American Mineralogist, Volume 87, pages 753–764, 2002

The new mineral baumstarkite and a structural reinvestigation of aramayoite
and miargyrite
HERTA EFFENBERGER,1,* WERNER HERMANN PAAR,2 DAN TOPA,2 ALAN J. CRIDDLE,3 AND
MICHEL FLECK1
1

Institut für Mineralogie und Kristallographie, Universität Wien, Althanstrasse 14, A-1090 Vienna, Austria
2Institut für Mineralogie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
3
Department of Mineralogy, The Natural History Museum, London SW7 5BD, U.K.

ABSTRACT
Baumstarkite is a new mineral found coating miargyrite from the San Genaro mine, Huancavelica
Department, Peru. It is triclinic and the third naturally occurring modification of AgSbS2 besides
monoclinic miargyrite and cubic cuboargyrite. The composition is usually close to the ideal formula. However, some grains of baumstarkite show zoned lamellae with As contents up to 11.5 wt%
and accords to Ag3(Sb,As)2SbS6. Baumstarkite is isotypic with aramayoite [end-member composition Ag3Sb2BiS6; solid solutions require the extended formula Ag3Sb2(Bi,Sb)S6]. Single-crystal Xray structure investigations were performed for baumstarkite [type locality, a = 7.766(2), b = 8.322(2),–c = 8.814(2) Å, a = 100.62(2), b = 104.03(2), g = 90.22(2)∞, Z = 2{Ag3Sb3S6}, space group P1,
2
R1(F) = 0.057, wR2(F ) = 0.128], aramayoite [Armonia mine, El Quevar, Argentinia: a = 7.813(2), b= 8.268(2), c = 8.880(2) Å, a = 100.32(2), b = 104.07(2), g = 90.18(2)∞, Z = 2{Ag3Sb2S6}, space–group P1, R1(F) = 0.034, wR2(F2) = 0.084], and miargyrite associated with baumstarkite type material [a = 12.862(3), b = 4.409(1), c = 13.218(3) Å, b = 98.48(2)∞, Z = 8{AgSbS2}, space group C2/c,
R1(F) = 0.031, wR2(F2) = 0.082]. The space-group symmetries of aramayoite and miargyrite wererevised, and the refinements unambiguously showed that the three investigated minerals are centrosymmetric.
In baumstarkite and aramayoite each three atomic sites are occupied by Ag and M = As, Sb, Bi,
respectively. The Ag atoms have two short bonded ligands (Ag-S is 2.51 to 2.58 Å). The M1 andM2 sites are [3 + 3] coordinated and are predominantly occupied by (Sb, As) atoms (M-S = 2.44
to 2.54 Å and > 3.09 Å). The [2 + 2 + 2] coordination of the M3 atom differs in the two mineral
species: the two shortest bond lengths in baumstarkite are smaller (2.51 Å) than in aramayoite(2.64 Å) to allow for the different sizes of the Sb and Bi atoms, respectively; the medium bondlengths are similar (2.75 to 2.82 Å) and the longest bond lengths are >3.02 Å. Considering only
the nearest-neighbor environments, baumstarkite and aramayoite feature zigzag chains parallel to[010], which are linked together to form layers parallel to (001). In miargyrite [2 + 2] and [2] coordinated Ag atoms are linked by SbS3 pyramids to form a three-dimensional network.

INTRODUCTION
Several minerals with the chemical formula AgMS2 (M =As, Sb, Bi) have previously been described: (1) M = As: monoclinic smithite (Hellner and Burzlaff 1964), trigonal
trechmannite (Matsumoto and Nowacki 1969); (2) M = Sb:
monoclinic miargyrite (Hofmann 1938; Knowles 1964; Smithet al. 1997), cubic cubargyrite (Walenta 1998); and (3) M =Bi: (probably) trigonal matildite (Geller and Wernick 1959;
Harris and Thorpe 1969). Schapbachite is identical withmatildite and was discredited by Ramdohr (1938). In generalthese minerals have end-member compositions or show only
minor substitutions at the M sites. The triclinic mineralaramayoite exhibits a partial solid solution M = (Sb, Bi) (Spencer 1926; Yardley 1926; Berman and Wolfe 1940; Mullen andNowacki 1974). Although different type structures have beenfound for these minerals, many of the above cited authors* E-mail: herta.silvia.effenberger@univie.ac.at0003-004X/02/0506–753$05.00

753

mentioned the PbS (or rock salt) type as the parental structure (see also Graham 1951 and Wernick 1960). However,according to the classification of sulfosalts by Makovicky(1993) the type structures of the AgMS2 minerals smithite,
trechmannite, miargyrite, and aramayoite have to be considered as derivatives of the SnS archetype. High-temperature
modifications are pseudocubic or cubic and tend to form solid
solutions with galena (Wernick 1960); the low-temperature
modifications feature distinct distortions caused by the crystal-chemical behavior of the cations. Except for cubargyrite,
all these minerals have been known for many decades from alarge number of localities and ore deposits. Partial knowledgeof these minerals dates back to the 19th century. Miargyrite isan economically important Ag ore of many silver deposits,
whereas aramayoite is rare and cubargyrite has been only documented in trace amounts. However, the available structural data
for AgMS2 compounds are limited. The semiconducting properties of the selenide and bismuth analogues are mentioned byGeller and Wernick (1959); the corresponding minerals are

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EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

bohdanowiczite, AgBiSe2 (Banas et al. 1979), and volynskite,
AgBiTe2 (Bezsmertnaya and Soboleva 1965).
As part of an ongoing project on complex Ag-Sn mineralization in northwest-Argentinia (Paar et al. 1996, 2000) a suiteof high-grade silver ore specimens from the San Genaro mine,
Huancavelica Department, Castrovirreyna District, Peru, wasinvestigated. One specimen composed of miargyrite was partially overgrown by small crystals and crystal aggregates of amineral labeled “aramayoite.” The crystals are triclinic, andmost of them are twinned. Electron-microprobe analyses revealed a Bi-free AgSbS 2 composition, isochemical withmiargyrite and cuboargyrite. Subsequent research proved
isotypy with aramayoite. A limited solid solution is observedfor M = (Sb, As) whereas aramayoite shows a partial solid
solution for M = (Sb, Bi).
Possible further occurrences of baumstarkite are worth
mentioning. (1) Kittel (1927) refers to two wet-chemical
analyses of aramayoite from Chocaya, Bolivia, one of which
exhibits As and Bi contents of 0.9 and 2.4 wt%, respectively.
Recalculation of his analyses assuming admixed chalcopyrite and pyrite leads to a composition close to that ofbaumstarkite. (2) A triclinic phase with composition AgSbS2
was reported as a rare associate of miargyrite from the abandoned Ag mine of Saint-Marie-aux-Mines, France (Saint
Jaques vein, level –143 m, Giftgrube) by Bari (1982). (3)Sugaki and Kitakaze (1992) described a Bi-free “aramayoite”
with As/(As + Sb) ratios between 0.08 and 0.28 from theepithermal quartz vein of the Koryu Au-Ag mine, Hokkaido,
Japan.
The new mineral baumstarkite is named for the German
mineralogist Manfred Baumstark (born 1954) who first provided crystals of “aramayoite” from San Genaro (MBB
98.36.01) for study purposes and drew attention to the occurrence of triclinic AgSbS2 at Saint-Marie-aux-Mines. Both themineral and the mineral name have been approved by the
I.M.A. Commission of New Minerals and Mineral Names (no.
99-049). The holotype material is deposited at the Mineralogical Institute, University of Salzburg, Austria (catalogue
nos. 14524, 14525) and at the Natural History Museum, London, U.K. (BM 2000,32 and 33). The present paper deals withthe mineralogical description of baumstarkite. To facilitate
the definition of baumstarkite and its chemical separation withrespect to aramayoite, and to enable a structural comparison
with miargyrite, the crystal structures of these two minerals
were refined and their space-group symmetries were revised.

