American Mineralogist, Volume 86, pages 1087–1093, 2001

Sicherite, TlAg2(As,Sb)3S6, a new sulfosalt mineral from Lengenbach (Binntal, Switzerland): Description and structure determination
STEFAN GRAESER,1, * PETER BERLEPSCH,2 EMIL MAKOVICKY, 2 AND TONČI BALIĆ -Z ǓNIĆ 2
1Mineralogisch-Petrographisches Institut, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, and Natural History Museum, CH-4001
Basel, Switzerland
2
Department of Mineralogy, Institute of Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark

ABSTRACT
Sicherite, TlAg2(As,Sb)3S6, is a new mineral species from the famous sulfosalt locality at
Lengenbach, Binntal, Switzerland. It occurs in small cavities in a dolomitic rock of Triassic age,associated with abundant realgar, and various other sulfosalts, mainly Tl-bearing species such as
hutchinsonite, hatchite, and jentschite. Sicherite is orthorhombic, space group Pmnb, Z = 4, a =12.479(3), b = 15.522(4), c = 5.719(4) Å; V = 1107.8(6) Å3. The strongest powder diffraction lines[dobs (Å), (hkl), I/I0] are as follows: 2.822, (340, 331, 012), 100; 3.363, (301), 50; 3.118, (141), 27;
3.210 (041), 26; 3.29, (240, 311), 23; 2.540, (341), 17. Sicherite is dark metallic grey, with metallic
luster, and appears completely opaque. Individual crystals reach approximately 0.4 mm, but aggregates may exceed 1–2 mm. The streak is dark brown-red, no cleavage was observed, fracture is
uneven to conchoidal. Microhardness VHN10 = 57–59 kg/mm2, corresponding to a Mohs hardness ofabout 3. Dcalc = 5.26 g/cm3. In polished section the mineral appears pure white, with extremely weakanisotropy. Sicherite, as well as the other Tl-minerals in Lengenbach, presumably represent products of a late stage activity of Tl-As-bearing hydrothermal solutions during Alpine metamorphism.The
crystal structure of sicherite was solved and refined to R1 = 4.62%. It is composed of slices of an–
SnS-like structure cut out parallel to (101)SnS and (101)SnS which are related by unit-cell twinning.
This is similar to, but more complex than, the simpler case of emplectite. The slices are parallel to(010) of the unit cell of sicherite. They are limited by zigzag surfaces composed of short portions of–≈ (001)SnS, and (301)SnS and (301)SnS planes, respectively. Adjacent slices are related by n-glide planes
parallel to (010) of sicherite and interconnected via coordination polyhedra of Tl and Ag which forma very distorted PbS-like array.

INTRODUCTION
The Lengenbach quarry in Binntal (Valais Canton, Switzerland) is well known for its very uncommon sulfosalt minerals. They are mainly Pb-Cu-Ag-Tl-(As,Sb)-sulfides occurringin small cavities of white dolomitic rock of Triassic age. With
the exception of some periods during the two world wars, thesulfosalt minerals have been mined almost continously for morethan 150 years. It was mainly scientific interest in the sulfosalt
minerals that was responsible for their exploitation. Beginningin 1833, the extraction of the minerals was conducted in theold quarry which by 1987 covered an area of approximately 30
× 20 m and reached a depth of about 20–25 m. In 1987 the oldquarry was abandoned due to a lack of attractive material, anda new site was opened at a location about 20 m farther to theeast, and at a higher elevation (20–25 m above the old quarry).
Though the distance from the old quarry is very short, the mineralization at the new site is remarkably different. The new site
is marked by much higher concentrations of Tl and As and has
yielded a considerable number of rare and new mineral species. Sicherite is one of several new Tl-sulfosalts discovered* E-mail: stefan.graeser@unibas.ch0003-004X/01/0009–1087$05.00

within the last decade. All the material used for this study wasprovided by the “Arbeitsgemeinschaft Lengenbach, AGL,” asyndicate composed mainly of several Swiss museums anduniversities, and additionally some private individuals, whowere active in Lengenbach from 1958 to 1998. Subsequently a
new syndicate was founded (“Interessengemeinschaft
Lengenbach, IGL”) with the aim of continuing the mining ofminerals at Lengenbach for research and collectors.
The mineral was named in honor of Valentin Sicher (born1925), active member in both Lengenbach syndicates since 1963
who contributed greatly to the specimen recovery efforts.
The mineral and mineral name have been approved by theCommission on New Minerals and Mineral Names (IMA). Typematerial is preserved at the Department of Mineralogy, Natural History Museum, Basel, Switzerland; the samples used for
X-ray, microprobe and ore microscopic studies remain at theMineralogisch-Petrographisches Institute, University of Basel.

