parameters were also
observed, but showed the high level of Fe2+ in pyrite (96.65%).

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Figure 2. Room temperature 57 Fe Mössbauer spectra of low-spin Fe2+ in pyrite and high-spin Fe2+ on
the lattice of silicates (Jen’s Vent 1 and Oil Pool).
Table 2. Mössbauer spectroscopic parameters of hot spring sediments.
SampleMineralISa (mm/s)

QSb (mm/s)

Area (%)

Jen’s Vent 1

FeS2
Silicate

0.31
1.06

0.55
2.13

56.66
43.34

Oil Pool

FeS2
Silicate

0.30
1.22

0.60
1.99

96.65
3.35

a. IS = isomer shift. b. QS = quadrupole splitting.

Eh-pH diagrams were plotted based on geochemical data of Jen’s Vent 1 hot springs (83 ◦ C,
activity [Fe2+ ] = 10−6.7 , [SO4 2- ] = 10−2.8 , according to [65]) which show the thermodynamic stabilities
of sulfur species with the current geochemical conditions of these hot springs (Figure 3). It can be seenthat the inﬂuences of the temperature and ion concentrations on thermodynamic equilibrium in the hotsprings studied were insigniﬁcant. The electrochemical potentials of Kamchatka hot springs (Table 1)
favor the stability of Fe(II) in silicates and pyrite, which is consistent with the Mössbauer spectroscopic
results. If only ideal thermodynamic equilibrium is taken into account (Figure 3b), elemental sulfur
seems unable to exist in the current springs. However, elemental sulfur was commonly observed in
the sediments. The SEM observations showed that some of the elemental sulfur crystals were irregular(Figure 4a) and needed conﬁrmation by EDS analysis (Figure 4b), while others could only be detectedbased on EDS microanalysis (Figure 4c,d). We also observed a small number of monoclinic sulfur
crystals with well-developed crystal faces (Figure 4e) that were chemically conﬁrmed by EDS analysis(Figure 4f). They imply that conditions in favor of sulfur deposition have existed previously.

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Figure 3. Eh-pH diagrams calculated using Geochemist’s Workbench illustrating [69]: (a) Eh, pH,temperature and estimated ion concentrations in Kamchatka hot springs Jen’s Vent 1, Vent 2, Zavarzin
and Burlyashii (83 ◦ C, Activities: Fe2+ = 10−6.715 , SO4 2− = 10−2.796 ); (b) The thermodynamic stability ofelemental sulfur under the current geochemical conditions (83 ◦ C, Activity: SO4 2− = 10−2 ).

Figure 4. Scanning electron microscope (SEM) images (a,c,e) and energy-dispersive X-ray spectroscopy(EDS)-measured chemical composition of element sulfur (b,d,f) in diﬀerent morphologies observed in
Kamchatka hot springs. The signal of Pd in the spectrum of (b) should be ignored because it was fromthe instrumental background.

Anaerobic chemoorganoheterotrophic and chemolithoautotrophic bacteria and archaea have been
identiﬁed and isolated from the Kamchatka hot springs [43,56,70]. Thermophilic sulfate-reducing
bacteria (e.g., Thermoanaerobacterium aciditolerans), sulfur-reducing bacteria (e.g., Thermanaerovibrio
velox), sulfur-reducing archaea (e.g., Thermoproteus uzoniensis, Thermoplasmatales), along with the otherthermophilic microorganisms, build up a biological system that interplays with the geochemical systemin those hot springs [43,61,71]. Besides those bacterial sulfur redox processes, Thermoanaerobacter

