Mineralogical Magazine, December 2013, Vol. 77(8), pp. 3105–3112

Dervillite, Ag2AsS2, from Lengenbach quarry, Binn valley,
Switzerland: occurrence and crystal structure
L. BINDI1*, F. NESTOLA2, L. DE BATTISTI3 AND A. GUASTONI4
1

Dipartimento di Scienze della Terra, Università degli Studi di Firenze,Via G. La Pira 4, I-50121 Florence, Italy
Dipartimento di Geoscienze, Università di Padova, Via Gradenigo 6, I-35131, Padua, Italy
3
FGL (Forschungsgemeinschaft Lengenbach), Via dello Storno 18, I-20147 Milan, Italy
4
Museo di Mineralogia, Università di Padova, Via Giotto 1, I-35122, Padua, Italy
2[Received 24 September 2013; Accepted 22 October 2013; Associate Editor: A. Christy]
ABSTRACT

Dervillite, As2AsS2, has been found in a sample from the Lengenbach quarry in the Binn Valley,
Valais, Switzerland. It occurs as very rare crystals up to 200 mm across on jordanite. Dervillite is
opaque with a metallic lustre and shows a dark brown streak. The structure is monoclinic, space groupPc, with a = 9.6155(7), b = 12.9331(8), c = 6.8616(5) Å, b = 99.352(8)º, V = 842.0(1) Å3 and Z = 8. In
the crystal structure [R1 = 0.060 for 2370 reﬂections with I > 2s(I)], silver adopts various coordinations
extending from quasi linear to quasi tetrahedral whereas arsenic forms very peculiar crystal-chemical
environments, such as As(S2As) and As(S2AsAg). Such metalloid–metalloid or metal–metalloid bonds
account for the apparent charge imbalance observed in the chemical formula.
K EY WORDS : dervillite, silver, Lengenbach, Switzerland.

Introduction
DERVILLITE was described ﬁrst by Weil (1941) as anatural compound of lead, sulfur and antimony,and possibly also bismuth. A reinvestigation of
the original sample by Bari et al. (1983) provedthat the chemical data given by Weil (1941) were
erroneous, because they were based on chemical
tests on material that was apparently not extracted
from the dervillite crystals. Unfortunately, given
wartime conditions, the study could not becompleted. By means of X-ray single-crystal
precession photographs and an X-ray powder
diffraction study of the holotype, Bari et al.
(1983) found that dervillite is monoclinic, spacegroup P2/a, with a = 6.833, b = 12.932, c =9.638 Å, b = 99.33º, V = 1715.9(7) Å3 and Z = 8.
New electron microprobe analyses indicated thepresence of only Ag, As and S, giving rise to the
Ag2AsS2 chemical formula. However, a full

* E-mail: luca.bindi@unifi.it
DOI: 10.1180/minmag.2013.077.8.05# 2013 The Mineralogical Society

structural study was never carried out and thedervillite structure remains unknown.
In the course of a research project dealing with
Ag-rich minerals from the Lengenbach quarry,
Binn Valley, Canton Valais, Switzerland (Nestola
et al., 2010; Bindi et al., 2010a, 2012a,b, 2013;
Guastoni et al., 2012) we discovered a dervillite
crystal (Fig. 1) in a sample coming from the so-

FIG. 1. SEM-BSE image of dervillite on jordanite.

L. BINDI ET AL.

A chemical analysis using energy dispersive
spectrometry (EDS) performed using a SEM
JEOL-5610 LV at the Museum of Natural
History of Milan showed the presence of Ag, As
and S only, with stoichiometry nearly identical toAg2AsS2.

called zone 0 (Graeser et al., 2008), collected in
1998. This zone is particularly enriched injordanite, galena and Ag-rich sulfosalts such as
marrite, quadratite, proustite and xantoconite.
Here we report the description of the new
occurrence of dervillite, together with thedetermination of its crystal structure.