OCCURRENCE AND PHYSICAL PROPERTIES
Baumstarkite occurs as a rare constituent in high-grade
Ag ores at the San Genaro mine, Huancavelica, Peru (Crowley
et al. 1997). Among the associated minerals miargyrite isdominant; pyrargyrite, stannite, and kesterite are less frequent.
Andorite, diaphorite, robinsonite, galena, chalcopyrite,
sphalerite, and pyrite were found as trace components (identified by electron-microprobe analyses; see Fig. 1).
Baumstarkite is of hypogene origin and was obviously precipitated from hydrothermal fluids with high activities of Ag,
Sb, and S. Individual crystals do not exceed 3 mm in size.
Crystal aggregates up to 40 ¥ 10 mm coat miargyrite. Anhedral

inclusions of baumstarkite in miargyrite up to several mm indiameter are common.
The megascopic color is iron-black but grayish-black on
fresh surfaces. The streak is grayish-black, and the luster ismetallic. Crystals of baumstarkite are opaque. In thin sections
and at crystal edges baumstarkite is transparent and deep bloodred. The VHN25 ranges from 71.3–98.5 (mean 89.3) which corresponds roughly to a Mohs’ hardness of 2.5. The cleavage isperfect parallel to {001} and less perfect parallel to {100}.
Parting was not observed; the material is sectile, somewhat
pliable, with an even fracture. The measured density is 5.33 g/
cm3 (as determined from a fragment weighing 20.43 mg by theBerman microbalance method) and corresponds with the calculated density of 5.39 g/cm3. The habit is equant and the only
crystallographic forms are pinacoids due to the crystal sym–––
metry 1: {001}, {101}, {201}, {010}, and {011} are most frequent, and {100} is subordinate. The {001} form often
represents cleavage faces. Twinning occurs occasionally, andthe twin plane is (001) (Fig. 2); indexing is based on the orientation determined by X-ray investigations.

OPTICAL PROPERTIESIn plane-polarized reflected light individual grains ofbaumstarkite are weakly to moderately bireflectant from grayto white. The mineral is not pleochroic. Red internal reflections are uncommon in plane polarized light and occur only
along fractures within the mineral where grains are very thin.
However, strong fiery red internal reflections are abundant
between crossed polars. The rotation tints of the most anisotropic grains are almost monochrome from bright white togray (with a brownish tint) to a very dark blue. Other, lessanisotropic grains, most of which are twinned, have vari-colored tints ranging from a dull greenish yellow through brownto mauve to dark blue. All of these properties are enhanced
upon immersion in oil.
The visible spectrum reflectances of the mineral were
measured at intervals of 10 nm (from 400 to 700 nm) with aZeiss MPM 800 microscope spectrophotometer. The bandwidth of the grating monochromator was set at 5 nm, the effective numerical apertures of the 50¥ objectives used werelimited to 0.28, the reflectance standard was WTiC (Zeiss314), and the oil used for immersion measurements was Zeiss
ND = 1.515.
Test measurements were made on some weakly anisotropic grains. Their undispersed bireflectance values rarely exceeded 2% and corresponded closely to the higher reflectancevalues of the most anisotropic grain measured. The data collected from the latter (compared with data for aramayoite
and miargyrite) are given in Table 1 and are shown graphically in Figure 3. Baumstarkite reveals monotone spectra for
R1 and R2: in air the bireflectance is almost constant at 8%
absolute, 22% relative, i.e., it is undispersed. The color values calculated relative to the standard illuminants A (2856
K) and C (6774 K) of the International Commission on Illumination (CIE 1971) show nearly constant dominant wavelengths (ld) or hue, and very weak excitation purities (Pe%) or
saturation. It is apparent that differences in lightness (or luminance, Y%) alone that account for the perceived white-gray

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

755

FIGURE 1. Backscattered electron images. (a) Fragments of baumstarkite with dark lamellae with high As content. (b) Baumstarkite crystals(ba) with weak compositional zonation and associated with miargyrite (mi), andorite (an), a robinsonite type material (ro), and traces of pyrargyrite(py). (c) Compositionally zoned baumstarkite, with local transition to aramayoite (ar), randomly intergrown with miargyrite (mi, Bi or As rich)and pyrargyrite (py). (d) Aramayoite with weak compositional zonation, surrounded (replaced) by a myrmekitic intergrowth of diaphorite (di)and galena-matildite s.s. (ga-ma) (lower part) and an andorite (an) like phase (upper part); owyheeite (ow) occurs as needle-shaped crystals
penetrating the intergrowth. Samples in a and b are from San Genaro, Huancavelica (MBB 98.36.01, 99/49a-WHP301), c is from Pirquitas(Veta-vein Potosi; WHP1716), and d is from Armonia mine, El Quevar (EQ 95/9).

bireflectance. In oil, the ratio of the bireflectance is 6% absolute and 30% relative. When compared with the air values,
it is the lower luminance values combined with the almost
doubled relative values of excitation purity that explain theenhancement in the perceived bireflectance.
A polished sample of miargyrite associated withbaumstarkite from the San Genaro mine, Huancavelica, Peru,
was measured under the same conditions as the baumstarkite
type specimen. The dispersion of the R1 and R2 and the imR1
and imR2 values closely match those of baumstarkite. In fact,
although our data show the mineral to be less strongly

bireflectant than baumstarkite, they also show that the R2 andim
R2 values for the two minerals are practically indistinguishable. It is, of course, not surprising that the optical propertiesof the two minerals are similar given their compositional similarity. The tabulated data for miargyrite match closely thoseof Caye and Pasdeloup (QDF3, 369,1993) for a sample from
Hiendelaencina, Spain, while Picot and Johan’s (1982) data for
an unlocalized sample show that it is appreciably morebireflectant, and are a better fit for our data for baumstarkite.
In the absence of compositional data in Picot and Johan’s(1982) compilation, it is tempting to speculate that they mea-

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EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

the Portugalete and Armonia mine samples. In both cases,
the reflectance values could not be reproduced over a periodof time. The wavelength scanning procedure we usually employ involves repeated measurement of the sample at 31 wavelengths in plane polarized light at orientations corresponding
to extinction positions between crossed polars. This takes
several minutes for each extinction position. The fact is thatsome samples of aramayoite are very light sensitive and thatin the time taken to measure their reflectance spectra theirreflectance values were falling. A revised procedure wasadopted which involved the collection of data from single
spectral scans on freshly polished surfaces. When this wasdone, the data collected from the Portugalete and Armonia
mine samples were consistent. A representative set of datafrom the Armonia mine are included in Table 1 and Figure 3.
Obviously, these differences are related to composition, allied perhaps to the structural orientation of grains and grainboundaries, but it is intriguing to note how light alone isenough to reveal them—it is also now quite clear why thereflectance spectra of this mineral measured on different
samples should be approached with caution. Nevertheless, thespectra and color of aramayoite—even samples that have been
affected to a limited extent by light—are different from thoseof miargyrite and baumstarkite, neither of which is in any
case nearly as light sensitive.
FIGURE 2. Sketch of a twinned crystal of baumstarkite; the twin
plane is (001) (program SHAPE, Dowty 1999a).