SAMPLE DESCRIPTION
The first sample containing the new mineral was collected
in 1992 by the AGL-syndicate. It was the black metallic character that drew our attention to the small xenomorphous ore

1087

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GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERAL

minerals associated with abundant realgar (most minerals fromthis area occurring together with realgar are Tl-sulfosalts whichin general display a dark red color). A powder diffraction studyof the material produced a diagram that did not match any other
Lengenbach mineral. Unfortunately, the material available atthat time was not suitable for a single-crystal X-ray study, andthe work had to be discontinued. A second find of the samemineral (AGL number L 27′ 345) in 1996 provided appropriate crystals for a fundamental study.
Both samples containing sicherite were discovered in zones
with high As contents (easily recognizable by the presence ofabundant realgar). Some of the sicherite aggregates occur as
inclusions in realgar. These local As enrichments also represent zones of high Tl contents. Sicherite is closely associatedwith Tl-minerals like hutchinsonite (TlPbAs5S 9), hatchite[TlPb(Ag,Cu)As2S5], and rarely jentschite (TlPbAs2SbS6). All
the Lengenbach Tl-sulfosalts are interpreted as late stage minerals formed through hydrothermal activity during Alpine metamorphism.
Sicherite samples found so far consist of aggregates up to 1
mm across. The aggregates are composed of numerous minutecrystals in subparallel orientation. One nearly euhedral 0.4 mm
single crystal was used for single-crystal X-ray diffraction andfor the optical measurements.

PHYSICAL AND OPTICAL PROPERTIESThe physical and optical properties of sicherite are summarized in Table 1. Sicherite is elongated parallel to a; the morphology of the crystal is dominated by the bipyramid {141},with the forms {001}, {010}, {031}, {301} of minor importance (Fig. 1). The axial ratio calculated from the goniometricmeasurement for {141} is: a : 1 : c = 0.798 : 1 : 0.370, in close
agreement with the ratio calculated from the lattice parameters:a : 1 : c = 0.804 : 1 : 0.368.
The black metallic color, lack of cleavage, and uneven fracture of sicherite are very similar to the common Lengenbachminerals tennantite and seligmannite. Presumably, sicherite
occurs much more frequently, but has been mistaken for otherminerals.
Reflectance measurements were made from 400 to 700 nm
using a Zeiss MPM800 spectrophotometer, and a SiC reflectance standard on a freshly polished grain at 23 °C (Table 2).
In polished section sicherite appears pure white, with very few
dark red internal reflections and extremely weak anisotropy
effects. Among the Lengenbach Tl-sulfosalts, sicherite has thehighest reflectivity values. Microhardness was measured on arandomly oriented surface using a Leitz Durimet microscope.
TABLE 1. Physical and optical properties of sicherite
Morphology
Forms
Color
LusterStreakCleavage
Fracture
Twinning
Hardness
Densitybipyramidal, elongated along a-axis{141}; {031}, {001}, {010}, {301}dark grey to black, opaque
metallic
dark brown red (darker than most of the other Tlsulfosalts)
not observed
uneven to conchoidal
not observed= 58.3 kg/mm2 (range: 57.2–59.2)
VHN10
Mohs
≤ 3 (according to VHN)
5.26 g/cm3 (calculated)

FIGURE 1. Idealized drawing of a sicherite crystal based on opticalgoniometry.

The VHN values obtained correspond to a Mohs hardness ofapproximately 3 (Table 1).