X-ray crystallography and crystal-structure
determination

Occurrence and chemical data
Dervillite occurs as very rare, well developed
crystals on jordanite (Fig. 1). The mineral does
not show any inclusions of, or intergrowths with,other minerals. The maximum grain size ofdervillite recovered in the sample studied is~200 mm. It is dark grey in colour and shows adark-brown streak. The mineral is opaque intransmitted light and exhibits a metallic lustre. No
cleavage is observed and the fracture is uneven.

T h e s a m e c r y s t a l f ra g m e n t (8 5 mm 6
170 mm6180 mm) used to obtain the EDS
chemical data was selected for the X-ray singlecrystal diffraction study that was done using a
STOE-STADI IV CCD single-crystal diffractometer (Table 1). The mineral was found to be
monoclinic, with a = 9.6155(7), b = 12.9331(8), c= 6.8616(5) Å, b = 99.352(8)º, V = 842.0(1) Å3
and Z = 8 [we preferred to adopt a and c

TABLE 1. Crystallographic data and reﬁnement parameters for dervillite.
Crystal data
Ideal formula
Crystal system
Space group
Unit-cell parameters (Å, º)
Unit-cell volume (Å3)
Z
Crystal size (mm)

Ag2AsS2
Monoclinic
Pc (#7)9.6155(7) 12.9331(8) 6.8616(5), 99.352(8)842.0(1)8
0.08560.17060.180

Data collection
Diffractometer
Temperature (K)Radiation, wavelength (Å)
2y max for data collection (º)
Crystal-detector dist. (mm)h, k, l ranges
Axis, frames, width (º), time per frame (s)Total reflections collected
Unique reflections (Rint)
Unique reflections I > 2s(I)Data completeness to ymax (%)
Absorption correction method

STOE-STADI IV CCD
298(3)
MoKa, 0.71073
75.02
50–15 14, –20 20, –11 11o, 503, 1.00, 20
13927
3788 (0.067)
2370
99.1
X-RED and X-SHAPE

Structure refinement
Refinement method
Data/restraints/parameters
R1 [I > 2s(I)], wR2 [I > 2s(I)]R1 all, wR2 allGoodness-of-fit on F2
Largest diff. peak and hole (e /Å3)

Full-matrix least-squares on F2
2370/0/182
0.0599, 0.1424
0.0884, 0.1592
0.888
3.73, –2.42

Rint = (n/n 1)1/2[F2o Fo (mean)2]/SF2o
R1 = S||Fo| |Fc||/S|Fo|; wR2 = {S[w(F2o F2c )2]/S[w(F2o)2]}1/2;
GooF = {S[w(F2o F2c )2]/(n p)}1/2. Where n is the number of reﬂections and p is the number of reﬁned parameters.

3106

x/a

0.2371(2)0.0416(2)0.6931(2)0.3575(2)0.5650(2)0.9615(2)0.8387(4)0.9610(3)0.3125(2)0.6178(2)0.3096(2)0.6260(2)0.1530(5)0.7918(5)0.1924(5)0.7333(6)0.8474(5)0.5142(5)0.0916(5)0.4272(5)