sured a mineral nearer in composition to that of baumstarkite
than miargyrite.
Our optical investigation of aramayoite revealed some interesting problems. The first sample studied, the type specimen (BM 1926,292) from the Animas mine in Bolivia, proved
to be very weakly bireflectant and anisotropic. Other samples,
including some from Portugalete, Bolivia (BM 1936,1160)
and the Armonia mine, El Quevar, Salta Province, northwest
Argentinia, conform more closely with the descriptions of
Uytenbogaardt and Burke (1971) and Picot and Johan (1982)
of a mineral with clearly defined cleavages parallel to twinlamellae. These differences in the reflectance and bireflectance
between twins are accompanied by even stronger differencesin anisotropic rotation tints. At a color temperature of ~3200 K
the tints associated with the most anisotropic material vary froma reddish brown to a pale bright mauve to steel gray.
Uytenbogaardt and Burke (1971) described the tints as lightpink to steel blue, while Picot and Johan (1982) recorded orange-brown to green polarization colors. Color descriptions
are notoriously inexact so how real these differences are isdoubtful, however, spectral reflectance values from well-polished minerals should be more reliable. The reflectance dataand color values of Caye and Pasdeloup (QDF3.15.1993) area good fit for our data for the type specimen, but both ofthese sets of data are much lower in reflectance than those of
Picot and Johan (1982). The probable explanation for these
discrepancies was found when measurements were made of

CHEMICAL COMPOSITION
Quantitative chemical analyses of baumstarkite, aramayoite, and miargyrite were obtained by electron-microprobe (JEOL
Superprobe JXA-8600, LINK-EXL software including an online ZAF correction, acceleration voltage of 25 kV with a beam
current of 30 nA). The following natural and synthetic standards were used: stephanite (AgLa), stibnite (SKa, SbLa), arsenopyrite (AsKa), and bismuthinite (BiLa). The results arecompiled in Table 2.
Most crystals and grains of baumstarkite are almost pure
Ag-Sb sulfides or contain minor amounts of arsenic. Traceamounts of Cu but no Bi were detected in the material available for study. However, several crystals contain sharply defined and chemically zoned lamellae (Figs. 1a and 1b) whereAs (up to 11.5 wt%) substitutes for Sb. The empirical formulaof As-poor baumstarkite is based on the analytically detectedvalues recalculated to a total of four apfu: Ag0.99(Sb0.97As0.03)S=1.00
S2.01. The empirical formula matches well with the simplified
formula AgSbS2. From crystal structure refinements, the crystal
chemical formula accounting for the partial solid solution between Sb and As is Ag3(Sb,As)2SbS6.
Aramayoite was chemically characterized using samples
originating from Bolivia (Animas mine, Chocaya), Argentinia(Armonia mine, El Quevar, Salta Province; Pirquitas mine,
Jujuy Province), and Austria (Altenberg, Lungau District,
Salzburg Province, Putz 2000). The material from Bolivia isfrom the type locality and represents co-type aramayoite (BM1940,10), donated by the late Harry Berman to the Natural
History Museum, London, U.K. All samples of aramayoite
show a distinct substitution of Sb by Bi (Figs. 1c and 1d;Table 2). The Sb:Bi ratio is usually between ~3:1 to ~4:1, but
it varies significantly at Pirquitas between ~4.4:1 and ~1.5:1,

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

757

TABLE 1. Reflectance measurements of baumstarkite, aramayoite, and miargyritel (nm)
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700

R1
33.15
32.70
32.15
31.60
31.10
30.40
29.95
29.35
28.60
28.10
27.60
27.05
26.45
25.90
25.45
25.15

baumstarkite
im
R2
R1
40.95
19.20
40.75
18.40
40.20
17.60
39.65
17.00
39.50
16.40
38.80
15.80
38.10
15.25
37.40
14.70
36.80
14.10
36.45
13.65
35.70
13.20
34.65
12.80
33.60
12.30
32.70
11.95
32.40
11.75
31.25
11.70

im

R2
25.95
25.25
24.30
23.80
23.30
22.60
21.80
21.30
20.75
20.50
19.80
18.85
17.90
17.25
16.60
16.30

R1
38.90
38.85
38.50
38.40
38.20
38.05
37.80
37.50
37.30
37.10
37.00
36.70
36.60
36.20
36.10
36.30

aramayoite
im
R2
R1
43.40
23.60
42.50
23.70
41.90
23.40
41.60
23.00
41.10
22.85
40.90
22.65
40.50
22.40
40.25
22.10
40.05
21.80
40.10
21.50
40.10
21.40
39.90
21.10
39.50
20.85
39.10
20.55
39.00
20.00
39.80
20.20

im

R2
25.92
26.20
26.10
25.50
25.35
24.80
24.50
24.20
24.00
23.90
23.85
23.60
23.40
22.90
22.30
22.50

R1
37.30
37.10
36.90
36.50
35.85
34.95
34.05
33.30
32.75
32.10
31.30
30.55
29.90
29.25
28.80
28.60

miargyrite
im
R2
R1
39.80
22.00
40.00
21.85
39.45
21.55
39.05
21.10
38.60
20.35
38.05
19.45
37.30
18.60
36.55
17.95
35.90
17.40
35.30
16.80
34.75
16.20
34.00
15.50
33.20
14.90
32.35
14.40
31.85
14.05
31.55
13.95

im
R2
24.90
24.95
24.40
23.85
23.35
22.60
21.90
21.20
20.50
19.95
19.50
18.75
18.00
17.30
16.85
16.75

470
546
589
650

31.30
29.20
27.80
26.15

39.65
37.30
36.10
33.05

16.60
14.60
13.35
12.15

23.60
21.15
20.10
17.60

38.35
37.45
37.00
36.40

41.40
40.20
40.00
39.40

23.00
22.05
21.50
20.70

25.60
24.15
23.90
23.20

36.10
33.10
31.70
29.65

38.85
36.35
35.00
32.75

20.75
17.75
16.50
14.65

23.60
21.00
19.75
17.65

x
y
Y%
ld
Pe%

0.435
0.405
28.40
491
3.1

0.436
0.424
0.407
0.402
36.40
13.90
491
491
2.8
5.8

0.428
0.405
20.40
491
4.6

0.444
0.407
37.20
491
0.85

CIE A
0.445
0.440
0.406
0.406
40.10
21.70
487
491
0.8
1.8

0.438
0.408
20.60
495
2.3

0.434
0.405
32.30
491
3.4

0.436
0.406
35.55
491
2.8

0.423
0.402
17.03
491
6.0

0.428
0.404
20.20
491
4.9

x
y
Y%
ld
Pe%

0.297
0.306
28.80
480
5.8

0.299
0.287
0.309
0.295
36.90
14.30
482
479
4.8
10.8

0.292
0.301
20.90
480
8.3

0.307
0.314
37.40
487
1.5

CIE C
0.307
0.303
0.312
0.310
40.25
21.90
475
481
1.7
3.4

0.303
0.309
24.10
476
3.2

0.296
0.305
32.80
480
6.3

0.299
0.308
36.00
481
5.0

0.286
0.295
17.50
479
11.0

0.291
0.3
20.70
480
8.9

COM

FIGURE 3. Reflectance spectra of baumstarkite, aramayoite, and miargyrite.