CHEMICAL COMPOSITION
Preliminary semi-quantitative analyses obtained using a
HITACHI SEM S-2500 scanning electron microscope showed
Tl, Ag, As, Sb, S, with no additional element with atomic number ≥ 11. Quantitative analyses of the mineral were carried out
using an electron microprobe (EMP). Tests for interferences
yielded 2.94 wt% As on Sb metal, thus all results for As werecorrected according to the equation As' [wt%] = As [wt%] –0.0294⋅Sb [wt%]. Chemical data are reported in Table 3.
The analyzed sample consisted of an aggregate approximately 0.5 mm in diameter. A total of 103 analyses were obtained with a CAMECA SX-50 electron microprobe, operated
at 15kV and 10 nA, with a point-focused beam. The following
standards, X-ray lines, and detector crystals were used: TlSe
(TlMα, PET), Ag metal (AgLα, PET), Cu metal (CuKα, LiF),
GaAs (AsLα, TAP), Sb metal (SbLα, PET). Pb was sought,
but was below the detection limit. The point analyses show
considerable substitution of As by Sb (with negative correlation coefficient R = –0.995, see Fig. 2). Element distribution
maps for As and Sb clearly indicate an enrichment of Sb in theapical parts of the aggregate, presumably representing a laterstage of growth (Fig. 3). The As-Sb substitution extends over awide range (from Sb/(As + Sb) = 0.14 to 0.58, with rare Sb
dominant parts) suggesting the existence of an isomorphic solid

GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERAL

1089

TABLE 2. Observations in reflected light
Color
Internal reflections

Birefringence
Pleochroism
Anisotropy

in airin oilpure whitepure whitedark red (visible only dark red (distinct)
along cracks fromhardness indentations)
not observed
not observed
not observed
not observed
extremely weak
distinct

Reflectance values
Wavelengths
R1
R2
400 nm
33.76
36.15
410
33.53
35.82
420
33.33
35.54
430
33.00
35.04
440
32.70
34.63
450
32.31
34.19
460
31.92
33.85
*470
31.43
33.43
480
30.99
33.00
490
30.54
32.62
500
30.07
32.25
510
29.59
31.80
520
29.23
31.44
530
28.88
31.08
540
28.52
30.74
*546
28.31
30.52
550
28.17
30.37
560
27.88
30.05
570
27.61
29.71
580
27.34
29.38
*589
27.10
29.11
590
27.07
29.08
600
26.79
28.75
610
26.53
28.46
620
26.24
28.15
630
26.02
27.91
640
25.76
27.68
*650
25.57
27.44
660
25.33
27.25
670
25.08
26.97
680
24.81
26.70
690
24.90
26.63
700
24.63
26.41
* = COM wavelenghts, WTiC standard.

imR1
17.95
17.73
17.67
17.45
17.09
16.77
16.44
15.98
15.65
15.22
14.81
14.49
14.18
13.89
13.62
13.48
13.39
13.13
12.94
12.76
12.54
12.52
12.30
12.14
11.90
11.73
11.54
11.36
11.16
11.09
11.08
10.96
10.89

imR2
21.27
20.93
20.56
20.14
19.63
19.22
18.89
18.41
18.07
17.64
17.26
16.95
16.59
16.29
15.96
15.80
15.69
15.37
15.07
14.87
14.56
14.53
14.34
14.07
13.84
13.60
13.43
13.17
12.93
12.88
12.73
12.64
12.53

FIGURE 2. Plot of the number of As atoms per formula unit (pfu)vs. Sb atoms pfu from EMP analyses, normalized on the basis of 12
atoms.

TABLE 3. Chemical composition of sicherite
Tl
Ag
Cu
As
Sb
S
Total

Analyzed*
23.63
23.98
0.22
19.08
10.96
21.65
99.53

Range
21.63–24.77
22.60–24.76
0.08–0.43
10.46–22.98
5.90–23.18
20.60–22.30
99.00–100.65

Structural formula (normalized on 12 atoms pro formula)
Tl
1.02
Ag
Cu

1.96
0.03

Σ = 1.99

As
Sb

2.24
0.80

Σ = 3.04

Ideal†
23.36
24.66
18.85
11.14
21.99
100.00

1.0

FIGURE 3. Element distribution maps for sicherite intergrown with
realgar (r). The Sb-rich apical portions of the grain might represent
later stages of growth. Presumably, metasomatism of the original Asrich crystal by Sb led to the observed pattern. Each map has anindividual intensity scale which cannot be compared directly to theothers. Analytical conditions: AsLα, SbLα, AgLα, CuKα, 15kV, 70nA.