Atom

Ag1
Ag2
Ag3
Ag4
Ag5
Ag6
Ag7
Ag8
As1
As2
As3
As4
S1
S2
S3
S4
S5
S6
S7
S8

0.5959(1)0.9139(2)0.9071(1)0.8929(1)0.6059(1)0.7898(2)0.4927(2)0.6868(2)0.6854(1)0.6817(1)0.8309(1)0.8216(1)0.5813(3)0.9187(3)0.7455(3)0.7506(3)0.6291(3)0.5672(3)0.8957(4)0.9360(3)y/b0.4229(2)0.7262(3)0.1224(2)0.6293(3)0.8956(3)0.3072(3)0.6518(4)0.8480(5)0.9386(2)0.3513(2)0.1799(2)0.6035(2)0.0500(7)0.4999(6)0.6573(6)0.8916(6)0.4149(7)0.5224(7)0.1043(7)0.0120(7)z/c0.051(1)0.052(1)0.0372(9)0.046(1)0.043(1)0.0360(9)0.109(3)0.031(1)0.0161(8)0.0194(9)0.0204(8)0.0158(8)0.024(2)0.021(2)0.024(2)0.033(3)0.020(2)0.021(2)0.017(2)0.022(2)U110.0282(8)0.044(1)0.0283(7)0.0234(7)0.0251(7)0.068(1)0.072(2)0.073(2)0.0131(7)0.0211(8)0.0183(7)0.0159(7)0.014(2)0.017(2)0.019(2)0.022(2)0.020(2)0.018(2)0.030(2)0.017(2)U220.0254(7)0.0353(9)0.0284(8)0.0425(9)0.045(1)0.057(1)0.068(2)0.113(2)0.0189(8)0.0131(7)0.0126(7)0.0169(8)0.018(2)0.018(2)0.015(2)0.011(2)0.021(2)0.029(2)0.019(2)0.022(2)U33–0.0029(6)0.0075(7)0.0014(6)0.0062(7)0.0067(7)0.027(1)0.048(1)0.016(2)0.0000(5)–0.0003(6)–0.0008(5)0.0020(6)0.003(1)0.001(1)–0.002(1)–0.001(1)0.001(2)–0.003(2)0.004(2)0.001(2)U230.0049(7)–0.0025(8)0.0012(7)0.0145(8)0.0152(8)0.0108(9)–0.004(2)0.008(1)0.0037(6)0.0020(6)0.0027(6)0.0039(6)0.003(2)0.006(2)0.003(2)0.001(2)0.006(2)0.010(2)0.007(1)0.007(2)U13

U12–0.0037(7)0.0149(8)–0.0030(6)–0.0062(7)–0.0036(7)–0.013(1)–0.019(2)0.021(1)0.0021(6)–0.0037(6)–0.0029(6)0.0012(6)–0.002(1)–0.003(1)0.003(2)0.004(2)–0.001(1)–0.006(2)–0.002(2)–0.006(2)TABLE 2. Atoms, fractional atom coordinates (Å) and atomic displacement parameters (Å2) for dervillite.

0.0350(4)0.0449(5)0.0318(4)0.0366(4)0.0367(4)0.0531(6)0.085(1)0.0728(8)0.0160(3)0.0179(3)0.0171(3)0.0160(3)0.0185(8)0.0185(8)0.0193(8)0.0221(9)0.0199(8)0.0221(9)0.0218(8)0.0198(8)Uiso

CRYSTAL STRUCTURE OF DERVILLITE

3107

L. BINDI ET AL.

parameters exchanged with respect to those given
by Bari et al. 1983) see below]. The analysis of
the systematic absences (h0l: l = 2n and 00l: l =2n) led to the choice of the space groups Pc andP2/c, the latter being the space group proposed byBari et al. (1983) (i.e. non standard P2/a in theirorientation of the unit cell). Although thestatistical tests on the distribution of |E| values(|E2 – 1| = 0.793) indicated the absence of an
inversion centre, suggesting the choice of thespace group Pc, the structure was initially solvedin the P2/c space group. A residual R1 = 0.21
value was quickly achieved. However, thepreliminary structural model obtained indicateda large spreading of the silver electron density anda large atomic disorder. The structure model was
subsequently optimized and an ordered model
was sought, but no improvement in R could beachieved. At this point, a thorough analysis of thestructure (essentially based upon the observationof the very large atomic displacement parameters
for particular atoms) suggested that somesymmetry element of the P2/c space groupshould be removed. The reﬂection and atomic
position data sets were then adapted to the Pc
space group (showing the same systematic
extinctions) and the structure reﬁned. Afterseveral cycles, an ordered solution with full site
occupations was ﬁnally determined by carefully
removing atoms with low site occupations and/or
non-realistic distances with neighbouring atoms
and adding signiﬁcant positions found in the
difference Fourier syntheses. The structure could
be smoothly reﬁned in Pc without any damping
factor or restrictions by the program SHELXL(Sheldrick, 2008). The occupancy of all the sites
was left free to vary (Ag vs. &; As vs. &; S vs.
&) but all the positions were found to be fully
occupied. Neutral scattering curves for Ag, As
and S were taken from the International Tables
for X-ray Crystallography (Ibers and Hamilton,
1974). At the last stage, with anisotropic atomic
displacement parameters for all the atoms and no
constraints, the residual value settled at R1 =0.0599 for 2370 observed reﬂections [2s(I) level]and 182 parameters and at R1 = 0.0884 for all
3788 independent reﬂections.
It is noteworthy that the acentric structuralmodel we obtained does not show large values in
the correlation matrix between pairs of atoms
which are equivalent in the centrosymmetricspace group P2/c. To test whether the acentricmodel is preferred to the centric one we also test
the presence of twinning by inversion (twin law:[01̄0 / 01̄0 / 001̄]) in the non-centrosymmetricstructure reﬁnement. Indeed, as is well known, acentrosymmetric structure that is reﬁned as noncentrosymmetric will show a twin scale factor,
equivalent to the Flack parameter in the case of
inversion twinning (Flack et al., 2006; Müller etal., 2006), that reﬁnes to 50% within analyticaluncertainty. We found the racemic twin-component scale factor reﬁned to 0.12(3), consistent
with a highly asymmetrical distribution of theenantiomorphic components and indicating theacentric model as the right choice.
Experimental details and R indices are given in
Table 1. Fractional atomic coordinates and atomic
displacement parameters are reported in Table 2.
Bond distances are given in Table 3.