758

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITETABLE 2. Results of electron-microprobe investigations of baumstarkite and aramayoite
N*

Ag

Sb

Bi
baumstarkite
San Genaro matrix (range) ‡ 22
35.6–36.7
38.8–40.9
San Genaro matrix (average) 2236.3(3)40.2(5)San Genaro lamellae 1
1238.3(3)27.9(17)San Genaro lamellae 2
1437.4(3)33.1(8)
Pirquitas (range) ||16
35.1–36.2
31.4 – 33.9
6.8 – 10.0
aramayoite
Chocaya (range, BM1940.10) 9
33.8–34.7
29.1–31.6
12.7–16.2
Chocaya (average, BM1136.1160) 934.3(3)30.6(7)14.1(1)El Quevar (range) ‡3
35.0–35.3
26.8–28.8
15.2–17.8
El Quevar (average)3
35.1(2)27.5(11)16.3(13)
Pirquitas (range) #20
32.9–36.8
21.8–30.5
11.9–25.0
Altenberg (average)3
32.5(1)22.6(6)25.4(6)* Number of analyses (values in wt%, standard deviations in parentheses).
† Ratio of atoms.
‡ A crystal out of these samples was used for X-ray investigations (EQ 95/9).
§ Included Cu 0.1 wt%.
|| Included Pb 0.4, Cu 0.2, Fe 0.6 wt%.
# Included Pb 0.4–4.0, Cu 0.1–0.3, and Fe 0.1–0.6 wt%.
** Included Pb 1.33, Te 0.12 wt%.

and is ~1.5:1 at Altenberg. In two cases the ratio at Pirquitas islarger than 5.5:1, indicative of a more or less complete solid
solution between aramayoite and baumstarkite. Samples withboth As and Bi substituting for Sb on a larger scale have notyet been found.
Miargyrite associated with baumstarkite in the San Genaro
mine was used for optical, chemical, and structural investigations (sample no. 00/10). Electron-microprobe analyses
showed that all the grains are chemically inhomogeneous.
The minor elements are Cu, As, Pb, and Hg. Copper and Asvary between 0.12 and 0.16 wt% and between 0.4 and 1.5
wt%, respectively. Lead (between 0.8 and 2.86 wt%) was detected at seven, and Hg (between 0.3 and 2.2 wt%) at six of a
total of nine points measured on several grains.

As

S

Total

0.3–2.0
0.7(4)
10.1(12)6.2(5)
0.2 – 2.7

21.9–22.3
22.0(1)23.1(2)22.6(2)20.9 – 21.5

99.4 §
99.4
99.3

0.1–0.3
0.2(1)
1.2–1.2
1.2(1)
0.2–0.9

20.6–21.0
20.8(1)
18.7–19.1
18.9(2)
19.5–20.9
19.5(1)

Sb/Bi †

5.47 – 7.51

100.0
99.0
101.5**

3.08–4.27
3.69
2.58–3.25
2.91
1.44–4.41
1.5

TABLE 3. Comparison of X-ray diffraction powder patterns ofbaumstarkite and aramayoite (the six strongest peaks
are listed)baumstarkite

h klaramayoite

d (obs) (Å) I (obs) d (calc) (Å) I (calc) d (obs) (Å) I (obs)

X-RAY INVESTIGATION

0 2 1
3.425
8
3.429
70
3.429
10
–
0 2 2
3.258
3
3.259
58
3.247
17
–
2 0 2
3.224
6
3.230
45
3.236
15
2 0 1
3.158
4
3.159
40
3.175
13
–
2 2 1
2.844
80
–
2.841
8
2.842
29
2 2 1
2.837
88
0 0 3
2.798
100
2.802
100
2.814
100
2 2 2
2.013
5
2.013
28
2.021
8
–
2 2 4
1.971
5
1.975
28
1.977
6
0 0 6
1.39946
1.40078
1.4073
7
Notes: Powder diffractometer data (Philips X’Pert diffractometer) (CuKa1,2radiation, internal standard Si). The powder diffraction pattern was calculated according to the results of structure refinements with the program
LAZY PULVERIX (Yvon et al. 1977).

X-ray powder patterns of baumstarkite and aramayoite aresimilar and show very strong texture effects caused by theexcellent cleavage of the minerals parallel to (001). The strongest lines detected are compared to those calculated from thestructural parameters for baumstarkite in Table 3. Single-crystal X-ray structure investigations were performed from small
crystal chips of baumstarkite, aramayoite, and miargyrite.
Details of the structure refinements are compiled in Table 4,
and final structural parameters are given in Table 5.
Baumstarkite. Single-crystal X-ray data collection was
performed on the type material. The atoms were located bydirect methods and difference Fourier maps. The refinement–
converged for space group P1. The low anisotropy of the displacement parameters and the good agreement between theobserved and calculated structure amplitudes supports
centrosymmetry. The Ag and M atoms occupy three crystallographically independent atomic sites each. For the refinements the M sites were at first assumed to be fully occupied
by Sb atoms. However, the equivalent isotropic displacement
parameters indicated the possibility of a slightly smaller scattering power at the M2 site and a slightly higher one at theM3 site. Small amounts of As and Bi, respectively, substituting for Sb were considered.
Aramayoite. An earlier structural investigation ofaramayoite by Mullen and Nowacki (1974) was not in agreement with the structure refinement of baumstarkite. Only theprinciple features of the atomic arrangements corresponded
to each other, and some coordination polyhedra contradicted
crystal chemical experience (e.g., Ag-S bond lengths of 2.05
and 2.11 Å). Despite a refinement of aramayoite in space group
P1 by the former authors some sites were partially or mixed
occupied. Mullen and Nowacki’s (1974) faulty structure model
was probably caused by the insufficient quality of their data:
the accuracy of X-ray data available at that time (photographic
non-integrated Weissenberg film data, multiple-film methods)
was obviously overestimated. Their data did allow them to determine the structure type, but though they tried to deduce details on the order-disorder phenomena between Ag, Sb, and Bi
they were unsuccessful. However, the main features of the structure were determined correctly.
To make a crystal-chemical comparison of the two isotypicmineral species a structure investigation of aramayoite was
performed using material from different localities. The results compare well. The data presented in this paper weretaken from a refinement of a sample from the Armonia mine,
El Quevar, Argentinia, because of the higher accuracy of thestructure refinement due to its better crystal quality. The struc-

