135
The Canadian Mineralogist
Vol. 44, pp. 135-140 (2006)GABRIELITE, Tl2AgCu2As3S7, A NEW SPECIES OF THALLIUM SULFOSALTFROM LENGENBACH, BINNTAL, SWITZERLAND
STEFAN GRAESER§Natural History Museum Basel, CH-4001 Basel, and Mineralogical Institute,
University of Basel, CH-4056 Basel, Switzerland

DAN TOPA
Department of Geology and Mineralogy, University of Salzburg, Hellbruner Str. 34/III, A-5020 Salzburg, Austria

TON ČI BALI Ć-ŽUNI Ć AND EMIL MAKOVICKY
Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark

ABSTRACT
Gabrielite, Tl2AgCu2As3S7, is a new species of sulfosalt mineral occurring in the famous Lengenbach locality, at Binntal,
Canton Valais, Switzerland. It was discovered in association with numerous other As sulfosalts, such as hutchinsonite, hatchite,
edenharterite, trechmannite, tennantite, and abundant realgar and rathite, generally occurring as idiomorphic crystals in cavities of
a Triassic dolomitic rock. The whole region was transformed during the Alpine orogeny to the greenschist to garnet-amphibolitefacies of metamorphism. Gabrielite occurs as pseudohexagonal crystals; its true symmetry is triclinic, space group P 1̄, Z = 6, a
12.138(3), b 12.196(3), c 15.944(4) Å,  78.537(5),  84.715(4),  60.470(4)°, V 2013(1) Å3 (from the crystal-structure determination). The strongest powder-diffraction lines [dobs in Å(Iobs)(hkl)] are 15.63(100)(001), 3.143(90)(214̄, 11̄4), 3.531(80)(300),
2.911(70)(422, 2 1̄ 4̄), 2.520(60)(2 2̄3), and 2.978(60)(324). Gabrielite is opaque with a grey to black color, metallic luster andblackish red streak. Individual crystals reach up to approximately 0.4 mm, and aggregates may exceed 1 mm across. Euhedral
crystals are short prismatic to platy parallel to (001), with excellent cleavage along this face. The microhardness VHN,18 kg/
mm2 (load 10 g), corresponds to a Mohs hardness of 1½–2. The chemical composition (electron microprobe, mean result of 11
analyses, in wt%) is Tl 37.30, Cu 12.50, Ag 8.47, As 18.90, Sb 1.79, S 20.64, total 99.70 wt%, from which the empirical formula,Tl5.95(Ag2.56Cu6.46)9.02(As8.23Sb0.48)8.71S21, was derived. The simplified formula is Tl2AgCu2As3S7. The calculated density Dcalc
is 5.38 g/cm3. In reflected light, gabrielite is white, has common red internal reflections, shows weak anisotropy and moderate
reflectivity [%Rmax in air (COM wavelengths in nm)]: 30.53(470), 29.10(546), 27.94(589), 26.35(650). The crystal structureis layered parallel to (001), which explains the habit and cleavage. The twin law with (100) as a twin plane is due to the local
symmetry properties of the structural layers. A presence of metal–metal (Tl–Tl and Tl–Cu) and metal–semimetal (Tl–As) bondsin the structure explains the low hardness and metallic character. Substitutions of Cu for Ag in the tetrahedral coordinations, Ag
for Cu in the trigonal–planar coordinations, and Sb for As in isolated AsS3 coordination polyhedra and As4S8 rings lead to thestructural formula Tl6(Ag,Cu)3IV(Cu,Ag)6III[(As,Sb)S3]3[(As,Sb)2S4]3; the empirical formula obtained from the microprobe data is
Tl5.95(Ag2.56Cu6.46)9.02(As8.23Sb0.48)8.71S21. The name of the mineral honors Walter Gabriel, of Basel, Switzerland, well-known
mineral photographer and expert of Lengenbach minerals.
Keywords: gabrielite, new mineral species, sulfosalt, thallium, Lengenbach, Switzerland.