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ethanolicus and Carboxydocella manganica sp. nov. isolated from the hot springs in Kamchatka canreduce Fe(III) for respiration [70,72].
The coexistence of pyrite crystals with a variety of sizes and morphologies were observed andcharacterized in all the studied hot spring sediments. The size of single euhedral crystals is in arange from ~100 nm to ~40 μm, with various crystal habits including cubic {100}, pyritohedral {210},octahedral {111}, pyritohedral {310} forms, and their combinations. Irregular or spherical aggregates ofpyrite crystals appearing loosely or tightly were also common in the sediments. Most pyrite crystals
showed smooth surfaces, while rough surfaces were observed on pyrite covered by clay minerals,organic matter, or the even ﬁner pyrite nanocrystals. However, the framboidal structure of pyrite thatis very common in sedimentary rocks and modern marine or lake sediments [19,29,73] was absent in
the hot spring sediments. Below are some detailed descriptions of crystal habits.
3.1. Single Crystals
The combination of a{100} and o{111} forms making cubo-octahedron crystals is the most common
combination form of pyrite crystals (Figures 5 and 6a,b). The dominant forms show a trend oftransformation between a{100} and o{111} forms (Figure 5). Such crystals are sometimes elongatedcubes because one pair of faces developed to a greater extent (e.g., Figure 5c). The size of single crystals
is in a range from ~200 nm (Figure 5h) to >40 μm (Figure 5d). Some crystals have smooth surfaces(e.g., Figure 5a), some are covered by clay minerals (e.g., Figure 5d), and others show etched structureson the surface, especially on faces of o{111} (e.g., Figure 5c). Sometimes incomplete transformations
between two forms result in face going missing during crystal development (Figure 6a,b).

Figure 5. SEM images of single pyrite with combinations of a{100} and o{111} faces and theircorresponding crystal shapes (color drawings on the right), indicating preferential orientation growthin the (100) and (111) directions from (a–h). (a) Pyrite crystals have smooth surfaces. (b,d,g) Pyrite
crystals covered by clay minerals. (c,e) Etching pits on the pyrite crystal surface, especially on faces ofo{111}.

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Figure 6. SEM images of single pyrite with combinations of a{100}, o{111}, e{210}, and f{310} faces andtheir corresponding crystal shapes (color drawings on the right) which show incomplete transformations
between two forms result in face going missing during crystal development (a–f).

Combination forms of a{100}, o{111}, e{210}, and f{310} were also observed (Figure 6c–f).
The octahedral form is dominant in these crystal habits and shows combinations with the e{210}form (Figure 6c) or with both a{100} and e{210} forms (Figure 6e). The pyritohedral form is dominant
in only a few crystals (Figure 6d). A combination of a{100}, o{111}, and f{310} forms in one crystal of
pyrite was observed, though the edges were ambiguous owing to its ultraﬁne size (300 nm, Figure 6f).
3.2. Pyrite Crystal Aggregates
Except those crystals that appeared clearly as single crystals in the sediments, most of them are in
the form aggregates. Five types of pyrite crystal aggregates were observed in the spring sediments:
I

II

III

IV

yrite crystals (single crystals with their sizes ranging from 5 to 10 μm) forming aggregates of~20 μm together with clay minerals (Figure 7a,b). The pyrite crystals in these aggregates havecombinations of {100}, o{111}, and e{210} habits. This type of aggregate, with sizes ranging from10 to 100 μm, was commonly found in the samples studied.
Parallel intergrowths of pyrite nanocrystals (<300 nm) were observed, which attach to, or nucleateon the o{111} surface of larger pyrite crystals (>10 μm) (Figure 7c,d). The habits of these pyrite
nanocrystals are mostly dominated by their o{111} form, which is sometimes slightly modiﬁedby e{210}.
Pyrite crystal aggregates attaching to the other minerals (Figure 7e,f). They are tiled on thesurfaces of the other larger crystals and commonly appear in irregular crystal morphologies.
These larger minerals oﬀer surfaces for small pyrite crystals to stick onto.
Massive pyrite nanocrystals (<100 nm) were observed to attach to, or crystallize on the surfaceof large pyrite crystals (>5 μm) (Figure 8a–g). The habits of these nanocrystals are octahedral(Figure 8d), cubic (Figure 8g), and irregular (Figure 8b). Pyrite nanocrystals do not just attach
to or overgrow some surfaces of larger crystals like type II, they also tile the surface. Diﬀerent
stages of pyrite nanocrystal development (irregular crystals with or without obscure edges) are170

Crystals 2020, 10, 535

Vshown in Figure 8b. Octahedral nanocrystals prefer to attach to o{111} faces while cubic ones
prefer a{100} faces (Figure 8e–g). Some of them grow in a certain direction (Figure 8g).
Irregular pyrite nanocrystals aggregate as spherulites (Figure 8e,f,h). Some of the small single
aggregates (~500 nm) attach to the surfaces of large pyrite crystals (white arrows in Figure 8e,f).
Some large aggregates (1–5 μm) attaching to other mineral surfaces are covered by thin ﬁlms
containing organic carbon and sulfur, as measured by EDS (Figure 8h).