}

{

}

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

759

TABLE 4. Single-crystal X-ray data-collection and structure refinements
baumstarkite
aramayoite
Miargyrite
Ag3(Sb,As)2SbS6
Ag3Sb2(Sb,Bi)S6
AgSbS2
7.766(2)7.813(2)12.862(3)8.322(2)8.268(2)4.409(1)8.814(2)8.880(2)13.218(3)100.62(2)100.32(2)–104.03(2)104.07(2)98.48(2)90.22(2)90.18(2)–542 Å3547 Å3741 Å3
–
–
P1
P1
C2/c
2
2
8
5.39
5.88
5.26
55 ¥ 30 ¥ 105
40 ¥ 160 ¥ 20
60 ¥ 60 ¥ 50
5 < 2J < 60
5 < 2J < 60
5 < 2J < 60
413
445
425
270
80
80
13.9
27.3
13.4
0.077
0.030
0.058
0.057
0.034
0.031
0.128
0.084
0.082
0.0019(3)0.0071(8)0.0038(2)crystal shape
crystal shape
crystal shape
0.363 to 0.687
0.090 to 0.531
0.443 to 0.567
3147
3158
1080
2867
3043
1002
113
113
40
1.19
1.05
1.12
0.02390.02820.0343
11.95
4.73
3.94
£0.001£0.001£0.001–3.08 to +2.60–0.87 to +1.09–1.41 to +2.08
Note: NONIUS four-circle diffractometer equipped with a CCD detector and a fiber optics collimator, Mo tube, graphite monochromator, j-scans for
distinct w-angles, Dj = 2∞/frame, frame size: binned mode, 621 ¥ 576 pixels, detector-to-sample distance: 28 mm; range of data collection: ±h ±k ±l. Unitcell parameters were obtained by least-squares refinements of accurate 2J values. Corrections for Lorentz and polarization effects and absorption
effects; neutral-atomic complex scattering functions (Wilson 1992), programs SHELXS-97, and SHELXL-97 (Sheldrick 1997a, 1997b). w = 1 / { s 2(Fo2)+ [P1*P]2 + P2*P}; P = ([max(0,F o2)] + 2*Fc2) / 3].

Chemical formula
a (Å)
b (Å)
c (Å)
a (∞)
b (∞)
g (∞)
V (Å3)
space groupZ
dcalc (g/cm3)
crystal dimensions (mm)range of data collection (∞)
number of frames
scan time (s/∞)
m(MoKa) (mm–1)
Rint = S|Fo2–Fo2(mean)|/SFo2
R1 = S(||Fo|–|Fc||)/SFowR2 = [Sw(Fo2–Fc2)2/S˚wFo4]1/2
extinction parameter absorption correction
transmission factors
observed unique reflections (n)reflections with Fo > 4s(Fo)variable parameters (p)GooF = {S[w(Fo2–Fc2)2]/(n–p)}0.5
P1
P2
max D/s
final difference Fourier map (eÅ–3)

tural refinement was begun using atomic coordinates obtained
for baumstarkite. A markedly different occupation for the Msites was observed. The M1 and M2 sites are chiefly occupied by Sb atoms, and only small amounts of Bi substitutefor the Sb atoms. However, the M3 site is predominately occupied by Bi atoms, with only small amounts of Sb. Our refinement clearly showed that aramayoite is centrosymmetric.
–
No hints for a reduction of the space-group symmetry P1 were
found. The anisotropies of all the displacement parameters
are unconspicuous. In addition, our results are in accordancewith the chemical analyses of aramayoite which scatter around
Ag:(Sb + Bi):S ratios of 3:3:6, but show a significant Bi content up to Bi:Sb = 1:2. In contrast, the structural formula derived by Mullen and Nowacki (1974) for aramayoite is
Ag5Sb3.75Bi2S12 (based on structural data without a chemical
analysis, the origin of the investigated samples is not mentioned).
Miargyrite. Miargyrite is isochemical with baumstarkite;
therefore special attention was given to its crystal structure.
The first examination based on Weissenberg-film data andpacking considerations was performed by Hofmann (1938).
Knowles (1964) found from single-crystal two-circle
diffractometer data the extinction symbol C1c1 and tried refinements in space groups C2/c and Cc. Due to significant
deviation of the atomic coordinates from centrosymmetry, anddue to a lower R value, the authors finally described the crystal structure of miargyrite in the acentric space group Cc.
Quite recently, Smith et al. (1997) reported on a structurerefinement of miargyrite in space group C2. They mentioned
the approximate C2/c symmetry, but found 60 reflections conflicting with the c-glide plane. Neither the method for detecting the reflections violating the extinction rule h0l: l = 2n (filmmethods or diffractometer data) nor the ratios I/sI were given.
Smith et al. (1997) used a larger crystal than in the previous
and present studies (0.15 ¥ 0.13 ¥ 0.11 mm) and a 8 kW rotating anode source for data collection. It cannot be excluded withcertainty that the intensity of reflections violating the extinction conditions result from the l/2 effect and/or from
“Umweganregung” (Renninger reflections). Any significant
deviation from centrosymmetry is not seen in their structure
model.
A careful re-examination of the crystal structure of
miargyrite gave no hints for a violation of the glide plane(the size of the crystal used was only 0.05 ´ 0.06 ´ 0.06 mm
to reduce the absorption). The check of extinction rules wasdone using Weissenberg-film methods (CuKa radiation) and afour-circle diffractometer. It is worth noting that even investigations with the CCD area detector and a conventional Mo tube
operating at 1.9 kW gave no hint of violations of the extinction
rule h0l: l = 2n; this method improves the ratio signal to background and consequently allows the recognition of weaker intensities as compared to conventional detectors. The structurerefinements were performed in the three space groups C2/c,
Cc, and C2. The resulting R values are practically the same,
but both refinements in acentric space groups exhibit large elements in the correlation matrix. The symmetry reductions

760

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

cause an increase in the number of free variables from 40 inC2/c to 74 and 72 in C2 and Cc, which is not justified from therefinement. The anisotropy of the displacement parameterscompares especially well in all three refinement models. Consequently the crystal structure of miargyrite is best describedin the centrosymmetric space group C2/c.