SOMMAIRE
La gabrielite, Tl2AgCu2As3S7, est une nouvelle espèce de sulfosel découverte à la carrière Lengenbach, localité bien connue
à Binntal, Canton de Valais, en Suisse. Elle a été découverte en association avec de nombreux autres sulfosels d’arsenic, parexemple hutchinsonite, hatchite, édenharterite, trechmannite, tennantite, avec réalgar abondant et rathite, en général sous forme decristaux idiomorphes dans des cavités d’une roche dolomitique triassique. La région entière a été transformée par métamorphisme
alpin au faciès schistes verts ou amphibolite à grenat. La gabrielite se présente en cristaux pseudohexagonaux; par contre, savraie symétrie est triclinique, groupe d’espace P 1̄, Z = 6, a 12.138(3), b 12.196(3), c 15.944(4) Å,  78.537(5),  84.715(4), 
60.470(4)°, V 2013(1) Å3 (maille établie par affinement de la structure). Les raies les plus intenses du spectre de diffraction X(méthode des poudres) [dobs en Å(Iobs)(hkl)] sont 15.63(100)(001), 3.143(90)(21 4̄, 1 1̄4), 3.531(80)(300), 2.911(70)(422, 2 1̄ 4̄),2.520(60)(2 2̄3), et 2.978(60)(324). La gabrielite est opaque, grise à noire, avec un éclat métallique et une rayure rouge noirâtre.

§E-mail address: stefan.graeser@unibas.ch

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THE CANADIAN MINERALOGISTLes cristaux individuels atteignent en général environ 0.4 mm, et les aggrégats peuvent dépasser 1 mm. Les cristaux idiomorphes
sont des prismes courts ou des plaquettes parallèles à (001) et au clivage le long de cette face. La microdureté VHN,18 kg/mm2(charge de 10 g), correspond à une dureté de Mohs de 1½–2. La composition chimique, établie avec une microsonde électronique(résultat moyen de 11 analyses, en %, base pondérale) est: Tl 37.30, Cu 12.50, Ag 8.47, As 18.90, Sb 1.79, S 20.64, total 99.70
wt%, ce qui mène à la formule empirique: Tl5.95(Ag2.56Cu6.46)9.02(As8.23Sb0.48)8.71S21. La formule simplifiée est Tl2AgCu2As3S7.
La densité calculée Dcalc est égale à 5.38 g/cm3. En lumière réfléchie, la gabrielite est blanche; elle montre de nombreusesréflections internes, et possède une faible anisotropie et une réflectivité moyenne [%Rmax mesurée dans l’air (longueurs d’ondede COM en nm)]: 30.53(470), 29.10(546), 27.94(589), 26.35(650). La structure cristalline possède des couches parallèles à(001), ce qui explique la morphologie et le clivage. La loi de macle, avec (100) comme plan de macle, découle des propriétés
locales de symétrie de ces couches. La présence de liaisons de métal à métal (Tl–Tl et Tl–Cu) et de métal à semimétal (Tl–As)
rend compte de la faible dureté et du caractère métallique. Les substitutions de Cu au Ag sur le site tétraédrique, de Ag au Cu
sur le site trigonal–planaire, et du Sb au As dans les polyèdres isolés AsS3 et les anneaux As4S8 mènent à la formule structurale
Tl6(Ag,Cu)3IV(Cu,Ag)6III[(As,Sb)S3]3[(As,Sb)2S4]3; la formule empirique obtenue à partir des données de microsonde électronique est Tl5.95(Ag2.56Cu6.46)9.02(As8.23Sb0.48)8.71S21. Le nom de l’espèce honore Walter Gabriel, de Bâle, en Suisse, photographede minéraux bien connu et expert des minéraux de Lengenbach.
(Traduit par la Rédaction)
Mots-clés: gabrielite, nouvelle espèce minérale, sulfosel, thallium, Lengenbach, Suisse.

