SCHWEIZ. MINERAL. PETROGR. MITT. 78, 489-494, 1998

Quadratite Ag(Cd,Pb)(As,Sb)S3 - a new sulfide mineralfrom Lengenbach, Binntal (Switzerland)by Stefan Graeserl,2, Wim Lustenhouwels and Peter Berlepsch 2Abstract

With the mineral quadratite another new sulfide phase has been identified in the famous Lengenbach locality (Binntal, Switzerland). Like the numerous other, mainly As-sultidc minerals at this locality, quadratite occurs in small vugs
in a white dolomitic rock of presumabl y Triassic age. Different from the new minerals discovered within the last fewyears, which all belong to the very special group of thallium sulfosalts and therefore always occur in close association with As-rich phases, quadratite exclusively occurs together with minerals poor in arsenic. Therefore it is closely
associated with minerals like jordanite and galena, whereas realgar is clearly absent. In general, quadratite forms
crystals of characteristic quadratic, sometimes octagonal shape: always tabular along the basis (001). The size is normally about 0. 1 mm across, crystals with a diameter of more than 1 mm are extremely rare. With few exceptions themineral is lJntwinned, one or two samples show an oriented intergrowth which may be interpreted as contact twinsalong (016). Small crystals are brown-red translucent, larger specimens are opaque with grey metallic luster, streakis reddish-brown. Single-crystal X-ray diffraction yielded a tetragonal cell with a = 5.499 (5) and c = 33.91 (4) A. V =1025 (2) A', space group 14/amd, Z = 8. The strongest lines in the powder diagram are (d ob' in A. lob" hkl): 2.77 (100)(200), 1.96D (80) (2.0.12),1.679 (70) (2.0.16),1.598 (70) (2.2.12;3.0.11),1.274 (60) (4.0.10),3.19 (50) (116). Chemicalcomposition (electron microprobe, mean of 6 analyses, wt%) is: Ag 26.30, Cu 0.05, Tl 0.49, Cd 21.42, Pb 10.17, As18.06, Sb D.27, S 23.61, total 100.37 wt%. The simplified formula is close to AgCdAsS). The calculated density D~,lo =5.31 g/cm J • From the microhardness VHN (load 10 g) of 57-72 kg/mm2 a Mohs hardness of::; 3 may be derived. Quadratite has a perfect cleavage along (001\ and distinct along [110).
The name is derived from the conspicous quadratic shape of the mineral, unique among Lengenbach sulfosalts,
so far.

Keywords: quadratite, new Ag-Cd-As-sulfide mineral, single-crystal data, microprobe analyses, Lengenbach,
Switzerland.

1. Introduction

The Lengenbach quarry in Binntal (Ct. Valais,Switzerland) is well known for its very uncommon sulfosalt minerals, mainly Pb-Cu-Ag-Tl(As,Sb )-sulfides occurring in small cavities in thewhite dolomitic rock. With exception of some periods during the two world wars, the sulfosalt mineralisation has been mined for minerals almost
professionally since more than 150 years. It waspurely the scientific interest to study these minerals that was responsable for their exploitation.
Starting with 1958, a Swiss syndicate consisting ofseveral scientific institutions (Museums, Mineralogical Institutes but also private people) has
been active in Lengenbach and organized themineral extraction until 1998. From 1958 to 1986,
the works were carried out in the old classicalquarry, which was abandoned in 1987 clue to lackof attractive material. Since the new site wasopened in 1987, a remarkable number of newmineral species has been discovered, which mainly belong to the special family of thallium sulfosalts (see e.g. GRAESER and EDEN HARTER, 1997).
In contrast to the mostly longprismatic TI-Assulfosalts, quaclratite, with Cd, Ag, As, besides sul-

1 Naturhistorisches Museum Basel, Augustinergasse 2, CH-4001 Basel, Switzerland.

<graesers@ubac!u.unibas.ch>
2 Mineralogisch-petrographisches Institut, UniversiUit Basel, CH-4056 Basel, Switzerland.

] Institut voor Aardwetenschappen, Vrije Universiteit Amsterdam, NL-1081 HV Amsterdam, Netherlands.

490

ST. GRAESER, W. LUSTENHOUVERAND P. BERLEPSCH

found in 1993, with crystals of more than 1 mm
(e.g. Markus Ecker, Spiesen, Germany). Thesecrystals enabled derivation of a complete set ofdata for the new mineral. Subsequently, the proposal for the new mineral was submitted to theLM.A. commission, which accepted name and description of the new species (LM.A. No. 94-038).
Type material is preserved at the Natural History
Museum Basel, and at the Mineralogical Institute,
University of Basel.
2. Description
2.1. PHYSICAL AND OPTICAL PROPERTIES

Quadratite crystal (0.45 mm 0), grown on jordanite. Sample M. Siegel. photograph by W. Gabriel.