TABLE 3. Selected bond distances (Å) for dervillite.
Ag1 S1Ag1 S3Ag1 S1Ag1 S6<Ag1 S>2.562(5)2.596(5)2.626(5)2.667(5)2.613

Ag2 S7
Ag2 S2Ag2 S7
Ag2 S3<Ag2 S>2.572(5)2.640(5)2.669(5)2.701(5)2.646

Ag3 S8Ag3 S2Ag3 S2Ag3 S4<Ag3 S>2.573(5)2.614(5)2.635(5)2.638(5)2.615

Ag4 S8Ag4 S3Ag4 S8Ag4 As4<Ag4 M>

2.484(5)2.508(5)2.661(5)2.774(3)2.608

Ag5 S4Ag5 S6Ag5 S6Ag5 As1<Ag5 M>2.478(5)2.479(5)2.577(5)2.696(3)2.558

Ag6 S7Ag6 S5Ag6 S2<Ag6 S>2.438(5)2.516(5)2.809(5)
2.588

3108

Ag7 S5Ag7 S5<Ag7 S>2.387(5)2.410(5)2.399

Ag8 S4Ag8 S1Ag8 S3<Ag8 S>2.403(6)2.522(5)2.861(6)2.595

As1 S3As1 S1As1 As3
As1 Ag5<As1 M>

2.222(5)2.265(5)2.510(2)2.696(3)2.423

As2 S6As2 S5
As2 As4<As2 M>

2.223(5)2.283(5)2.496(3)2.334

As3 S8As3 S7
As3 As1<As3 M>

2.208(5)2.239(5)2.510(2)2.319

As4 S2As4 S4As4 As2
As4 Ag4<As4 M>2.235(5)2.270(5)2.496(3)2.774(3)2.444

CRYSTAL STRUCTURE OF DERVILLITE

Structure factors (Table 4) have been deposited
with the Principal Editor of Mineralogical
Magazine at http://www.minersoc.org/pages/
e_journals/dep_mat_mm.html.
Results and discussion
In the crystal structure of dervillite (Fig. 2) there
are eight Ag positions which adopt various
coordinations ranging from quasi linear to quasi
tetrahedral (Table 3). In detail, one Ag atom (i.e.
Ag7) can be considered to be in linear
coordination, with an overall mean Ag S of
2.40 Å, although the S Ag S angle departs
slightly from 180º [i.e. 173.0(2)º]. Two Ag atoms(i.e. Ag6 and Ag8) may be considered as threefold coordinated, with an overall mean Ag S
distance of 2.59 Å, in good agreement with the
Ag S distance found for the Ag(1) position in the
crystal structure of stephanite, Ag5[S|SbS3](2.54 Å: Ribár and Nowacki, 1970), that found
for the Ag position in the crystal structure ofpyrargyrite, Ag3[SbS3] (2.573 Å: Engel andNowacki, 1966) and those observed for thedifferent polytypes of pearceite (i.e. 2.56, 2.58
and 2.59 Å for the -Tac, -T2ac, and -M2a2b2c
polytypes, respectively; Bindi et al., 2006, 2007).
The last 5 silver atoms (i.e. Ag1, Ag2, Ag3, Ag4
and Ag5) adopt a close-to-tetrahedral coordination. Ag1, Ag2 and Ag3 coordinate four S atoms
with an overall mean Ag S distance of 2.62 Å,
which matches that found for the Ag(3) positionin the crystal structure of stephanite, Ag5[S|SbS3](2.68 Å: Ribár and Nowacki, 1970) and that found
for the Ag(3) polyhedron in the crystal structureof billingsleyite (2.643 Å: Bindi et al., 2010b). In
addition, the value of 2.62 Å for AgS4 comparesreasonably well with the value of 2.66 Å
extrapolated from Shannon’s (1976) tables. On
the other hand, Ag4 and Ag5 coordinate three S

