Mineralogical Magazine, June 2015, Vol. 79(3), pp. 687–694

Eckerite, Ag2CuAsS3, a new Cu-bearing sulfosalt from
Lengenbach quarry, Binn valley, Switzerland: description
and crystal structure
L. BINDI1,2,*, F. NESTOLA3, S. GRAESER4, P. TROPPER5 AND T. RABER6
1

Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy
CNR  Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via G. La Pira 4, I-50121 Firenze, Italy
3
Dipartimento di Geoscienze, Università degli Studi di Padova, Via Gradenigo 6, I-35131 Padova, Italy
4
Mineralogisches Institut, Universität Basel, Bernoullistrasse 30, CH-4056 Basel, Switzerland
5
Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52f, A-6020 Innsbruck, Austria
6
FGL (Forschungsgemeinschaft Lengenbach), Edith-Stein-Strasse 9, D-79110 Freiburg, Germany
2[Received 6 October 2014; Accepted 7 December 2014; Associate Editor: A. Christy]
ABSTRACT

Eckerite, ideally Ag2CuAsS3, is a new mineral from the Lengenbach quarry in the Binn Valley, Valais,
Switzerland. It occurs as very rare euhedral crystals up to 300 mm across associated with realgar, sinnerite,
hatchite, trechmannite and yellow, ﬁbrous smithite. In thick section eckerite is opaque with a metallic
lustre and shows a dark orange-red streak. It is brittle; the Vickers hardness (VHN25) is 70 kg/mm2 (range:
6478) (Mohs hardness of ~23). In reﬂected light, eckerite is moderately bireﬂectant and weakly
pleochroic from light grey to a slightly bluish grey. Internal reﬂections are absent. Under crossed nicols, itis weakly anisotropic with greyish to light blue rotation tints. Reﬂectance percentages for Rmin and Rmax
are 27.6, 31.7 (471.1 nm), 22.8, 26.1 (548.3 nm), 21.5, 24.5 (586.6 nm) and 19.4, 22.3 (652.3 nm),respectively.
Eckerite is monoclinic, space group C2/c, with a = 11.8643(3), b = 6.2338(1), c = 16.6785(4) Å, b =110.842(3)º, V = 1152.81(5) Å3, Z = 8. The crystal structure [R1 = 0.0769 for 1606 reﬂections with Fo >4s(Fo)] is topologically identical to that of xanthoconite and pyrostilpnite. In the structure, AsS3 pyramids
are joined by AgS3 triangles to form double sheets parallel to (001); the sheets are linked by Cu(Ag) atoms
in a quasi-tetrahedral coordination. Among the three metals sites, Ag2 is dominated by Cu. The mean
metalS distances reﬂect well the Ag $ Cu substitution occurring at this site.
The eight strongest powder X-ray diffraction lines [d in Å (I/I0) (hkl)] are: 3.336 (70) (3̄12); 2.941 (100)(3̄14,114); 2.776 (80) (400,2̄06); 2.677 (40) (312); 2.134 (50) (4̄21); 2.084 (40) (2̄08,206); 2.076 (40)(420); 1.738 (40) (2̄28,226). A mean of ﬁve electron microprobe analyses gave Ag 52.08(16), Cu 11.18(9),Pb 0.04(1), Sb 0.29(3), As 15.28(11), S 20.73(13), total 99.60 wt.%, corresponding, on the basis of a totalof 7 atoms per formula unit, to Ag2.24Cu0.82As0.94Sb0.01S2.99. The new mineral has been approved by theInternational Mineralogical Association Commission on New Minerals, Nomenclature and Classiﬁcation(2014063) and named for Markus Ecker, a well known mineral expert on the Lengenbach minerals formore than 25 years.
K EY WORDS : eckerite, new mineral, Ag-sulfosalts, Lengenbach, Switzerland.