INTRODUCTION

THE MINERALS OF LENGENBACH

Gabrielite, ideally Tl2AgCu2As3S7, is a new speciesof sulfosalt mineral found at Lengenbach, in Binntal,
Ct.Valais, Switzerland, a locality well known for its
remarkable occurrence of numerous rare and uniquesulfosalts. It has provided mineralogists and crystallographers all over the world with interesting researchmaterial for more than 200 years. The main mineralizedzone is small, not exceeding a volume of about 50 
30  30 meters within a dolomitic rock of presumably
Triassic age. The host rock displays mostly a snowwhite color, it is partly a compact, marble-like rock,yet, on the other hand, it may occur as a soft materialof powdery consistency that can be pulverized evenbetween the fingers (“sugary” dolomite). Undoubtedly,hydrothermal activity played an important role in thealteration of the host rock as well as in the formation ofcavities and the crystallization of rare sulfosalt minerals
within them. The arsenic necessary for the formation of
the various As-rich sulfosalts in the dolomite presumably originated in the adjoining gneisses and migrated
into the dolomite either during Alpine metamorphism,
as postulated by the first author (Graeser 1965, 1998),
or at a much earlier time, during the diagenesis of the
dolomitic sediments (Hofmann & Knill 1996).
The name of the new mineral species honors Walter
Gabriel, of Basel (b. 1943), a chemical technician
who, in his spare time, has photographed thousands ofLengenbach specimens for books and publications. He
has studied Lengenbach minerals for decades, and has
gained considerable knowledge of these uncommon
minerals.
The new mineral and its name have been approvedby the Commission on New Minerals and Mineral
Names, I.M.A. (IMA 2002–053). Type material ispreserved at the Natural History Museum Basel andthe Institute of Mineralogy, University of Basel (X-ray
samples) under the catalogue number S113.

The locality has yielded more than 120 different
mineral species so far, and it is remarkable that almost
half of these minerals are sulfides and sulfosalts.
The sulfosalt minerals of Lengenbach belong almost
exclusively to the group of arsenic sulfosalts, hosting
additional elements like Pb, Tl, Cu, Ag; about 26 speciesof them were discovered here for the first time (typelocality). Fourteen Tl-sulfosalts have been discovered
here during the past 20 years, of which 11 are newminerals. A recent description of the locality and its
minerals is given in Graeser (1998).
Mineral collecting at Lengenbach dates back at least
to the year 1797 (proven by a dated sample preservedin the Natural History Museum in Basel). Systematic exploitation of minerals for research by various
syndicates and companies started around 1833, andhas continued with interruptions caused by the twoworld wars up to the present time. Currently, works inLengenbach are being carried out by the “Forschungsgemeinschaft Lengenbach”, a syndicate consisting ofvarious mineral collectors and mineralogists, under thesupervision of the municipality of Binn.

OCCURRENCE
A sample (L21913) collected in 1989 by the syndicate working at that time in Lengenbach (“Arbeitsgemeinschaft Lengenbach”), contained some crystals
with a peculiar shape that caught our attention. An
X-ray powder-diffraction pattern demonstrated that itis different from any other Lengenbach mineral known
so far. Unfortunately, the crystals on the sample were
not suitable for a thorough study, either with respect toquality or to quantity. Further finds in 1990, and 1995,
in particular, provided material more suitable for study,
and the results are based mainly on this material.

GABRIELITE, A NEW SULFOSALT SPECIES FROM LENGENBACH

With a Tl content of 37.4 wt%, gabrielite is among
the minerals richest in Tl occurring at Lengenbach.
Only imhofite, Tl3As7.67S13, with 37.6 wt%, contains
slightly more Tl. As in the case of the other mineralsof Tl, gabrielite is only found in the zone with elevatedTl and As contents, in the southern part of the quarry,and consequently it is commonly accompanied byrealgar, rathite, trechmannite, tennantite, hutchinsonite,
hatchite, edenharterite and other Tl-rich sulfosalts. All
these minerals crystallized in small irregularly shaped
cavities in the dolomitic rock. The Tl-rich minerals, inparticular, are the products of the latest stage of hydrothermal activity at Lengenbach.

137

material used for analytical work had to be verified firstby X-ray diffraction (Gandolfi camera). However, thepseudohexagonal shape (where present), the perfectcleavage, the black metallic appearance and the reddish
streak (clearly visible on broken edges) might give someindication of the kind of mineral. With a Mohs hardnessof 1½–2 (according to VHN indentations measured
on a Leitz Durimet microscope), gabrielite has thelowest hardness among all Lengenbach Tl-sulfosalts. In
polished section under reflected light, gabrielite displaysa white color with common red internal reflections.
Anisotropy is weak; no bireflectance or pleochroism
was observed. Reflectance values are moderate andclose to those of most other Tl sulfosalts (Table 2).