Figure 7. SEM images of pyrite aggregates. (b,d,f) are ampliﬁcations of the highlighted areas in images(a,c,e), respectively.

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Figure 8. SEM images of pyrite aggregates. (b) showed octahedral pyrite crystals highlighted in (a).
(d) An aggregate of octahedral pyrite crystals on the surface of one bigger crystal in (c). (f,h) Irregularpyrite nanocrystals aggregate as spherulites. (g) The linear arrangement of pyrite nanocrystals on thesurface of a big pyrite crystal (e).

3.3. Intergrowth Texture
Crystal Intergrowths Appear in Four Types:
I

Intergrowth of single crystals. The cubical pyrite intergrowth texture was very common in the
hot spring sediments. Cubical pyrite crystals with a size range of 5 to 10 μm show intergrowthwith each other, which are sometimes coated by clay minerals (Figure 9a). The octahedral crystals
ranging from 300 nm to 1 μm were observed to have intergrowth with each other (Figure 9b) andwere covered by thin bioﬁlms, as indicated by EDS measurements.
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Crystals 2020, 10, 535

II

III

IV

Intergrowth of crystal combinations. Combination crystals of o{111} and a{100} show intergrowthwith each other (Figure 9c). The individual crystals are around 3 μm in size. All faces of a{100}form are striated in a speciﬁc direction.
Twin crystals appear as mirror images across the boundary where each crystal is combined withoctahedron and cube habits (Figure 9e). The size of a whole crystal is about 3 μm. There are also
other small pyrite crystals attached to the edges of twin crystals.
Parallel growth with relative smooth a{100} forms and rough o{111} forms, which can be coveredby a thin layer of clay minerals (Figure 9f). The dimension of a whole crystal is about 700 nm.

Figure 9. SEM images of diﬀerent types of intergrowths of pyrite crystals (a–f). The inset at thebottom-left of (b) was the ampliﬁcation of the highlighted area. (c) Pyrite crystals intergrowth. (e) Pyrite
twin crystals. (f) Pyrite crystal parallel growth.

The single forms commonly observed that make pyrite polyhedrons include: cube a{100},
pyritohedron e{210}, and octahedron o{111} [74]. There are other single forms, such as pyritohedronf{310}, {210}, {211}, {321}, and a small quantity of {221}. Combination forms of pyrite crystals were foundto be common in these hot spring sediments. As reported in previous studies, cubical pyrite crystals
are the most common, while octahedrons are rare among all single forms [75], yet the octahedron form
is not rare in the hot spring sediments studied. Normally, the habit of a crystal is conﬁned by the
crystallographic structure and defects, and its crystallization environments (mainly temperature andthe degree of supersaturation, e.g., [16]). In terrestrial hot springs, the ﬂuctuation of temperature in
microenvironments is uncommon, and therefore it has negligible impacts on crystal habits [14].

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3.4. Spherulite Pyrite Crystals
Pyrite spherules were observed in sediments of all hot springs (Figure 10). The size of pyrite
spherules is usually around 500 nm to 1 μm, aggregated either loosely (Figure 10a) or tightly(Figure 10b). Their assemblages are diﬀerent from framboidal pyrite, which is made of more euhedral
pyrite nanocrystals arranged in a particular order [20]. Framboidal pyrites dominate the crystal formsin black shales (a hydrothermal environment without biologically induced pyrite mineralization,e.g., [76–78]). However, no pyrite framboid was observed in any of the studied hot springs. All ofthese pyrite spherules are covered by a relatively thick crust of clay rich in organic matter, as indicatedby EDS measurements (Figure 10c–f).

Figure 10. SEM images and the EDS result of spherulitic pyrite crystals that are characterized by bioﬁlm
covering materials (a–e), aggregated either loosely (a) or tightly (b). (f) is the EDS proﬁle of the pyrite
crystal in (e).