* E-mail: luca.bindi@unifi.it
DOI: 10.1180/minmag.2015.079.3.13# 2015 The Mineralogical Society

Introduction
THE Lengenbach quarry, the type locality for 32
new mineral species (Roth et al., 2014), exploits a
Triassic dolostone overlying gneiss basement, atthe northern front of the Monte Leone Nappe inL. BINDI ET AL.

sinnerite (Fig. 1). The mineral exhibits a subhedral to anhedral grain morphology, and shows no
inclusions of, or intergrowths with, otherminerals. The maximum grain size of eckerite is~300 mm.
In thin plates, the mineral is transparent and redin colour, it shows an orange-red streak and anadamantine lustre. On the contrary, in thicksections it is opaque in transmitted light andexhibits a metallic lustre. No cleavage is observed
and the fracture is irregular. The calculated
density (for Z = 8) for the empirical formula(see below) is 5.313 g/cm3. Unfortunately, thedensity could not be measured because of thesmall grain size. Micro-indentation measurements
carried out with a VHN load of 25g give a meanvalue of 70 kg/mm2 (range: 6478) corresponding to a Mohs hardness of ~23.
In plane-polarized incident light, eckerite ismoderately bireﬂectant and weakly pleochroic
from light grey to a slightly bluish grey. The
mineral shows no internal reﬂections. Between
crossed nicols, eckerite is weakly anisotropic withgreyish to light-blue rotation tints.
Reﬂectance measurements were performed in
air by means of a MPM-200 Zeiss microphotometer equipped with a MSP-20 system processor
on a Zeiss Axioplan ore microscope. The ﬁlament
temperature was ~3350 K. An interference ﬁlter
was adjusted, sequentially, to select four wavelengths for measurement (471.1, 548.3, 586.6 and652.3 nm). Readings were taken for specimen andstandard (SiC) maintained under the same focus
conditions. The diameter of the circularmeasuring area was 0.1 mm. Reﬂectance percen-

the Penninic domain of the Alps. The rocks weremetamorphosed to upper greenschist–lower
amphibolite facies. The dolostone is 240 m thickat Lengenbach; mineralization occurs in theuppermost part of the sequence, 180 to 200 m
above its base, close to the contact with theoverlying Jurassic to Lower Cretaceous‘Bündnerschiefer’. Hofmann and Knill (1996)
recognized four major types of mineralization;
stratiform layers of pyrite, with minor galena,
sphalerite and xenomorphic sulfosalts; massive tointerstitial sulfosalt accumulations; discordantsulfosalt and sulﬁde veins; and idiomorphic
minerals within druses and open ﬁssures.
Eckerite was observed in the second type ofmineralization, corresponding to the ‘‘Zone 1’’ ofGraeser et al. (2008), on a sample with the ofﬁcial
number L-27381 of the AGL [Arbeitsgemeinschaft Lengenbach (Lengenbach Working
Association), active from 1958 until 1997]. The
dolomite in this zone shows a very saccharoidal
texture. It hosts the largest pyrite concentration
and baumhauerite, sartorite and realgar are themain arsenic mineral phases present. Minerals
associated with eckerite are realgar, sinnerite,
hatchite, trechmannite and yellow, ﬁbroussmithite.
Eckerite was approved as a new mineral by theCommission on New Minerals, Nomenclature andClassiﬁcation of the International Mineralogical
Association (2014-063, Bindi et al., 2015). The
mineral name honours Markus Ecker (b. 1966), awell known mineral expert on the Lengenbachminerals for more than 25 years. He has foundseveral new mineral species from Lengenbachand has published many articles about the quarry
and its minerals. He is currently a Member of theTechnical Commission of the Forschungsgemeinschaft Lengenbach (FGL, Lengenbach
Research Association).
The holotype material is deposited in themineralogical collections of the Museo di Storia
Naturale, Università degli Studi di Firenze, Italy(under catalogue number 3144/I) and at the
Natural History Museum, University of Basel,
Switzerland (under catalogue number S169).
In this study we report the description of the
new mineral eckerite together with the determination of its crystal structure.
Physical and optical properties
Eckerite occurs as very rare crystals growing on
dolomite, sometimes growing on smithite or

FIG. 1. Eckerite (reddish-brown) in association withyellowish, ﬁbrous smithite and realgar.