RESULTS AND DISCUSSION
In baumstarkite, aramayoite, and miargyrite the environment of the Ag atoms is roughly characterized by the coordination numbers [2], [2 + 2], [4], or [2 + 2 + 2] (relevantinteratomic bond lengths and bond angles are compiled inTable 6). Such coordinations are in agreement with common
crystal chemical experience (Burdett and Eisenstein 1992).
The gaps between the first and second coordination spheres
differ but correlate with the shortest of the Ag-S bonds. Ag-S
bond lengths begin for Ag[2] atoms at 2.39 Å, and for Ag[2+2]/Ag[4]/Ag[2+2+2] atoms between 2.51 and 2.58 Å. Additional S
ligands exhibit a wide spread distribution of bond distances.
Most differences are in the environments of the Ag atoms in
miargyrite: the Ag1 atom features four similar bond lengths,
and the Ag2 atom has a distinct twofold-coordination with ahuge gap to the next ligands. The S-Ag-S bond angles between
the two nearest neighbors are 146∞ to 163∞, but for the Ag2[2]atom in miargyrite the bond angle is 180∞ due to the site sym–
metry 1 of the central atom. Linear S-Ag-S bond angles between the two shortest bonds are rare.
An analysis of the displacement parameters of the Ag atoms shows that they correlate with the bond length distribution rather than with the S-Ag [2] -S bond angles. The
displacement of the Ag1 atom in miargyrite is relatively regular in accordance with the coordination figure which tends toa strongly distorted tetrahedron (principal mean-square atomic
displacements of U are 0.045, 0.037, and 0.024 Å2). In contrast, the Ag2 atom shows a large anisotropy. The principal
mean-square atomic displacements of U are 0.070, 0.045, and
0.017 Å2; the smallest axis is approximately in the direction of
the two shortest bonds. This anisotropy might be considered asa possible reason for the lowering of the space-group symmetry C2/c. A reduction of the symmetries to both C2 or Cc abol–ishes site symmetry 1 and consequently allows a bent S-Ag2-S
bond angle. However, refinements in both the acentric spacegroups result in structure models with anisotropies just as large
as in the centrosymmetric model and therefore they do not justify a refinement in any lower symmetry. The anisotropy of theAg2 atom in miargyrite is comparable to that of the Ag1 andAg2 atoms in baumstarkite/aramayoite: the principal meansquare atomic displacements of U are 0.069, 0.066, 0.026/0.065,
0.057, 0.024 Å2 and 0.069, 0.058, 0.028/0.070, 0.047, 0.025
Å2. Consequently the anisotropy of the Ag1 atom in miargyrite
should be considered as chiefly influenced by the pronounced
gap between the two nearest and next ligands. The environments of the three Ag atoms in baumstarkite/aramayoite support that (1) mainly the bond-length distribution controls theanisotropy of the displacement parameters of the Ag atoms andthat (2) the influence of a large or even linear angle S-Ag[2]-S is
minor. Despite the large S-Ag3[2]-S angle of 163∞ the principal
mean-square atomic displacements of U (0.047, 0.037, 0.030/

TABLE 5. Structural parameters (e.s.d.s in parentheses) forbaumstarkite, aramayoite and miargyrite

Ag1
Ag2
Ag3
M1 = Sb
M2 = Sb
M3 = Sb
S1
S2
S3
S4
S5
S6

0.10810(16)0.29748(15)0.01456(12)0.63539(8)0.17282(8)0.48601(8)0.8531(3)0.3751(3)0.7238(3)–0.0219(3)0.4430(3)0.2447(3)yzbaumstarkite
0.42584(14) 0.14892(15)0.07224(13) 0.84390(14)0.74128(12) 0.49807(10)0.41859(7)0.16628(7)0.91999(7)0.17039(7)0.24662(7)0.49470(8)0.2103(3)0.2033(3)0.2545(3)0.1731(3)0.5306(3)0.4623(3)0.7031(3)0.1989(3)0.7655(3)0.1869(3)1.0324(3)0.4631(3)

Ag1
Ag2
Ag3
M1 = Sb
M2 = Sb
M3 = Bi
S1
S2
S3
S4
S5
S6

0.11503(12)0.30433(11)0.00914(7)0.64238(5)0.16700(5)0.49374(3)0.85745(19)0.38019(19)0.73044(19)–0.02532(20)0.43666(19)0.23938(20)aramayoite
0.42287(9)0.14742(10)0.07450(8)0.84722(9)0.74334(7)0.49858(6)0.41787(4)0.16524(4)0.91841(4)0.16979(4)0.24679(3)0.49421(3)0.20464(17) 0.20003(18)0.25024(18) 0.16822(17)0.52180(17) 0.46193(17)0.69719(18) 0.19690(18)0.76178(18) 0.18244(17)1.02194(17) 0.46306(17)

atom

x

Uequiv
0.0536(3)0.0518(3)0.0377(3)0.0218(2)0.0217(2)0.0239(2)0.0241(5)0.0242(5)0.0232(5)0.0265(5)0.0239(5)0.0235(5)0.0485(2)0.0474(2)0.03067(17)0.01789(15)0.01789(15)0.01903(10)0.0208(3)0.0200(3)0.0198(3)0.0226(3)0.0198(3)0.0200(3)miargyrite
Ag1
0.0
0.02945(14) 0.25
0.03479(17)
Ag2
0.0
0.5
0.5
0.04345(19)M = Sb0.25550(2) –0.03358(7)0.62662(2)0.01811(14)S10.14337(7)0.3510(2)0.69954(7)0.0183(2)S20.11173(7)0.1796(2)0.41767(7)0.0212(2)Notes: The anisotropic displacement parameters are defined as: exp[–2p2S3i=1S3j=1 Uij a*i a*j hi hj], Beq according to Fischer and Tillmanns (1988).
The occupation factors were refined from scattering power by leastsquares methods. Baumstarkite: Sb:As is 0.994(11):0.006(11) for M1 and
0.966(11):0.034(11) for M2, Sb:Bi is 0.974(6):0.026(6) for M3; aramayoite:
Sb:Bi is 0.937(4):0.063(4), 0.963(4):0.037(4), and 0.097(5):0.903(5) for
M1, M2, and M3.

0.039, 0.029, 0.025 Å2) are comparable to that of the Ag1 atomin miargyrite. The other S-Ag[2]-S angles scatter about 150∞. In
general, the shortest axes correspond with the directions of theshortest bonds. As in miargyrite, by way of trial a refinementof baumstarkite and aramayoite in space group P1 also resulted
in large anisotropies of the displacement parameters of the sites
Ag1 and Ag2.
In minerals with formula AgMS2 the M = As, Sb, Bi atoms exhibit a one-sided nearest neighbor environment as characteristic for atoms with steric active lone-pair electrons. The
displacement parameters indicate only moderate anisotropies
for all the M atoms. Two different coordination geometries
with different importance in sulfosalts were observed(Makovicky 1981, 1989). The M1 and M2 atoms inbaumstarkite and aramayoite and the M atom in miargyrite
show the usual [3 + 3] coordination. The sizes of the MS3
pyramids are similar and indicate agreement with structuralrefinements, i.e., a predominant occupation by Sb atoms. Individual M-S bond lengths for the first coordination sphereare 2.44 to 2.54 Å, and the average bond lengths are 2.463 to
2.492 Å. Next ligands have M-S > 3.09 Å. However, a detailed comparison of the average bond distances inbaumstarkite and aramayoite show moderate differences:

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITE

761

TABLE 5.–continued

Ag1
Ag2
Ag3
M1
M2
M3
S1
S2
S3
S4
S5
S6

0.0526(6)0.0518(6)0.0357(5)0.0232(3)0.0229(3)0.0248(4)0.0242(10)0.0237(10)0.0239(10)0.0266(11)0.0252(10)0.0255(10)0.0438(6)0.0422(6)0.0465(6)0.0183(3)0.0177(3)0.0193(4)0.0200(11)0.0241(11)0.0189(10)0.0223(11)0.0219(11)0.0195(10)U33baumstarkite
0.0660(7)0.0643(7)0.0311(5)0.0247(4)0.0241(4)0.0272(4)0.0287(12)0.0251(11)0.0248(11)0.0305(12)0.0254(11)0.0253(11)

