Structural role of tellurium in the minerals of the pearceite-

polybasite group

L. BINDI1,*, P. VOUDOURIS

2AND P. G. SPRY3

1 Dipartimento di Scienze della Terra, Universita degli Studi di Firenze, Via La Pira, 4, I-50121, Florence, Italy2 Department of Mineralogy-Petrology, Faculty of Geology & Geoenvironment, National and Kapodistrian

University of Athens, Panepistimiopolis, G-15784, Athens, Greece3 Department of Geological and Atmospheric Sciences, 253 Science I, Iowa State University, Ames,

Iowa 50011-3212, USA

[Received 27 February 2013; Accepted 21 March 2013; AE: S. Mills]

ABSTRACT

The crystal structure of a Te-rich polybasite has been refined by means of X-ray diffraction data

collected at room temperature (space group P3m1; R = 0.0505 for 964 observed reflections and 94

parameters; refined formula Ag14.46Cu1.54Sb1.58As0.42S9.67Te1.33). The structure comprises stacking of

[(Ag,Cu)6(Sb,As)2(S,Te)7]2� A and [Ag9Cu(S,Te)2(S,Te)2]

2+ B layer modules in which Sb forms

isolated SbS3 pyramids, as occurs typically in sulfosalts, Cu links two S atoms in a linear coordination

and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The silver d10

ions are found in the B layer module along two-dimensional diffusion paths and their electron densities

evidenced by means of a combination of a Gram-Charlier development of the atomic displacement

factors and a split model. The Te-for-S substitution occurs at the same structural sites that Se

substitutes for S in selenopolybasite and the Te occupancy at one of these sites is 0.49, thus suggesting

the possibility that ‘telluropolybasite’ could be found in nature.

KEYWORDS: polybasite, tellurium, crystal structure, ionic conductivity, structural disorder, Kallianou

mineralization, Greece.

Introduction

SULFOSALTS belonging to the pearceite-polybasite

g r o u p e x h i b i t t h e g e n e r a l f o rmu l a

[(Ag,Cu)6M2S7][Ag9CuS4], with M dominated

by As in the case of pearceite or by Sb in the

case of polybasite. The three polytypes occurring

in these minerals (i.e. Tac, T2ac and M2a2b2c)

were recently characterized structurally (Bindi et

al., 2006a,b, 2007a,b,c; Bindi and Menchetti,

2009; Evain et al., 2006a,b). Their structure can

all be described as a regular succession of two-

layer modules stacked along the c axis (Fig. 1): a

first layer module A with composition

[(Ag,Cu)6(As,Sb)2S7]2–, and a layer module B

with composition [Ag9CuS4]2+. The (As,Sb)

forms isolated (As,Sb)S3 pyramids such as

typically occur in sulfosalts, Cu links two S

atoms in a linear coordination, and Ag occupies

sites with coordination ranging from quasi-linear

to almost tetrahedral. In the B layer, the silver

cations are found in various sites corresponding to

the most pronounced probability density function

locations of diffusion-like paths (e.g. Bindi et al.,

2006a).

Although sulfur is the usual anion in members

of the pearceite-polybasite group, large amounts

of Se (Harris et al., 1965; Barrett and Zolensky,

1986) and Te (Warmada et al., 2003; Jelen et al.,

2007; Voudouris and Spry, 2008; Kovalenker et

al., 2011; Voudouris et al., 2011) have been

reported from types of hydrothermal ore deposits.* E-mail: luca.bindi@unifi.itDOI: 10.1180/minmag.2013.077.4.02

Mineralogical Magazine, June 2013, Vol. 77(3), pp. 419–428

# 2013 The Mineralogical Society

The structural role of Se was investigated by

Bindi et al. (2007d) on a crystal from the De

Lamar Mine, Owyhee County, Idaho, USA. The

structural determination proved that Se occupies

specific atomic positions, leading to the definition

of a new mineral species of the group which was

named selenopolybasite (Bindi et al., 2007d). In

contrast, no structural studies have been carried

out on Te-rich crystals, mainly because of the

very small size of the crystals.