688

ECKERITE, Ag2CuAsS3, A NEW Cu-BEARING SULFOSALT FROM LENGENBACH QUARRY

TABLE 1. Data and experimental details for the selected eckerite crystal.
Crystal data
FormulaCrystal size (mm)Form
ColourCrystal system
Space groupa (Å)b (Å)c (Å)b (º)
V (Å3)Z

Ag2CuAsS3
0.08060.09060.110
block
red
Monoclinic
C2/c (#15)
11.8643(3)6.2338(1)16.6785(4)110.842(3)1152.81(5)8Data collection
Instrument
Radiation type
Temperature (K)Detector-to-sample distance (cm)Number of frames
Measuring time (s)Maximum covered 2y (º)
Collected reflections
Unique reflections
Reflections with Fo > 4s(Fo)Rint
Rs
Range of h, k, l

STOE-STADI IV CCD
MoKa (l = 0.71073 Å)
293(3)
5
1563
40
69.26
17,4452488
1606
0.05810.0355
18 4 h 4 18, 10 4 k 4 10, 26 4 l 4 26

Refinement
Refinement
Final R1 [Fo > 4s(Fo)]Final R1 (all data)
Number refined parameters
Drmax (e Å3)
Drmin (e Å3)

Full-matrix least squares on F2
0.07690.0825
67
3.76
3.59

tages for Rmin and Rmax are 27.6, 31.7 (471.1 nm),
22.8, 26.1 (548.3 nm), 21.5, 24.5 (586.6 nm) and
19.4, 22.3 (652.3 nm), respectively.
X-ray crystallography and crystal-structure
determination
A crystal fragment (80 mm690 mm6110 mm)
was selected for the X-ray single-crystal diffraction study that was undertaken with a STOESTADI IV CCD single-crystal diffractometer(Table 1). Data were integrated and corrected
for standard Lorentz polarization factors with theCrysAlis RED (Oxford Diffraction, 2006) software package. The program ABSPACK in
CrysAlis RED (Oxford Diffraction, 2006) was
689

used for the absorption correction. The observed
reﬂection conditions (hkl: h+k = 2n; h0l: h,l = 2n;0kl: k = 2n; hk0: h+k = 2n; 0k0: k = 2n; h00: h =2n; 00l: l = 2n), together with the statistical tests
on the distribution of |E| values that strongly
indicated the presence of an inversion centre (|E2
 1| = 0.896), pointed to the choice of the spacegroup C2/c. The structure solution was theninitiated in this space group. The positions ofmost of the atoms (all the metal positions andmost of the S atoms) were determined by meansof direct methods (Sheldrick, 2008). A leastsquares reﬁnement on F2 using these heavy-atom
positions and isotropic temperature factors
produced an R factor of 0.134. Three-dimensional
difference Fourier synthesis yielded the positions

L. BINDI ET AL.

TABLE 2. Atom, site occupancy factors (s.o.f.), fractional atom coordinates and equivalent isotropic atomic
displacement parameters (Å2) for eckerite.
Atoms.o.f.