Fig. 1

fur as main constituents, represents a completely
new type of sulfide minerals in Lengenbach, a factwhich is documented by the special morphologyof the mineral, too. First specimens of the mineral
were discovered by the mineral collectors Monikaand Walter Gabriel (Basel) on the dump of thequarry in 1989. These crystals reached about
0.1 mm in diameter, a size that was suitable to establish a new species but did not allow to derive
all the data necessary for the submission of a proposal to the Commission on New Minerals andMineral Names, LM.A. Additional material was

104

It was the very uncommon morphology - thin tabular crystals of unmistakable quadratic shape, unlike any other Lengenbach mineral - that drew
the attention to the unknown mineral. The thinplaty crystals occur always as octogonal tablets,
the smaller edge clearly corresponding to (hhl)faces (Fig. 1). One of the small crystals could bemeasured on an optical goniometer from which
the following faces were calculated: a large pinacoid (001), two bipyramids of second order (103)and (104), which are always dominant over thetwo additional small bipyramids of first order(111) and (116). One unique sample (found by theGerman collector Ralph Cannon) displayed avery conspicous twinning law.1110ugh the twinned
crystals could not be measured goniometrically,an interpretation as penetration twins along (016)
is obvious. Viewed with the naked eye, the two
crystals cross at (nearly) 90 therefore the angle pfor the twinning-plane must be about 45 0 , and0,001
116

Fig.2 Shape crystal drawings (DOWTY, 1997) based on optical goniometry. Left: single crystal; right: penetration
twin along (016).

QUAD RATITE Ag(Cd.Pb)(As,Sb)S-, - A NEW SULFIDE MINERAL FROM LENGENBACH

491

Tab. 1 Physical and optical properties of quadratite from Lengenbach.
Habit:
Forms:
Twinning:
Colour:Streak:Cleavage:
Tenacity:
Hardness:
Density:extremely thin quadratic (octogonal) laminae, often translucentpinacoid (DOl), always strongly dominant
first order bipyramids {103} and {l04}, mostly small
second order bipyramids {Ill} and {116). dominant
penetration along (016), see figure 2
in thin laminae reddish translucent, otherwise grey-metallic, opaquereddish-brown(DOl) perfect (micaceous); (110) distinct
thin laminae are ductile to flexible
Molls: ~ 3 - according to micro indentation VHN lOg = 63 (52-72)
5.31 g/cm-' (calculated)

Ore microscopic data (MPV-STAT by WILD-LEITZ)
Colour:Internal reflcxions:
Pleochroism:
Bireflectance:

greyish-white with bluish tint
dark red reflexions along cleavage cracks (oil immersion)
not observed (polished section ~ parallel to (001))
not observed (polished section ~ parallel to (001))

Reflectance values:

WTiC standard
WavelengthAir

Oil470nm
543 nm587nm
657 nm

30.5-32.3
29.6-30.8
28.7-29.7
26.3-28.0

12.5-14.0
12.2-13.3
12.0-12.9
11.3-12.3

since the angle (001'016) = 45.6°, the twinningplane undoubtedly is (016), see figure 2. The extremely thin laminae appear clearly translucent
and under a transmission polarizing microscopeproduce an indistinct nearly uniaxial figure whosecharacter is not interpretable. With increasing
thickness the crystals turn to an opaque. metallic
character. In polished section under the metal microscope, quadratite appears greyish-white, with aslight bluish tint. Due to the perfect, micaceous
cleavage along the basis (001), polished sections
could only be prepared parallel to this face. Therefore, they provided few information on
anisotropy, bireliectance, and pleochroism. A fullset of physical and optical properties is compiledin table 1.
2.2. X-RAY DIFFRACTION STUDY