Ag1
Ag2
Ag3
M1
M2
M3
S1
S2
S3
S4
S5
S6

0.0479(4)0.0500(4)0.0278(3)0.0182(2)0.0174(2)0.02031(14)0.0179(7)0.0185(7)0.0199(7)0.0208(7)0.0187(7)0.0208(7)0.0414(4)0.0400(4)0.0387(3)0.0163(2)0.0165(2)0.01733(14)0.0199(6)0.0224(7)0.0190(6)0.0224(7)0.0220(7)0.0198(6)aramayoite
0.0565(4)0.0530(4)0.0249(3)0.0195(2)0.0193(2)0.01897(14)0.0236(7)0.0187(7)0.0189(6)0.0241(7)0.0183(6)0.0185(6)0.0060(3)0.0019(3)0.0059(2)0.00364(14)0.00267(14)0.00252(9)0.0032(5)0.0037(5)0.0007(5)0.0031(5)0.0032(5)0.0001(5)0.0156(3)0.0194(3)0.0054(2)0.00499(14)0.00389(15)0.00456(9)0.0038(5)0.0039(5)0.0035(5)0.0053(6)0.0041(5)0.0053(5)0.0205(3)–0.0170(3)0.0014(2)0.00172(13)0.00243(14)0.00126(8)0.0046(5)–0.0013(5)0.0011(5)–0.0021(5)0.0062(5)0.0012(5)

Ag1
Ag2
Sb
S1
S2

0.0250(3)0.0346(3)0.0218(2)0.0180(4)0.0207(4)0.0439(3)0.0593(4)0.0184(2)0.0203(5)0.0237(5)miargyrite
0.0344(3)0.0392(4)0.0148(2)0.0170(4)0.0210(4)0.0–0.0106(3)–0.00050(8)–0.0004(3)0.0017(3)0.0005(2)0.0145(3)0.00464(12)0.0041(3)0.0084(3)0.00.0157(3)–0.00022(9)–0.0005(3)0.0012(4)

Atom

U11

U22

<M1-S> and <M2-S> are both slightly but significantly larger
in aramayoite (2.489 and 2.475 Å) than in baumstarkite (2.472
and 2.463 Å). This behavior is in accordance with the partial
substitution of Sb by As and Bi atoms in the two minerals,
respectively.
In baumstarkite and aramayoite, the positions M3 = Sb andBi feature a distinct environment with coordination number [2+ 2 + 2]. Furthermore, a huge difference in the short M3-S
bond lengths is observed for the two minerals due to the predominant occupation by Sb and Bi atoms, respectively. The
shortest and medium M3-S bond lengths in baumstarkite (~2.51
and 2.76 Å) are smaller than in aramayoite (~2.64 and 2.81 Å)
to allow for the different size of the Sb and Bi atoms, respectively. Two further ligands are at a distance of ~3.0 Å indicating only moderate chemical interactions. It is worth mentioningthat these fifth and sixth M3-S bond lengths are larger forbaumstarkite than for aramayoite. This causes a stronger distortion for the M3S6 polyhedron in baumstarkite (BiS6) than inaramayoite (SbS6). Probably steric reasons caused by the structure type are responsible for the reverse of the Bi-S/Sb-S bondlengths. In addition, the steric activity of the lone-pair electrons of the Sb3+ atoms seems to be more pronounced as compared to Bi3+ atoms.
The different size of the coordination of the M atoms reflects the chemical variability of baumstarkite and aramayoite.
Despite ideal compositions of Ag3Sb3S6 and Ag3Sb2BiS6, extensive substitution between As:Sb and Sb:Bi was found bychemical analysis. Due to the different size and geometry of
the M1[3]S3 and M2[3]S3 pyramids compared to the M3[2+2]S4
polyhedra any significant As content substitutes for Sb at the

U23

U13

U12

0.0089(5)0.0095(5)0.0097(4)0.0046(2)0.0035(2)0.0018(2)0.0049(9)0.0057(9)0.0013(9)0.0025(9)0.0051(9)0.0009(9)0.0186(5)0.0208(5)0.0068(3)0.0070(2)0.0053(2)0.0073(2)0.0075(9)0.0063(8)0.0043(8)0.0087(9)0.0072(8)0.0088(8)0.0228(5)–0.0126(5)0.0024(4)0.0027(2)0.0035(2)0.0020(2)0.0058(8)0.0002(8)0.0034(8)–0.0003(9)0.0064(8)0.0001(8)M1 and M2 position whereas the M3 position is occupied
either by Sb or by Bi atoms. Only small amounts of Bi substituting for Sb at the M1 and M2 position in aramayoite wereobserved. Both three and four ligands in the first coordination sphere of Sb and Bi atoms are known, whereas As[4] is
uncommon (for an exception see bernardite, TlAs5S8, Pas̆ava
et al. 1989). Consequently, the crystal chemical formula forthe structure type is roughly Ag3(As,Sb)2(Sb,Bi)S3. In approximate accordance with these crystal-chemical considerations,the analytically determined Sb:Bi ratio in aramayoite is ≥1.8:1.2
and the Sb:As ratio in baumstarkite is ≥ 1.9:1.1. Considering
the limiting ratios Sb:Bi ≥2.0:1.0 and Sb:As ≥ 1.0:2.0, the compositions of baumstarkite and aramayoite are Ag3(Sb,As)2SbS3and Ag 3 Sb 2(Bi,Sb)S 3, respectively. However, samples ofbaumstarkite with small contents of both As and Bi according
to Ag3(Sb0.95As0.05)2(Sb0.9Bi0.1)S3 were also detected (Table 2).

STRUCTURAL RELATIONSHIPS
Authors of earlier papers dealing with X-ray work on AgMS2
minerals have considered structural relations within this groupof compounds from the point of view of cell metrics, strongest
lines in the powder pattern, and from structural investigations(as far as they were available). Previous papers mentioned the
PbS type as the parental structure. Additionally, this structural
relation has been deduced from the phase transitions. Cubic
high-temperature modifications of AgMS2 are described as crystallizing in the PbS type; Ag and M atoms are octahedrally
coordinated and statistically occupy one atomic site. The lowtemperature modifications of these compounds exhibit complete ordering of Ag and M atoms. Order phenomena are

762

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITETABLE 6. Interatomic bond distances (in Å) and bond angles (in E) for baumstarkite (M1 = M2 = M3 = Sb) and aramayoite (M1 = Sb, M2 =Sb, M3 = Bi) and miargyrite
bond distances
Ag1–S2
Ag1-S4
Ag1-S12
Ag1-S47
Ag2-S511
Ag2-S18
Ag2-S49
Ag2-S25
Ag2-S64
Ag3-S111
Ag3-S4Ag3-S32
Ag3-S619
Ag3-S311
Ag3-S6
M1-S1
M1-S2
M1-S3
M1-S510
M1-S5
M1-S41
M2-S4
M2-S5
M2-S6
M2-S23
M2-S110
M2-S16
M3-S311
M3-S64
M3-S511
M3-S2
M3-S3
M3-S611baumstarkite
2.510(3)2.529(3)2.849(3)2.964(3)2.518(3)2.540(3)2.849(3)2.934(3)3.235(3)2.538(3)2.542(3)2.774(3)2.801(3)3.058(3)3.109(3)2.436(2)2.454(3)2.525(3)3.118(3)3.251(3)3.479(3)2.437(3)2.453(2)2.500(3)3.185(3)3.229(3)3.496(3)2.510(2)2.511(2)2.747(3)2.769(3)3.090(3)3.144(3)aramayoite
2.508(2)2.536(2)2.874(2)2.948(2)2.523(2)2.545(2)2.889(2)2.877(2)3.273(2)2.581(2)2.584(2)2.760(2)2.786(2)3.004(2)3.025(2)2.447(2)2.478(2)2.541(2)3.089(2)3.269(2)3.395(2)2.445(2)2.466(2)2.513(2)3.209(2)3.218(2)3.413(2)2.639(2)2.647(2)2.803(2)2.819(2)3.023(2)3.062(2)bond angles
S2-Ag1-S4