x/a

y/b

z/c

Ueq

Ag1
Ag2
Ag3
As
S1
S2
S3

Ag0.84(2)Cu0.16
Ag0.37(2)Cu0.63
Ag1.00As1.00
S1.00
S1.00
S1.00

0.1431(2)0.0586(2)0.3596(1)0.3536(1)0.1513(3)0.4260(3)0.1573(3)0.0350(3)0.9218(5)0.0264(3)0.0463(2)0.1797(5)0.1404(6)0.1253(5)0.21371(9)0.4328(2)0.1354(2)0.43332(8)0.0765(2)0.3294(2)0.3600(2)0.0668(8)0.084(1)0.0776(7)0.0258(3)0.0372(7)0.0347(7)0.0343(7)of the remaining sulfur atoms. The program
SHELXL (Sheldrick, 2008) was used for thereﬁnement of the structure. The occupancy of all
the sites was left free to vary (Ag vs. Cu for theAg sites; As vs. Sb; S vs. vacancy). The
occupancy of Ag3, As and the S sites was foundto be consistent with a pure occupation by Ag, As
and S, respectively, and then ﬁxed. Neutral
scattering curves for Ag, Cu, As, Sb and S weretaken from the International Tables for X-ray
Crystallography (Ibers and Hamilton, 1974). At
the last stage, with anisotropic atomic displacement parameters for all atoms and no constraints(except the named occupancies), the residual
value settled at R1 = 0.0769 for 1606 observed
reﬂections [Fo > 4s(Fo) level] and 67 parameters
and at R1 = 0.0825 for all 2488 independent
reﬂections. Inspection of the difference Fourier
map revealed that maximum positive and negative
peaks were 3.76 and 3.59 e/Å3, respectively.
Experimental details and R indices are given in
Table 1. Fractional atomic coordinates andisotropic displacement parameters are reported
in Table 2 whereas the anisotropic displacement
parameters are given in Table 3. Bond distances

are given in Table 4. The calculated powder
X-ray diffraction pattern, computed with the atom
coordinates and occupancies reported in Table 2,
is given in Table 5 together with the measured
pattern (114.6 mm diameter Gandolﬁ camera,
FeKa radiation). Unit-cell parameters reﬁned
from the powder data are: a = 11.871(6), b =6.2251(3), c = 16.635(7) Å, b = 110.89(2)º, V =1158.6(5) Å3.
Structure factors (Table 6) have been deposited
with the Principal Editor of Mineralogical
Magazine and are available at
http://www.minersoc.org/pages/e_journals/
dep_mat_mm.html.
Chemical composition
A preliminary chemical analysis using energy
dispersive spectroscopy performed on the crystal
fragment used for the structure study did notindicate the presence of elements (Z > 9) otherthan Ag, Cu, As and S. The chemical composition
was then determined using wavelength dispersive
analysis (WDS) by means of a JEOL 8100
SUPERPROBE electron microprobe. Major and

TABLE 3. Anisotropic atomic displacement parameters (Å2) for eckerite.

Ag1
Ag2
Ag3
As
S1
S2
S3

U11

U22

U33

U12

U13

U23

0.070(1)0.047(1)0.0335(7)0.0252(5)0.033(2)0.032(2)0.024(1)0.088(2)0.119(2)0.0460(8)0.0247(6)0.024(1)0.041(2)0.031(1)0.0403(9)0.069(2)0.144(2)0.0275(6)0.054(2)0.034(2)0.048(2)0.0109(9)0.030(1)0.0004(5)0.0022(4)0.001(1)0.001(1)0.001(1)0.0159(7)0.0009(9)0.0193(8)0.0095(4)0.015(1)0.017(1)0.012(1)0.0060(7)0.053(1)0.0002(9)0.0013(4)0.005(1)0.004(1)0.001(1)690

ECKERITE, Ag2CuAsS3, A NEW Cu-BEARING SULFOSALT FROM LENGENBACH QUARRY

TABLE 4. Selected bond distances (Å) for eckerite.
Ag1 S3
S1
S2
mean

2.451(4)2.494(4)2.612(4)2.519

Ag2 S3
S1
S2
S2
mean

2.338(4)2.341(4)2.568(4)2.922(5)2.542

Ag3S2S1
S3mean

TABLE 5. Measured and calculated powder X-ray
diffraction data for eckerite.

hkl
1̄11
112
202
004
3̄12
310
3̄14

— (a) —dmeas
Iest

— (b) —
dcalc
Icalc

5.444
4.148

10
10

5.43414.13223.9083

6
20
5

3.893

20

3.336
3.195

70
10

3.89673.3204
3.17922.9438

20
38
8
100

2.941

100
2.93372.772097742.75962.67162.60112.36022.12852.0847

46
26
11
8
19
17

2.08462.0714
1.954121
29
12

1.9484
1.94471.89151.859821220
161.8481
1.732917
91.73281.591517
91.58971.57271.2020