2.0

2.2
0.8

S
5.95
6.0
* Mean of 103 individual analyses (wt%).
† Theoretical composition for simplified formula (for As:Sb = 2.2/0.8).

solution series between sicherite and a hypothetical Sb endmember. However, almost all of the analyses (101 of 103) show
As > Sb which justifies the attribution of sicherite to the Asrich end-member. A similar substitution, though to a much lesser
degree, exists between the elements Ag and Cu. In contrast to

the related orthorhombic Tl-mineral edenharterite, TlPbAs3S6,(Graeser and Schwander 1992), where the substitution of As
by Sb leads to jentschite, TlPbAs2SbS6, (Graeser and Edenharter
1997; Berlepsch 1996), an ordered (As,Sb)-phase with a As:Sb
ratio of 2:1 with lower, monoclinic symmetry, the As-Sb substitution in sicherite seems to represent a more or less complete isomorphic solid solution series. However, the observed
substitutional range found in sicherite does not rule out thepossibility that somewhere above Sb/(As + Sb) = 0.58 ordering may occur. The imhofite-jentschite group of Tl-minerals
from Lengenbach and sicherite are closely related; this is clearlyshown by mineral formulae and selected unit-cell dimensions

1090

GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERAL(Table 4), although the relations between their crystal structures are far from simple.

X-RAY ANALYSIS
Preliminary Debye-Scherrer photographs were obtained in
1993. As this material turned out to be unsuitable for singlecrystal X-ray study, we decided to wait for better material. Suchmaterial was not available until 1996, when identical powder
diagrams were obtained from an unknown sulfosalt phase from
Lengenbach. The complete set of powder data and lattice parameters refined from d-values are reported in Table 5. Because the mineral cannot be distinguished visually from otherphases, each sample used for microprobe work, structure determination, etc. was first identified by the Gandolfi method.
TABLE 4. Relationships between several Tl-sulfosalts from
Lengenbach
Name
Formulaa (Å)
b (Å)
Imhofite*(Tl,)3As8S13
8.775 24.425
15.476 47.602
Edenharterite†
TlPbAs3S6
8.121 23.969
Jentschite§
TlPbAs2SbS6
Sicherite‡
TlAg2As3S6
15.522 12.479* Burri et al. (1965).
† Graeser and Schwander (1992).
§ Graeser and Edenharter (1997).
‡ this paper (a and b parameters exchanged).

c (Å)
11.478
5.849
5.847
5.719

β (°)
108.28°90
107.68°90

TABLE 5. Powder-diffraction data of sicherite
hkl
020
011
040
131
301
240

dobs
7.770
5.346
3.881
3.655
3.363
3.290

311
041
141
340
331
012
341
260

3.210
3.118
2.822
2.540
2.388

520
332
600

2.141
2.070

071
342
631
181
213
641

2.011
1.82791.7376471
272
1.6836
602
381
612
651
472

{
{
{
{
{
{

{

1.6536{dcalc*
7.760
5.359
3.882
3.666
3.363
3.296

Iobs†
9
11
11
16
50

Icalc§
6
9
17
75
10

Given the chemical data, which show considerable substitution of As by Sb, the fact that the powder data show a very
narrow variance was unexpected. This fact, too, proves thatthe description of the new mineral sicherite comprises the Asrich end-member exclusively.
The single-crystal study was carried out using both
Weissenberg and precession techniques which indicated spacegroups Pmnb or P21nb, of which the former was confirmed by
the full structure determination. The Pmnb orientation was chosen in accordance with the principle b > a > c for an orthorhombic unit cell (Donnay and Ondik 1973).

X-RAY DATA COLLECTION AND STRUCTURE
REFINEMENT

A small crystal fragment of sicherite was measured using a
Bruker AXS four-circle diffractometer equipped with a 1000
K CCD detector and a flat graphite monochromator using
MoKα-radiation from a fine focus sealed tube (Table 6). The
SMART system of programs was used for crystal lattice determination and X-ray data collection, SAINT+ for data reduction including intensity integration, background and Lorentz
polarization corrections, and SHELXTL for the structure solution and refinement (all Bruker AXS products). The program
XPREP from the SHELX package was used to make an empirical absorption correction based on reflection measurements
at different azimuthal angles. From the proposed space groups
Pmnb and P21nb the centrosymmetric space group Pmnb waschosen consistent with the orthorhombic symmetry of the lattice and intensity statistics (|E2–1| = 0.912).