PHYSICAL AND OPTICAL PROPERTIES
It was the very peculiar shape of some of the verysmall crystals (individual crystals not exceeding 0.3–0.4mm), that piqued scientific interest in the mineral; thecrystals show a conspicuous sixfold outline, thoughfrom the distinct striation parallel to the crystallographic
a axis on the largest face, hexagonal symmetry can beruled out (Fig. 1). In one case, an aggregate of looselyintergrown crystals attained a diameter of more than 1
mm. As is the case for most Tl-rich sulfosalts, it is notpossible to unequivocally identify the mineral basedon physical properties and visual inspection alone(Table 1). In its anhedral form, gabrielite is almost
indistinguishable from other Lengenbach Tl-minerals,
in particular sicherite, TlAg2(As,Sb3)S6. Therefore, any

FIG. 1. Gabrielite, Lengenbach. SEM picture showing the dominant (001) face withstriation along a axis and (presumable) twinning along (100).

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THE CANADIAN MINERALOGISTCHEMICAL COMPOSITION
Preliminary qualitative chemical anlayses using a
Hitachi S–2500 scanning electron microscope (SEM)
revealed the presence of Tl, Cu, Ag, As, Sb, and S only.
Quantitative analyses were performed using an electron
microprobe (JEOL JXA–8600; LINK–eXL). Chemical
data are reported in Table 3. From the observed ranges
and the results of the crystal-structure analysis, it can be
expected that Sb can substitute in even larger amounts
for As. It is, however, difficult to predict whether anincreased level of substitution would bring a structuralchange like in the case of the two Tl-sulfosalts edenharterite TlPbAs3S6 (Graeser & Schwander 1992) andjentschite TlPbAs2SbS6 (Graeser & Edenharter 1997),
where the ordered distribution of Sb in the structure
led to a lowering in symmetry from orthorhombic tomonoclinic. A more probable reason for a change ofsymmetry could be a different proportion of Cu and Ag
in the structure. The crystal-structure analysis pointed toan ordered partial substitution of these two elements onsome structural sites as the main reason for the loweringof symmetry from an idealized monoclinic to theobserved triclinic (Balić-Žunić et al. 2006). Although
no close structural relations are observed between them,some of the Tl sulfosalts from Lengenbach show aconspicuous geometrical relationship (Table 4).

X-RAY DIFFRACTION
The mineral yielded excellent powder patterns(Debye–Scherrer and Gandolfi cameras) that are unlike
those of any known mineral species. In combination
with SEM data, it is obvious that the mineral represents a new complex sulfosalt phase with Tl, Cu, Ag,
As, and Sb. Consequently, a preliminary single-crystal
study was carried out using Weissenberg and precession
techniques which, at first, did not lead to a satisfactory
result. Preliminary studies led to (pseudo) monoclinic
symmetry, presumably due to the presence of twinningphenomena; attempts to split the twins into unequivocal
single crystals were unsuccessful. The true triclinic
symmetry resulted from the subsequent structure-determination. The powder pattern was thereafter indexedon the basis of a triclinic cell. Cell parameters refinedfrom the powder data are in good agreement with thoseobtained from the structure determination. An indexed
powder-pattern is reported in Table 5, and unit-celldimensions are listed in Table 6.

GABRIELITE, A NEW SULFOSALT SPECIES FROM LENGENBACH

139

An extraordinary property of the crystal structure is
the presence of short metal–metal and metal–semimetalbonds (Tl–Cu of 3.03 to 3.08 Å, Tl–Tl of 3.09 Å, and
Tl–As of only 3.11 Å), all of which are the shortest
registered among sulfosalts so far. The metallic luster
and a very low hardness may be linked to the influenceof these metal–metal bonds in the structure.
The crystal structure contains isolated AsS3 groups
and As4S8 rings. Partial substitution of Sb for As isobserved only in some of the As sites. The substitution
is most probably limited by the size requirements andthe bonding scheme, because Sb is found only in sites
where the surrounding atoms enclose smaller volumes,
and is not recorded in sites with short As–Cu distances.
Cu and Ag atoms appear mixed in two types of coordinations with S, with Cu dominating in trigonal–planar,and Ag in tetrahedral ones. Owing to these preferences,
it is probable that both elements are necessary to stabilize the structure, and no pure Cu or Ag variants are
possible. The structural formula of gabrielite can be
written as Tl6IV(Ag,Cu)3III(Cu,Ag)6[(As,Sb)S3]3[(As,Sb)2S4]3, with IV and III denoting the tetrahedral andtrigonal planar coordinations, and As (Sb) grouped
according to the polymerization of the coordination
polyhedra. The shortened form of the formula, withminor substitutions ignored, is Tl2AgCu2AsS3As2S4.