9
17
4

114
400
2̄06
312
2̄23
4̄06
4̄21
2̄08
206
420
404
008
132
2̄27
133

2.776

80

2.677
2.606
2.348
2.134

40
20
10
50

2.084

40

2.076

40

1.949

20

1.893

10

1.851

10

6̄06
2̄28
1.738

6̄26
1̄110
626

1.595

30

1.573
1.204

10
10

As S3
S2
S1
mean

2.266(3)2.268(3)2.291(3)2.275

minor elements were determined at 20 kV
accelerating voltage and 10 nA beam current,
1 mm beam size and with counting times of 20 s
for peak and 10 s for background. For the WDS
analyses the Ka lines for all the elements wereused. The standards employed were: Ag-pureelement (Ag), Cu-pure element (Cu), galena (Pb),Sb-pure element (Sb), As-pure element (As) andtroilite (S). The crystal fragment was found to behomogeneous within analytical error. The averagechemical compositions (ﬁve analyses on different
spots) together with wt.% ranges of elements arereported in Table 7. On the basis of 7 atoms, theempirical formula of eckerite is Ag2.24Cu0.82
As0.94Sb0.01S2.99, which is in excellent agreementwith that obtained from the structure reﬁnement,
Ag2.21Cu0.79AsS3. The simpliﬁed formula is
Ag2CuAsS3, which requires Ag 47.90, Cu 14.11,
As 16.63, S 21.36, total 100.00 wt.%.
Results and discussion

40

226
227

2.502(4)2.501(4)2.512(4)2.505(a) = measured Gandolﬁ diffraction pattern (FeKa); (b) =diffraction pattern calculated for FeKa using the atom
coordinates and site occupancies reported in Table 2.

Description of the structure
The crystal structure of eckerite (Fig. 2) is
topologically identical to that of xanthoconite(Engel and Nowacki, 1968) and pyrostilpnite(Kutoglu, 1968), with one of three independent
Ag positions dominated by Cu (i.e. Ag2). In the
structure, AsS3 pyramids are joined by AgS3
triangles to form double sheets parallel to (001);
the sheets are linked by the metal position
dominated by copper in a quasi-tetrahedral
coordination. The mean bond distances for thethree-coordinated Ag-polyhedra (Ag1 and Ag3)
are in the range 2.452.61 Å (Table 4), whichmatches well those observed for triangularlycoordinated Ag atoms in the different polytypes
o f t h e p e a r c e i t e p o l y b a s i t e m i n e r a l s(2.432.62 Å, Bindi et al., 2006, 2007; Evain etal., 2006). The Ag2 position dominated by Cu ineckerite shows a quasi-tetrahedral coordination
with a mean bond distance of 2.542 Å comparedto 2.674 Å in Cu-free xanthoconite (Engel andNowacki, 1968). The ordering of copper mainly at

691

L. BINDI ET AL.

TABLE 7. Electron microprobe data (means and ranges in wt.% of elements),
atomic ratios (on the basis of 7 atoms per formula unit) with their standard
deviations (s) for the selected eckerite crystal.

Ag
Cu
Pb
Sb
As
S
Total

Wt.%

Range

Atomic ratios

S

52.08
11.18
0.04
0.29
15.28
20.73
99.60

50.6352.83
10.7311.82
0.000.10
0.220.34
15.1515.63
20.3021.05
99.4499.82

2.24
0.82
0.00
0.01
0.94
2.99

0.16
0.09
0.01
0.03
0.11
0.13

the Ag2 site in the eckerite structure reﬂects thepreference of Cu for tetrahedral environments.
Indeed, copper is usually hosted in tetrahedra inCu sulfosalts [e.g. enargite (Karanovic et al.,2002); luzonite (Pﬁtzner and Bernert, 2004);
sinnerite (Bindi et al., 2013)]. The shortest
AgAg contact in the eckerite structure is
Ag1Ag2 at 2.835 Å, slightly shorter than thatfound in fcc Ag metal (2.889 Å; Suh et al., 1988).
Bond-valence sums calculated from the data of
Brese and O’Keeffe (1991) are reported inTable 8. The valence sums obtained are in nearperfect agreement with the cation populations
assumed in the present study.