23
3.287
3.210
3.110
2.837
2.820
2.812
2.542
2.390

26
27
100
17

27
41
22
100
24
63
29
8

8
2.376
2.143
2.080

5

2
20
24

15
2.067
2.014
1.82851.81781.81071.7457

6
<5

4
14
3
3
2
20

8
1.7223
1.68701015<51.68201.68071.67221.65401.5278

9

4
6
16
3
7

1.5239<5163
1.52322
Note: 90 mm Debye-Scherrer camera, FeK α radiation.
* d-values calculated for: a = 12.479 (3), b = 15.522 (4), c = 5.719 (4) Å(refined from powder data).
† Intensities measured by densitometer.
§ Intensities calculated from structural data (Lazy Pulverix; Yvon et al.
1977).

TABLE 6. Summarized data collection and refinement parameters
for sicherite
X-ray power
Temperature
Detector to sample distance
Active detection area
Resolution
Sample size
Exposures
Rotation width per frame
Measuring time
Coverage of possible reflections
Average redundancy
Maximum covered 2Θ
Miller index limits
Measured reflections
Observed reflections
Unique reflections
Reflections > 2 σ(I)Rint before absorption correction
Rint after absorption correction
Min., max. transmission factors
Rσ
Number of l.s. parameters
Goof
R1 (Fo > 4σ[Fo])R1 (all data)wR2 (on Fo2)
Rint = Σ|Fo2 – Fo2(mean)| / ΣFo2
Rσ = Σσ(Fo2) / ΣFo2
R1 = Σ ||Fo| – |Fc|| / Σ|Fo|
wR2 = √(Σ w[Fo2 – Fc2]2 / Σw[Fo2]2)
Goof = √(Σ w[Fo2 – Fc2] / [n–p])
w = 1 / (σ2[Fo2] + [0.0305 * P]2)
P = ( F o2 + 2 F c 2 ) / 3

40 kV, 35 mA
293 K
6.0 cm
6.25 x 6.25 cm2
512 x 512 pixels
0.14 x 0.05 x 0.04 mm3
2880
0.3°
60 s
90.1%
5.4
72.84° (d = 0.60 Å)
h: ± 25; k: –20, +16; l: –3, +8
14133
13203
2617
1599
17.64%
12.59%
3.5%, 9.1%
9.58%
79
1.000
4.62%
9.16%
9.65%

GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERAL

The structure was solved by direct methods which revealed
the position of the Tl atom. In subsequent refinements the positions of the Ag, As, and S atoms were deduced from difference Fourier syntheses by selecting from among the strongest
maxima at appropriate distances from Tl. To account for Sb
the populations on the As sites were refined allowing for As
and Sb summing to full occupancy. In a later stage of the refinement the obtained site occupancies of As and Sb were fixed
and their positions allowed to refine individually. The final refinement was performed with anisotropic displacement parameters for all positions and an empirical extinction coefficient.
The refinement was stopped when the mean shift/esd for varied parameters dropped below 1. The highest residual peak was
3.2 e/Å3, 1.22 Å from Tl and the deepest valley was – 5.1 e/Å3,0.16 Å from Tl. Fractional atomic coordinates are listed in Table
7, anisotropic displacement parameters are given in Table 8,
and selected interatomic distances and angles are shown in Table
9 1.

1091

polyhedra. Layers hosting As alternate in the b direction with layers that contain Tl and Ag. The interconnection between adjacentlayers with Tl and Ag is made by strong As1-S3 bonds (Fig. 5).

COORDINATION POLYHEDRA
Tl is coordinated by six S atoms in the range 3.174(2) to
3.235(2) Å that form a distorted octahedron. Considering two
additional S atoms at 3.652(2) and 3.897(2) Å, the coordination polyhedron can alternatively be described as a distorted
cube. The ratio Vs/Vp for coordination number (CN) 8 (where
Vp is the volume of the polyhedron and Vs the volume of a sphere
fit to the ligands defining the coordination polyhedron) yields
a value of 2.545 which is lower than the ideal ratio of 2.7206
for a cube (Makovicky and Balić -Zˇunić 1998) which showsthat breaking tetragonal faces into triangular significantly distorts cubic coordination.