The special ductile nature of the mineral provided some problems for the X-ray study. Therefore,
powder data were derived mainly from diffraction
patterns carried out by the Gandolfi technique(on single crystals). The patterns established withthis device turned ot to be of superior quality
compared to those obtained by common DebyeScherrer-technique. TIle single-crystal investigation (Weissenberg and Precession methods) too,
was impeded by the mineral's special properties,
and was fully successful only with the better ma-terial found in 1993. The results of this study suggest some relationship to the mineral cernyite,
though the mineral formula and space group symmetry are quite different compared to quadratite.
We hope that in future we will be able to separate
adequate material for a complete structure determination which could solve this problem. The results of the X-ray diffraction study are given intable 2.
2.3. CHEMICAL COMPOSITIONFirst qualitative chemical data were obtained
from EDS analyses on a SEM device (Cambridge
Stereoscan Mark 2A, University of Basel), carriedout with extremely fine laminae of about 0.1 mmin diameter. These analyses clearly pointed outthat the mineral consisted of mainly Ag, Cd, As,and sulfur, a composition that was not known
from a natural mineral, so far. Quantitative determinations, however, became feasible only withlarger amounts of material discovered some years
later. Chemical data are reported in table 3.
With the composition Ag(Cd,Pb,TI)(As,Sb)S]
for quadratite, the mineral obviously represents
the first example of a Ag-Cd-As-sulfide in mineral systematics, altogether. Without a complete
structure determination to date it is not possible
to decide whether it belongs to the sulfosalt groups.str, (with AsS] pyramids) or to complex sulfides

ST. GRAESER, W. LUSTENHOUVER AND P. BERLEPSCH

492
Tab.2

X-ray diffraction data of quadratite from Lengenbach.
dolo, (in A)\Ivisucll

30
50
100
< 1010

4.23
3.19
2.77
2.39
2.33

10

2.145

10
80

2.090
2.040
1.960

10

1.878

io

10

1.827

30

1.783

30
70
40

1.759
1.679
1.664

70

1.598

20

I

1.536

30

1.435

10

1.380

30

1.299

60

1.274

dealeh k II\,lslIcll

4.239
3.203
2.750
2.403
2.312
{ 2.136{) 0 8116
200
213
215
2 0 10

2.119
2.091
2.055
1.970
{ 1.895

o 0 161.875{ 1.830

1 0 17301101.817{ 1.789

2 0 142 1 13

1.770
1.767
1.679
1.664
{ 1.602305
228
2 0 162 1 152 2 12

1.576{ 1.544

3 0 113 1 102030
20101030
20
20
20
30
40

1.524
{ 1.433
1.433
1.424
1.375
{ 1.309
1.308
1.292
1.274

3 2 1
1 1 222 2 161 0 23400415408332
4 0 1030
1020

1 0 151 1 142 0 12224

10

dolo, (in A)
1.245
1.237
1.224
1.210
1.177
1.156
1.136
1.131
1.0982
1.07381.06761.04591.03701.01980.98940.9816dealeh k 1{ 1.257

2 0 241.237{ 1.236

1 1 264 0 121.230{ 1.226420422

1.224
211
C·
1.202

1 027
3 3 10

C.l81

426
428

1.178

2 0 26

1.156

42 10

Gandolfi camcra 114.6 mm 0, FeK" radiation, intensities visually estimated

Crystal system:

tetragonal

Space group:

14/amdLattice parameters (cell constants refined from Gandolfi powder data):
a5.499 (5) AV'"

1025 (2) ;"'3c33.91 (4) AZ'"

8cia

6.165 (from lattice parametcrs)
6.19 (from optical goniometry)(with AsS 4 tetrahedrons), though from the general aspect of the mineral an affiliation to the complex sulfides appears more probable. Anotherproblem arises from the question if Cd may besubstituted partly by Pb, as postulated by the mineral formula. There are few minerals known in nature containing Cd as a major element (NICKELand NICHOLS, 1997), however, among about 10
well defined species, one of them seems to repre-sent exactly the same uncommon substitution: forthe sulfide mineral shadlunite (Fe,CuMPb,Cd)Ssa similar substutition is postulated by the authors(EVSTIGNEEVA et al., 1973).
Analytical work was carried out by means ofan electron probe on one relatively large quadratite crystal (about 1 mm2). The chemical data
are reported in table 3. A BSE-image of quadratite (Fig. 3) proofs a remarkably homogeneousQUAD RATITE Ag(Cd,Pb)(As,Sb)S3 - A NEW SULFIDE MINERAL FROM LEN GENBACH
Tal!. 3AgCuTlCdPbAsSbSTotal493

Chcmical composition of quadratitc,

26.30
0.05
0.49
21.42
10.17
18.06
0.27
23.61
100.37

IIIII

26.05- 26.88
0.04- 0.06
0.46- 0.57
21.08- 22.06
9.31- 10.65
17.74- 18.37
0.23- 0.32
23.29- 24.11
99.76-101.08

26.30
21.91
10.10
18.25
23.44
100.00

Structural formula (based on 24 sulfur atoms)
Ag
Cu
CdPb
TI

7.95
0.03
6.27
[,610.08As
Sb

7.92
0.07
24S

8.0
~ = 7.98

6.0
2.0~ = 7.96~=7.998.024

I mean of six analyses, leading to the empirical formula (Ag7YS Cu lI .ll3h" 7.')H( Cd".27 Pb J.61 Tl o.IIKh = 7.90 (AS 7.92