In summary, the diversity of pyrite crystal habits described in this study was found to be muchhigher than that of authigenic pyrites in deep-sea hydrothermal vents [41,79]. This diversity has
been preserved in those small hot springs for >40,000 years [52–54]. The record of the complex
crystallographic features over such a long time reﬂects secular interactions between the continuous
supply of energy and nutritional elements by the active hot springs and the metabolisms of the
microbial communities.
We can consider a complex assemblage of pyrite forms as the reﬂection of the interactions between
the microbial communities with their geochemical environments, even though there is no direct
record of a biologically mediated mineralization. The complex pyrite crystal habits is coincident
with the thriving of microbial communities in the Kamchatka volcanic hot springs through time.

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Further evaluation of the complexity may develop links between the microbial physiology (e.g., energy
metabolism, metal respiration, and nutritional cycles) and the evolution of iron sulﬁde mineralogy.
This complex set of pyrite crystals may also be a useful reference for the identiﬁcation of biogenic
iron sulﬁdes on Mars. Potential Martian biosignatures using crystal morphology and traits include thedeposition of the digenetic apatite ‘ﬂowers’ as the result of the biological cycle of phosphorus [48],single-domain magnetite formed at low temperatures [80–82], etched pits on the surface of crystals
that have cell characteristics [83], abnormally tiny crystal sizes (~2–10 nm) [6,81], unusual crystal
lengths in one or more dimensions [84,85], and mineral casts or encrustations preserving biologicalcharacteristics [6,85,86]. The formation of a jarosite-goethite-gypsum assemblage was considered to be
the result of oxidation of pyrite [87]. The Chemistry and Mineralogy (CheMin) x-ray diﬀraction (XRD)
analyses conﬁrmed the existence of pyrite in mudstone at John Klein by Curiosity rover [88]. However,detailed observations of pyrite crystal on Mars have been limited by in situ measurement methods sofar. Some of researchers have tried to look for clues in Marian meteorites. Euhedral pyrite crystals
have not often been observed in many meteorites [89,90]. Pyrites in Northwest Africa (NWA) 7533 arecubic or octahedral crystals with average grains sizes of 30–40 μm [91]. Euhedral octahedral pyrite
crystals (~50 μm) were observed in Marian meteorite NWA 7475 [92]. These assemblages of pyrite are
not as complex as those we observed in the Kamchatka hot springs and their possible hydrothermal
genesis [91] may have made the diﬀerence. Putative microbial activity in the hot spring environments
on Mars in the distant past [93,94] might also have formed similar complex pyrite deposits. Due to thelack of dynamic geological activity and the freezing temperatures on Mars, the microstructures of any
complex pyrite deposits in the near-subsurface sediments could be well-preserved for multiple billionsof years. However, to fully understand the validity of a well-preserved pyrite complex on Mars, further
experiments will be required. Such experiments will need to concentrate on the stability of complex
pyrite deposits under various environmental conditions, such as burial, desiccation, heating, and otherprocesses that are likely to have occurred throughout Martian history. Therefore, this presents a new
avenue in the search for signs of ancient biota on Mars, and these pyrite complexes should be added to
the list of potential signatures of Martian life.
4. Conclusions
The active volcanic hot springs on the Kamchatka Peninsula are extreme environments for
microbial ecosystems characterized by the persistent supply of nutrients and geothermal energy.
The electron microscopic observations presented in this paper show diverse crystal habits and a wide
range of pyrite crystal sizes in the hot springs studied. We propose that it is the continuous interplay
between the geochemical environments of the volcanic hot springs and the microbial ecophysiologicalactivities that sustain the continuous precipitation of pyrite and preserve the diverse crystal sizes
and habits. We suggest that the complexity of crystal habits of pyrite in those hot springs represents
a combined biological and geochemical contribution to the kinetics of pyrite mineralization and its
preservation, and thus implies a biologically mediated process.
Author Contributions: Conceptualization, Y.-L.L. and M.T.; methodology, M.T.; validation, Y.-L.L. and M.T.;
formal analysis, Y.-L.L. and M.T.; writing—original draft preparation, M.T.; writing—review and editing, Y.-L.L.
and M.T.; supervision, Y.-L.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by RGC, grant number 17312016, NSFC, grant number 31970122
and Joint Foundation Project between Yunnan Science Technology Department and Yunnan University,
grant number C176240210019.
Acknowledgments: We thank Juergen Wiegel of the University of Georgia for his generosity in sharing thoseprecious samples.
Conﬂicts of Interest: The authors declare no conﬂict of interest.

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