STRUCTURE DESCRIPTION
The crystal structure of sicherite can be described as being
composed of slices of SnS-like structure cut out parallel to–(101)SnS and (101)SnS which are related by unit-cell twinning.
This is similar to, but more complex than, the structure ofemplectite (Makovicky 1985).The slices are parallel to (010)of the unit cell of sicherite. They are limited by zigzag surfaces–
composed of short portions of ≈ (001)SnS, (301)SnS, and (301)SnS
planes. Adjacent slices are related by n-glide planes parallel to(010) of sicherite, and interconnected via coordination polyhedra of Tl and Ag which form a very distorted PbS-like array(Figs. 4 and 5). The slices are made up of AsS7 coordination

1
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Karlsruhe, Gesellschaft für wissenschaftlich-technische Information mbH, D-76344 Eggenstein-Leopoldshafen, Germany.

FIGURE 4. Crystal structure of sicherite projected along a. Circles
in order of decreasing size represent S, Tl, Ag, and As. Shading shows–
two (101)SnS slabs related by n-glide planes parallel to (010) and linked
through coordination polyhedra of Tl and Ag that form a very distorted
PbS-like array.

TABLE 7. Occupancy factors, fractional atomic coordinates, and equivalent isotropic displacement parameters for sicherite
Atom
Tl1
Ag1
As1
Sb1
As2
Sb2
S1
S2
S3
S4

Occ.
1
1
0.57
0.43
0.73
0.27
1
1
1
1

x/a
0.25000.07814(4)0.25000.25000.0950(2)0.0955(6)0.25000.25000.10710(10)0.97803(12)y/b0.23322(2)0.80769(4)0.9641(9)0.9589(8)0.0551(4)0.0524(7)0.14175(14)0.89253(13)0.06197(10)0.16637(10)z/c0.87013(8)0.15465(17)0.806(3)0.796(3)0.2949(10)0.3069(19)0.3705(4)0.1615(4)0.8866(3)0.3323(3)Ueq0.03965(14)0.0586(2)0.0257(18)0.031(2)0.0243(10)0.0292(19)0.0272(4)0.0273(5)0.0273(3)0.0347(4)TABLE 8. Anisotropic displacement parameters for sicherite
Atom
Tl1
Ag1
As1
Sb1
As2
Sb2
S1
S2
S3
S4

U11
0.03737(19)0.0418(3)0.019(2)0.038(3)0.0157(11)0.040(3)0.0218(7)0.0239(8)0.0214(5)0.0265(6)U220.0354(2)0.0397(3)0.033(4)0.0211(17)0.0248(15)0.031(3)0.0299(10)0.0258(10)0.0313(7)0.0294(8)U33
0.0462(3)0.0944(7)0.026(3)0.036(4)0.032(2)0.017(3)0.0299(12)0.0322(13)0.0290(8)0.0484(11)U230.00179(17)0.0171(3)–0.002(2)0.003(2)0.0023(12)–0.0035(19)–0.0050(9)0.0023(8)0.0022(6)–0.0006(7)

U13
0.000
–0.0176(3)
0.000
0.000
0.0000(9)–0.0008(18)
0.000
0.000
–0.0013(5)0.0017(6)