baumstarkite
149.29(10)Aramayoite
151.35(6)S511-Ag2-S18

145.70(9)147.79(6)S111-Ag3-S4

163.01(9)162.74(5)S1-M1-S2
S1-M1-S3
S2-M1-S3

97.69(8)89.55(8)95.35(8)97.26(5)88.98(5)95.14(5)S4-M2-S5
S4-M2-S6
S5-M2-S6

97.44(8)91.40(9)93.80(9)97.18(6)90.43(5)93.61(5)S311-M3-S64
S311-M3-S511
S311-M3-S2
S64-M3-S511
S64-M3-S2
S511-M3-S2

90.67(8)89.33(8)86.86(8)85.04(8)89.94(8)173.67(7)89.08(5)88.31(5)87.47(5)85.08(5)89.35(5)173.07(4)miargyrite
2.5448(11)S20-Ag1-S213
149.82(5)Ag1-S20,13
Ag1-S112,14
2.6498(10)S20,13-Ag1-S112,14
93.40(3)105.68(3)
9,17
12
14
3.4169(11)S1 -Ag1-S1
101.49(5)Ag1-S10,90
9Ag2-S22.3888(9)S2 -Ag2-S2
180
Ag2-S10,9
3.0587(11)3,123.5626(11)Ag2-S2
Sb-S2172.4471(10)S217-Sb-S1
97.00(4)Sb-S1
2.5082(9)S217-Sb-S113
93.57(3)18
18
2.5199(10)S1-Sb-S1
92.00(3)
Sb-S1
Sb-S2
3.2289(12)4Sb-S1
3.2835(11)Sb-S215
3.4157(11)Notes: Symmetry code: 0 = x y z (if specified in cases of multiple bonds); 1 = x + 1, y, z; 2 = x – 1, y, z; 3 = x, y + 1, z; 4 = x, y – 1, z; 5 = x, y, z + 1;
6 = x – 1, y + 1, z; 7 = –x, –y + 1, –z; 8 = –x + 1, –y, –z +1; 9 = –x, –y + 1, –z + 1; 10 = –x + 1, –y + 1, –z; 11 = –x + 1, –y + 1, –z + 1; 12 = –x, –y,–z + 1; 13 = –x, y, –z + 1/2; 14 = x, –y, z + 1/2; 15 = –x + 1/2, –y – 1/2, –z + 1; 16 = –x + 1/2, –y + 1/2, –z + 1; 17 = x, –y + 1, z – 1/2; 18 = –x + 1/2, y – 1/2, –z + 3/2; 19 = –x, –y + 2, –z + 1.

accompanied by distinct coordinations. The M atoms have onesided coordinations due to the steric activity of their lone-pairelectrons. Consequently the relation to the PbS type is only arough description. The differences in the M-S bonds no longer
justify an approximation by sixfold coordination. According
to detailed investigations of sulfosalts by Makovicky (1993)
the type structure of the AgMS2 minerals should be considered
as derivatives of the SnS archetype to account for their special
coordinations and the bond-strength distributions. Despite thederivation from one structure type, the linkage among the shortconnected atoms in the AgMS2 minerals is different.
Considering only the nearest-neighbor environment, the typestructure of the two isotypic minerals baumstarkite andaramayoite comprises zigzag chains parallel to [010] withformula {AgMS2} (Fig. 4). The MS3 pyramids are corner-connected to MS4 polyhedra; MS3 pyramids share two corners with
MS4 polyhedra and MS4 shares four corners with MS3 pyramids. M-Sshared is longer than M-Sunshared. The S atoms making
short bonds to only one M atom (S1 and S4) make two Ag[2]-Sbonds. Two of the S atoms sharing corners between MS3 andMS4 polyhedra are involved in one Ag[2]-S bond (S2 and S5)
whereas S3 and S6 are only weakly connected to Ag atoms. All
Ag[2]-S bonds are within the chains. The chains are interconnected by additional Ag-S and M-S bonds to layers parallel to(001). The Ag-S bonds between the layers are >2.93 Å, in agreement with the observed cleavage (see Fig. 5). Following Mullen
and Nowacki (1974) and Graham (1951), one of the (001) layers in baumstarkite and aramayoite might be seen as three sheetsEFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITEFIGURE 6. The crystal structure of miargyrite: (a) For Ag atoms
nearest neighbor environment with Ag-S bond lengths <2.70 Å isshown. The SbS3 pyramids are shaded. (b) One 1•{SbS2} chain
formed by the SbS3 pyramids running parallel to the b axis.

of a distorted PbS type structure.
Miargyrite is formed by chains of corner connected SbS3
pyramids running parallel to [010] (Figs. 6a and 6b). The M-Sbonds with the shared S1 atoms are longer than those with theunshared S2 atoms. The {MS2} chains are linked by Ag1[4] andAg2[2] atoms. The structural relation of miargyrite and the PbS
type have been described by many authors. Knowles (1964)
mentioned that only the cations correlate with the PbS structure whereas the S atoms are greatly displaced. According to
Makovicky (1993) the type structure of miargyrite is deduced
from a periodically twinned SnS-like array. The cell volumes
recalculated for one AgSbS2 unit indicate distinct packing densities. Miargyrite (92.9 Å3) is less densely packed as compared
to baumstarkite (90.3 Å3) and cubargyrite (90.2 Å3; Walenta
1998). It is worth mentioning that aramayoite has a volume of
only 91.2 Å3 for one AgSb2/3Bi1/3S2 unit, i.e., less than miargyrite.

▲

FIGURE 4. The zigzag 1•{AgMS2} chains running parallel to [010]in the crystal structures of baumstarkite and aramayoite and theirstacking within the (001) plane. The Ag atoms and the Ag-S bonds<2.60 Å are shown. The SbS3 pyramids are shaded, and the SbS4 and
BiS4 polyhedra are cross hatched (all structural drawings used the
ATOMS program, Dowty 1999b).

763

FIGURE 5. The crystal structure of baumstarkite and aramayoite.
The Ag atoms with their two nearest neighbors are shown. The SbS3
pyramids are shaded, and the SbS4 and BiS4 polyhedra are crosshatched.

764

EFFENBERGER ET AL.: BAUMSTARKITE, ARAMAYOITE, AND MIARGYRITEACKNOWLEDGMENTS
W.H.P. is especially thankful to the Austrian Research Council (FWF) for
providing financial support through grants P 11987 GEO and P 13974 GEO.
Financial support by the International Centre for Diffraction Data is gratefully
acknowledged (Grant 90/03 ET). We acknowledge technical assistance by W.
Waldhör during preparation of polished sections for microprobe analysis. The
authors thank R.C. Peterson and M.F. Fleet for helpful comments and remarksthat improved the manuscript.

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MANUSCRIPT RECEIVED APRIL 16, 2001
MANUSCRIPT ACCEPTED JANUARY 3, 2002
MANUSCRIPT HANDLED BY MICHAEL E. FLEET