ACKNOWLEDGEMENTS

DESCRIPTION OF THE STRUCTURE
The structure of gabrielite is the subject of acompanion paper (Balić-Žunić et al. 2006). Here wemention only the main features that are responsible forthe physical properties described.
The crystal structure is layered parallel to {001},
which explains both the platy habit and an excellent
cleavage parallel to this form. There are three different
types of layers present. Two of them are with internal
trigonal symmetry, whereas the third has an internal
monoclinic symmetry. The only symmetry elements thatare shared by all three layers are centers of symmetry,
resulting in an overall triclinic symmetry of the structure. This peculiar combination results in a crystal latticevery close to a monoclinic C-centered one, and is thereason for the pseudohexagonal outline of crystals, andfor the nearly equal lengths of the a and b periods and a angle close to 60°. The pseudosymmetry of the crystal
lattice is connected also to the twinning, with (100) as
the twin plane. The local symmetry properties of layers
suggest a possibility for polytypism. Both a monoclinic
and a trigonal polytype can be expected, with a doubled
and a tripled c axis periods, respectively.

Markus Ecker and Dr. Thomas Raber (membersof the “Forschungsgemeinschaft Lengenbach”) kindly
provided their samples of gabrielite for the study. We are
greatly obliged to R. Guggenheim, M. Düggelin and D.
Mathys (SEM laboratory, University of Basel) for their
help in acquiring the SEM pictures and performing thechemical analyses of the minerals). Eric Reusser (ETH
Zürich) performed the EMP analyses of gabrielite.
The first samples containing the new mineral species
were put at our disposal by the “Arbeitsgemeinschaft
Lengenbach”, the syndicate exploiting the Lengenbachminerals until 1998. We thank Yves Moëlo, Nantes, andFranklin F. Foit, Jr., of Washington State University,
and Associate Editor Nigel J. Cook for critical reviews
and constructive comments, which greatly improved
the manuscript.

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THE CANADIAN MINERALOGISTREFERENCESBALI Ć-ŽUNI Ć, T. & MAKOVICKY, E. (1993): Contributions
to the crystal chemistry of thallium sulphosalts. I. The
O–D nature of imhofite. Neues Jahrb. Mineral., Abh. 165,
317-330.
BALI Ć-ŽUNI Ć, T., MAKOVICKY, E., KARANOVIC, LJ., POLETI, D.
& GRAESER, S. (2006): The crystal structure of gabrielite,
Tl2AgCu2As3S7, a new species of thallium sulfosalt from
Lengenbach, Switzerland. Can. Mineral. 44, 141-158.
BURRI, G., GRAESER, S., MARUMO, F. & NOWACKI, W. (1965):
Imhofit, ein neues Thallium-Arsensulfosalz aus dem Lengenbach (Binnatal, Kt.Wallis). Chimia 19, 499-500.
GRAESER, S. (1965): Die Mineralfundstellen im Dolomit desBinnatales. Schweiz. Mineral. Petrogr. Mitt. 45, 567-795.

eral from Lengenbach (Binntal, Switzerland): description
and structure determination. Am. Mineral. 86, 1087-1093.
GRAESER, S. & EDENHARTER, A. (1997): Jentschite (TlPb
As2SbS6); a new sulphosalt mineral from Lengenbach,
Binntal (Switzerland). Mineral. Mag. 61, 131-137.
GRAESER, S. & SCHWANDER, H. (1992): Edenharterite (TlPb
As3S6): a new mineral from Lengenbach, Binntal (Switzerland). Eur. J. Mineral. 4, 1265-1270.
HOFMANN, B.A. & KNILL, M.D. (1996): Geochemistry andgenesis of the Lengenbach Pb–Zn–As–Tl–Ba–mineralisation, Binn Valley, Switzerland. Mineral. Deposita
31, 319-339.
YVON, K., JEITSCHKO, W. & PARTHÉ, E. (1977): Lazy Pulverix,a computer program for calculating X-ray and neutron diffraction powder patterns. J. Appl. Crystallogr. 10, 73-74.

GRAESER, S. (1998): Alpine minerals: a review of the most
famous localities of the central Swiss Alps. Rocks & Minerals 73, 14-33.
GRAESER,S., BERLEPSCH, P., MAKOVICKY, E. & BALI Ć-ŽUNI Ć,T. (2001): Sicherite, TlAg2(As,Sb)3S6, a new sulfosalt min-Received May 9, 2005, revised manuscript accepted
October 16, 2005.