FIG. 2. The crystal structure of dervillite projected down [001]. The vertical direction is the b axis. Ag, As and S aregiven as white, red and yellow spheres, respectively. The unit cell is outlined.

3109

L. BINDI ET AL.

atoms and the tetrahedral conﬁguration iscompleted by an As atom at a distance of~2.7 Å (Table 3).
The As atoms show very peculiar crystalchemical environments. If we plot only the Aspolyhedra along [010] (Fig. 3), it is evident that
the As atoms form columns along the a axis. As1
and As4 are in tetrahedral coordination with two S
atoms at 2.2 2.3 Å, one As at 2.5 Å and one Ag
at ~2.7 2.8 Å. The remaining As2 and As3 atoms
exhibit a ‘pyramidal’ coordination with two S andone As. The values observed for the As As
bonds are in keeping with that observed in lautite(Bindi et al., 2008) and those typically observedin As S molecular compounds (e.g. dimorphite,
realgar, uzonite; Bonazzi and Bindi, 2008).
Table 5 shows the X-ray powder patterncalculated using the structural data obtained inthis study in comparison with those reported byBari et al. (1983). There is an excellent match
between the two patterns except for the ﬁrst two
weak lines (i.e. 5.39 and 3.88 Å) given for thetype material.
Finally the chemical formula is considered.
Bari et al. (1983) proposed to write the formula ofdervillite as As8As4S8 (with Z = 2) because theyspeculated on the possible presence of As4S4molecular groups coexisting with acanthite-like
portions. They hypothesized a unit cell ﬁlled by
8Ag2S + 2As4S4. Such reasoning was driven by
the fact that the Ag2AsS2 formula is not chargebalanced if one considers monovalent silver andtrivalent arsenic. However, the structure solution

presented here demonstrates that there is some‘cation–cation’ bonding (mainly As As and
As Ag) in the coordination polyhedra of some
atoms in dervillite, explaining the apparent charge
imbalance observed.
The presence of dimeric [As2S4]4 ions with acentral As As bond in dervillite could lead us towrite the formula as [Ag +1 ] 4 [As 2 ] 4+ [S 2 ] 4 .
However, it is difﬁcult to analyse such polycationic compounds in strict bond-valence terms,
as the electronegativity of such elements lies
between that of common cations and common
anions. The weak Ag As bonds in dervillite aregood examples of the ‘anionic’ behaviour, which
could be explained through dative donation of the
As lone pair to the closed-shell d10 Ag cations.
Analogies with the dervillite structure can befound in tvalchrelidzeite, Hg3SbAsS3 (Yang et al.,2007), pääkkonenite Sb2AsS2 (Bonazzi et al.,1995) and chalcothallite Tl 2 (Cu,Fe) 6 SbS 4(Makovicky et al., 1980). In all of these structures
either metalloid metalloid or metal metalloid
bonds are present, or even entire antimonide
portions exist.
Acknowledgements
The manuscript beneﬁted from reviews by
Andrew Christy, Peter Leverett and Stuart Mills.
The research was supported by ‘‘progetto
d’Ateneo 2006, University of Padova’’ to FN
and by ‘‘progetto d’Ateneo 2012, University ofFirenze’’ to LB.