Sb U.(17 )}; ~ 7,{jl) S24
II range of data

III theoretical composition for the simplified formula
8 X Ag(CdoHPbo2)AsS3 (based on a zell content Z =8)

Microprobe: lEaL lXA-8800M (20 kY, 5 X 10-11 A),
lEaL on-linc ZAF correction using Berger & Seltzer'sliZ plus Philibert's and DlInc1ll11b & Reed's absorption
corrcction
Standards: Ag (purc), ClI (pure), TlSbS 2 (TI, Sb), CdS(Cd. S), PbS (Pb). InAs (As)chemical composition of the mineral, though it isvery closely intergrown with other sulfide minerai phases Uordanite, pyrite).

3. Occurrence, associated minerals
The mineral quadratite occurs in cavities formedby hydrothermal activity in the white Lengenbach
dolomite. Different from the various new TI-sulfosalts discovered within the past few years (seeGRAESER and EDENHARTER, 1997, e.g.), which arestrictly associated with minerals of extremely highcontents in arsenic (minerals like realgar, orpiment, etc.), quadratite exclusively occurs togetherwith phases poorest in arsenic, mainly in association with galena, jordanite (Pb1iAs,Sb)6S23),
pyrite, and, occasionally, sphalerite, lengenbachiteand few hatchite. In most cases, quadratite isFig. 3 BSE-image of quadratite (dark grey),jordanite(light grey), and pyrite (black idiomorphic inclusion).
Diameter of sample about 0.3 mm.

closely intergrown with jordanite, and even contains inclusions of this mineral (see Fig. 3). Quadratite is undoubtedly the product of a late hydrothermal activity by which galena, sphalerite,
and jordanite were leached and partly dissolved.
Silver was leached from galena (Ag-contents between 1'000-5'000 ppm), cadmium from sphalerite (Cd-contents between 1'000-2'000 ppm), arsenic and thallium presumably from j ordanite andpyrite (thallium). The concentrations of minor elements in galena and sphalerite are taken fromGRAESER (1969,1971). The characteristically corroded crystals of galena and sphalerite proof theleaching process by hydrothermal fluids.
Within the first years after its discovery, quadratite remained an extremely rare species inLengenbach, presumably as a consequence of itsvery small dimensions. In 1997, however, morethan 30 quadratite samples existed in the official
exploitation list of the "ArbeitsgemeinschaftLengenbach". Obviously the eyes of the collectors
became used to the mineral and, in particular,from the knowledge of the mineral's special occurrence, potential quadratite-bearing samples
were easily discernible.

494

ST. GRAESER, W. LUSTENHOUVER AND P. BERLEPSCH
Acknowledgements

References

Monika and Walter Gabriel, the first discoverers of theunknown mineral kindly put all their material at our disposal for investigation. Several other collectors (MarkusEcker, Ralph Cannon, e.a.) provided us with larger samples and thus enabled the accomplishment of the study
that led to the proposal for the new mineral species.
Mikosch Siegel put an excellent crystal at our disposal
for photographic purpose, splendidly carried out by W.
Gabriel. We are greatly obliged to R. Guggenheim, M.
DUggelin, and D. Mathys (SEM laboratory, Universityof Basel), who contributed the first chemical data of themineral. Martin Kunz (ETH ZUrich) provided valuable
recommendations for the improvement of the text.

DOWTY, E. (1997): SHAPE, a computer program fordrawing crystals, version 5.5.
EVSTIGNEEVA, T.L., GENKIN, A.D., TRONEVA, N.V., FILIMONOVA,AA and TSEPIN,Al. (1973): Shadlunite,a new sulfide of copper, iron, lead, manganese, andcadmium from copper-nickel ores. Zapiski Vses.
Mineral. Obshch.l02, 63-74.
GRAESER, S. (1969): Minor elements in sphalerite andgalena from Binntal (relations to coloration problems, isotopic compositions, etc.). ContI'. Mineral.
Petrol. 24,156-163.
GRAESER, S. (1971): Mineralogisch-geochemische Untersuchungen an Bleiglanz und Zinkblende.
Schweiz. Mineral. Petrogr. Mitt. 51,415-442.
GRAESER, S. and EDENHARTER, A. (1997): Jentschite(TlPbAs 2SbS o) - a new sulphosalt mineral fromLengenbach, Binntal (Switzerland). Min. Mag., Vol.
61,131-137.
NICKEL, E. and NICHOLS, M. (1997): The mineral database. Aleph Enterprises and CSIRO.
Manuscript received May 28, 1998; revision accepted July 1, 1998.