Eckerite as a Cu-rich member of thexanthoconitepyrostilpnite series
As is well known, Ag-Cu sulfosalts are an
important source of silver and occur in a variety
of hydrothermal ore deposits, in particular ﬁssurevein polymetallic base-metal sulﬁde deposits (e.g.
Loucks and Petersen, 1988; Chutas and Sack,
2004). Among these sulfosalts, some of the mostimportant are the proustite (Ag3AsS3)–pyrargyrite(Ag3SbS3) solid solution series, together with thatbetween xanthoconite and pyrostilpnite, the rare
low-temperature polymorphs (e.g. Lange et al.,1993). Eckerite is structurally identical to these
two latter minerals.

FIG. 2. The crystal structure of eckerite projected down [010]. The horizontal direction is the a axis. Ag, Cu (Ag2position), As and S are given as white, red, black and yellow spheres, respectively. The unit cell is outlined.

692

ECKERITE, Ag2CuAsS3, A NEW Cu-BEARING SULFOSALT FROM LENGENBACH QUARRY

TABLE 8. Bond valence (vu) calculation for eckerite*.
Atom

Bond valence

Ag1
Ag2
Ag3
As
S1
S2
S3

0.978(5)1.003(5)1.057(5)2.977(4)2.113(5)1.944(5)2.138(5)the geochemical distinctiveness of theLengenbach deposit, able to produce rare minerals
not discovered in other deposits.
Acknowledgements* Calculated from the bond-valence parameters of Breseand O’Keeffe (1991). For Ag1 and Ag2 sites theweighted sums Ag1xCux (x = occupancy of Cu) of thebond-valence parameters were used.

Given the importance of these minerals in
economic geology, a lot of attention has been paid
to the mixing properties of these solid solutions.
Bindi et al. (2010) have studied the crystallographic and chemical constraints on the proustite–pyrargyrite solid solution series by evaluating 32
natural samples of proustite and 27 samples ofpyrargyrite. Interestingly, no Cu was detected in
all the samples investigated. The same has been
observed for the minerals of the xanthoconite–
pyrostilpnite series, where Cu is only reported as a
trace element (Gaines et al., 1997). The lack ofcopper in minerals of the system proustite–pyrargyrite has been explained with thermodynamic considerations by Harlov and Sack (1995)
and Harlov (1999). On the one hand, these authors
showed that mixing on the AsSb site is nonideal and symmetric, and that complete solid
solution occurs down to ~105ºC, and perhaps as
low as 90ºC. On the other hand, they also pointed
out that the mixing on the AsSb site inproustite–pyrargyrite is more symmetric andmore ideal than that for mixing on CuAg sites,
where it is both asymmetric and signiﬁcantly nonideal (Harlov and Sack, 1995; Harlov, 1999). The
studies by Harlov (1999) reinforce the earlier
investigation by Harlov and Sack (1995) thatshows the greater stability of Ag when comparedto Cu on the AgCu sites for both proustite andpyrargyrite. Although eckerite belongs to thexanthoconitepyrostilpnite series, we think thatsimilar thermodynamic considerations can beapplied. This would explain the rarity of eckerite
and would justify why the mineral had not been
discovered until now. The exceptional conditions
needed to form such a unique phase emphasize

The paper beneﬁted from reviews by Tonči BalićŽunić, Peter Leverett and one anonymous
reviewer. Associate Editor Andrew G. Christy isthanked for efﬁcient handling of the manuscript.
Martina Tribus is thanked for help with microprobe analysis. The research was supported by‘‘progetto d’Ateneo 2012, University of Firenze’’
to LB.
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