FIG. 3. Portion of the crystal structure of dervillite projected down [010]. Only the coordination polyhedra of the As
atoms are depicted. The vertical direction is the c axis. Symbols as in Figure 2. The unit cell is outlined.

3110

CRYSTAL STRUCTURE OF DERVILLITETABLE 5. X-ray powder diffraction patterns for dervillite.
——— 1 ———
hkl
dcalc (Å) Icalc
100
110
–
1̄21
121
–
2̄21
1̄31
211
1̄02
131
012
040
230
300
221
310
140
102
2̄31
022
2̄02
1̄22
112
041
1̄41
320
3̄21
122
141
240
032
050
2̄41
202
212
150
132
3̄12
2̄32
241
222
1̄51
3̄22

9.488
7.650
–
4.39464.0194–3.53453.49923.48473.36623.30073.27493.23333.1904
3.1626
3.15783.07213.06043.03603.01592.99912.99532.98582.95572.91762.84532.84102.76782.74822.73552.67172.66252.58662.56672.56552.51642.49552.48222.47792.45982.41102.38472.37472.3518

5
6
–
5
6
–
17
12
7
15
11
65
19
6
29
8
100
21
12
95
27
18
16
40
23
44
39
9
17
33
5
63
5
18
6
10
5
6
15
10
15
7
5
6

——— 2 ———
hkl
dobs (Å) I/Io

111̄

5.39

1

130
221̄
131̄

3.88
3.53
3.493

1
1
1

112̄

3.251

3

300

3.170

2

310

3.075

10

102, 231̄

3.019

8

112

2.936

1

141̄, 320

2.843

5

321̄
122

2.778
2.742

1
2

032

2.659

3

241̄

2.565

1

312̄, 132

2.475

1

113̄, 420

2.234

1

hkl

——— 1 ———
dcalc (Å)
Icalc

340
3̄41
4̄21
3̄32
302
312
4̄02
341
4̄31
2̄23
052
421
350
033
3̄42
161
260
133
1̄43
261
5̄21
520
170
342
360
441
1̄71
511
004
2̄71
2̄04
432
243
271
3̄62
104
5̄13
1̄63
4̄43
620
4̄71
6̄52
660

2.26092.22352.21822.1786
2.14372.11492.11042.07332.07112.06582.05532.02562.0022
1.9994
1.98991.9873
1.96241.90201.8636
1.85191.83141.82081.8135
1.7867
1.78121.78051.76551.7386
1.6926
1.69251.68311.66831.65121.6455
1.63931.6214
1.57301.56651.5636
1.53601.4551
1.31771.2750

5
5
11
8
20
15
5
7
16
27
11
5
17
8
24
13
5
14
22
8
29
5
7
13
5
12
14
9
7
11
6
7
6
6
5
5
9
6
6
5
5
6
6

——— 2 ———
hkl
dobs (Å) I/Io

213̄, 123̄
402

2.142
2.114

1
1

341

2.070

2

061

2.055

1

033, 342̄

1.992

2

133, 511̄

1.894

1

143̄, 261
521̄

1.855
1.836

1
1

502, 342

1.785

1530, 422

1.739

1

1: calculated powder pattern and indexing for dervillite of this study. d values calculated on the basis of a =9.6155(7) Å, b = 12.9331(8) Å, c = 6.8616(5) Å, b = 99.352(8)º, and with the atomic coordinates reported in Table 2.
Intensities calculated using XPOW software version 2.0 (Downs et al., 1993).
2: observed powder pattern and indexing (converted to the current axes orientation) originally reported by Bari et al.
(1983).

3111

L. BINDI ET AL.

References
Bari, H., Cesbron, F., Moelo, Y., Permingeat, F., Picot,
P., Pierrot, R., Schubnel, H-J. and Weil, R. (1983) La
dervillite, Ag2AsS2, nouvelle déﬁnition de l’espèce.
Bullettin de Minéralogie, 106, 519 524.
Bindi, L., Evain, M. and Menchetti, S. (2006)
Temperature dependence of the silver distribution
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