U12
0.000
–0.0122(2)
0.000
0.000
0.0042(9)0.002(2)
0.000
0.000
–0.0031(5)0.0025(5)1092

GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERALTABLE 9. Selected interatomic distances (Å) and angles (°). In each table the distances from the central atom to the ligands are given on
the diagonal, the angles between two ligands and the central atom are listed in the upper right corner, and the distances
between ligands can be found in the lower left corner.
Tl1 (CN6 and CN8) bonded to:
S1
S1
S1
3.174(2)127.2(1)S1
5.690(3)3.177(2)S33.645(3)3.499(3)S33.645(3)3.499(3)S44.867(2)5.114(3)S44.867(2)5.114(3)S2
6.730(3)3.873(3)S2
3.873(3)7.032(3)Ag1 (CN6) bonded to:
S2
S4
S2
2.504(1)151.1(1)S44.856(2)2.511(2)S4
4.091(2)3.879(2)S4
4.140(2)3.879(2)S3
4.498(2)4.333(2)S1
3.873(3)3.406(2)As1 (CN7) bonded to:
S2
S3
S2
2.304(17)97.8(5)S3
3.525(2)2.375(10)S33.525(2)3.549(2)S4
4.091(2)3.883(2)S4
4.091(2)5.931(2)S1
5.919(3)3.645(3)S2
5.690(3)5.196(3)Sb1 (CN7) bonded to:
S2
S3
S2
2.318(16)95.6(4)S3
3.525(2)2.438(9)S33.525(2)3.549(2)S4
4.091(2)3.883(2)S4
4.091(2)5.931(2)S1
5.919(3)3.645(3)S2
5.690(3)5.196(3)As2 (CN7) bonded to:
S4
S3
S4
2.259(5)95.8(2)
S3
3.405(2)2.330(6)S1
3.406(2)3.499(3)S2
5.495(2)3.525(2)S3
3.883(2)3.521(2)S3
3.887(2)5.690(3)S4
5.504(2)5.769(2)Sb2 (CN7) bonded to:
S4
S1
S4
2.289(10)93.4(4)S1
3.406(2)2.390(9)S33.405(2)3.499(3)S2
5.495(2)4.024(3)S3
3.883(2)5.629(2)S3
3.887(2)3.645(3)S4
5.504(2)5.786(3)S2
163.7(0)69.1(0)122.7(0)122.7(0)74.0(0)74.0(0)3.625(2)5.690(3)S370.0(0)66.7(0)3.184(2)3.549(2)4.333(2)6.235(2)5.980(3)6.235(3)S370.0(0)66.7(0)67.7(1)
3.184(2)6.235(2)4.333(2)5.980(3)6.235(3)S4
98.8(0)105.8(0)84.9(0)152.5(0)3.235(2)5.663(3)4.140(2)4.091(2)S4
98.8(0)105.8(0)152.5(0)84.9(0)122.2(1)3.235(2)4.140(2)4.091(2)S4
98.5(1)91.7(1)2.885(2)5.690(3)3.405(2)4.867(2)S4
96.4(1)88.4(1)148.4(1)3.028(2)3.887(2)5.114(3)S3
107.3(1)101.6(1)69.8(0)79.3(0)3.064(2)6.369(2)S1
81.8(1)
69.6(1)102.7(1)106.9(0)168.7(1)3.336(2)S3
97.8(5)96.7(5)2.375(10)5.931(2)3.883(2)3.645(3)5.196(3)S4
85.6(4)78.9(3)174.9(5)3.562(9)5.663(3)5.786(3)4.140(2)S4
85.6(4)174.9(5)78.9(3)105.3(3)3.562(9)5.786(3)4.140(2)S1
160.7(5)70.0(3)70.0(3)105.7(3)105.7(3)3.695(16)4.024(3)S2
134.6(5)111.6(4)111.6(4)68.0(2)68.0(2)64.6(2)3.830(17)S3
95.6(4)93.4(4)2.438(9)5.931(2)3.883(2)3.645(3)5.196(3)S4
86.8(4)79.4(3)172.6(5)3.505(8)5.663(3)5.786(3)4.140(2)S4
86.8(4)172.6(5)79.4(3)107.8(3)3.505(8)5.786(3)4.140(2)S1
156.9(5)69.1(3)69.1(3)106.4(3)106.4(3)3.717(15)4.024(3)S2
138.0(5)112.4(4)112.4(4)69.5(2)69.5(2)65.2(2)3.752(17)S1
94.4(2)95.9(2)2.383(4)4.024(3)5.629(2)3.645(3)5.786(3)S2
171.5(2)76.4(1)89.8(2)3.251(5)4.498(2)5.196(3)4.140(2)S3
87.5(2)76.1(1)
171.9(2)87.4(1)3.260(5)5.485(2)3.887(2)S3
84.9(2)172.3(2)76.5(1)103.4(1)111.6(2)3.372(6)3.883(2)S4
116.1(2)124.4(2)123.4(2)67.1(1)62.3(1)61.5(1)
4.122(6)S3
93.1(4)93.9(3)2.400(11)3.525(2)3.521(2)5.690(3)5.769(2)S2
168.7(4)90.1(3)75.9(3)3.232(10)4.498(2)5.196(3)4.140(2)S3
86.9(3)169.0(4)75.1(3)87.6(2)3.264(9)5.485(2)3.887(2)S386.0(3)77.8(3)171.5(4)105.3(3)113.2(3)3.305(11)3.883(2)S4
117.6(3)125.8(3)124.7(3)68.2(2)63.1(2)62.7(2)4.054(11)

Ag is linearly coordinated by two S atoms at 2.504(1) and2.511(2) Å. Four additional S atoms in the range 2.885(2) to
3.336(2) Å complete a distorted octahedral coordination. Two
AgS6 polyhedra share a common S1-S2 edge and form mirrorrelated pairs extending along [100]. Adjacent pairs share common S4-S4 edges, are symmetry related by a screw-axis parallel
to a, and form an openwork layer (010) which accommodates
the TlS8 polyhedra, which are isolated from one another.
Both As1 and As2 have three close S atoms (Table 9) thatform a trigonal pyramid with As at the apex. Four distant S
atoms complete the AsS7 coordination polyhedra which in bothcases can be described as a monocapped trigonal prism.
For Sb the same situation exists as for As. The refined Sb1
site is 0.098 Å away from As1, and the Sb2 site is 0.080 Åfrom As2. The short Sb-S distances are typically larger thanS2
65.5(0)167.3(0)123.1(0)123.1(0)69.3(0)69.3(0)98.2(1)
3.897(2)the corresponding As-S distances but still smaller than anticipated for typical Sb-S short bonds. This is due to the strong
overlap of the As and Sb positions and to the unresolved S
positions belonging to the individual As and Sb polyhedra; these
represent averaged S positions, common for As and Sb. All As
positions contain significant amounts of Sb (Table 7). Considering the variable amount of As and Sb in sicherite the formula
TlAg2(As1.87Sb1.13)Σ=3S6, obtained from the structure refinement,
agrees well with the previously obtained simplified formula
TlAg2(As2.20Sb0.84)Σ=3.04S6 based on EMP data.

RELATIONSHIPS TO OTHER MINERALS
Although the chemical composition and the unit-cell parameters suggest a possible relationship between sicherite andthe hutchinsonite group of merotypes, no simple structural re-

GRAESER ET AL.: SICHERITE, A NEW SULFOSALT MINERAL

1093

lationship exists. In the unit-cell twinning principle, the crystal
structure of sicherite shows similarities to emplectite (Portheine
and Nowacki 1975). With respect to the disposition of coordination polyhedra, close relationships exist to marrite, PbAgAsS3(Wuensch and Nowacki 1967) and freieslebenite, PbAgSbS3(Ito and Nowacki 1974). These can be seen from the respective
unit-cell parameters (Tab. 10), and from the projection on(001)sicherite (Fig. 5).

ACKNOWLEDGMENTS
We are grateful to R. Guggenheim (University of Basel) for preliminary
SEM studies, to A. Criddle (The Natural History Museum, London) for performing the reflectivity measurements and to J. Spratt (The Natural History
Museum, London) for the assistance with the EMP work. This work was financially supported by the Freiwillige Akademische Gesellschaft (FAG) Basel andthe Swiss National Science Foundation (SNSF), grant no. 8220-053386 (bothgrants for P.B.). Additional financial support was provided by the Danish State
Research Council (Natural Sciences), grants 9701009 and 9901772.

REFERENCES CITEDFIGURE 5. Crystal structure of sicherite projected along c. Circles
in order of decreasing size represent S, Tl, Ag, and As. Coordination
polyhedra for individual cations are shaded.

TABLE 10. Unit-cell data for sicherite, marrite, and freieslebenite
Sicherite
Marrite Freieslebenite
Powder data
CCD data
Composition TlAg2(As,Sb)3S6 TlAg2(As,Sb)3S6 PbAgAsS3 PbAgSbS3
Crystalorthorhombic
orthorhombic monoclinic monoclinic
systemPmnb
P 2 1/ a
P 2 1/ aSpace group Pmnb
12.479(3)12.418(3)7.2705(6)7.518(1)
a (Å)
b (Å)
15.522(4)15.427(3)12.6319(4)12.809(4)c (Å)
5.719(4)5.6895(13)5.9853(3)5.940(1)β (°)
91.228(3)92.25(1)3
V (Å )
1107.81089.9
549.6
571.6
Z
4
4
4
4
Reference**†‡* This study.
† Wuensch and Nowacki (1967).
‡ Ito and Nowacki (1974).

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edenharterite (TlPbAs3S6) and jentschite (TlPbAs2SbS6) from Lengenbach,
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