During a re-investigation of samples from gold-

bearing veins in metamorphic rocks of the

Cycladic Blueschist Unit, Kallianou deposit,

Evia Island, Greece, which were studied

previously by Voudouris and Spry (2008) and

Voudouris et al. (2011), polybasite grains with

high Te contents (up to 7.4 wt% Te) and of

suitable size for an X-ray investigation were

discovered. Here we report the structural data of

the most Te-rich polybasite crystal yet discovered

and discuss the role of Te in the nomenclature of

the group.

The Kallianou mineralization

The Kallianou district (Fig. 2) is part of the

Attic�Cycladic�Pelagonian ore belt, which

includes base- and precious-metal skarn, intru-

sion-related and epithermal mineralization in

Lavrion (Attika), and the Evia, Sifnos, Mykonos,

Tinos, Kythnos and Milos Islands (Vavelidis and

Michailidis, 1990; Vavelidis, 1997; Skarpelis,

2002; Tombros et al., 2004, 2007; Neubauer,

2005; Spry et al., 2006; Bonsall et al., 2011;

Alfieris et al., 2013). These deposits are spatially

associated with arc-related magmatic rocks, whose

emplacement was partially controlled by exten-

sional kinematic conditions when the meta-

morphic core complexes were uplifted to near-

surface levels (Neubauer, 2005). The Kallianou

mining district is famous for the exploitation of

gold-silver-rich ore during ancient times. The

indicated mineral resource of the Kallianou

deposit is estimated to be 500,000 tonnes at an

average grade of 2.0�2.4% Pb, 0.7% Zn,

0.5�0.8% Cu, 35�60 g/t Ag and 5 g/t Au

(Alexouli-Livaditi, 1978; Katsikatsos, 1978).

Ore minerals in the Kallianou quartz veins

occur in masses (up to 10 vol.%) to dissemina-

tions, filling fractures or cementing brecciated

quartz fragments. The main gangue minerals

include quartz and calcite, whereas wall-rock

alteration consists of chlorite, muscovite, albite

and calcite. Metallic minerals include pyrite,

arsenopyrite, lollingite, sphalerite, chalcopyrite,

tetrahedrite, tennantite, galena, gold, pearceite,

sylvanite, argentite, electrum, native silver, a

cervelleite-like phase, two other members of the

sys tem Ag�Cu�Te�S [Ag2CuTeS and

(Ag,Cu)2TeS], hessite and Te-rich polybasite

(Alexouli-Livaditi, 1978; Vavelidis and

Michailidis, 1990; Voudouris and Spry, 2008).

Te-rich polybasite is considered to be a major

Ag-carrier in Kallianou mineralization. It occurs

together with Zn-rich tetrahedrite as small (up to

30 mm) inclusions in galena that formed after

pyrite (Fig. 3). Polybasite microprobe data

(Voudouris et al., 2011) point to the following

ranges in the chemical formula Ag14.6�15.2Cu1.1�2.0Sb1.6�1.9As0.1�0.4S9.2�9.5Te1.1�1.5,calculated on the basis of 29 atoms. The Cu

content is low (<5.3 wt.% Cu) in accordance with

the hypothesis of Bindi et al. (2007d) that the Cu

FIG. 1. Projection of the Te-rich polybasite structure

along the a axis. The figure emphasizes the succession

of the [(Ag,Cu)6(Sb,As)2(S,Te)7]2� A and [Ag9Cu(S,-

Te)2(S,Te)2]2+ B module layers. Ag, Cu, Sb/As and S are

given as grey, light blue, purple and yellow spheres,

respectively. The most Te-rich position is given in red.

The unit cell is outlined.

420

L. BINDI ET AL.

FIG. 2. (a) Simplified geological map of the Kallianou ore district showing the location of Kallianou district within

the Styra Nappe (modified after Ring et al., 2007) (b) Cross-section through the Cycladic Blueschist Unit on Evia

Island, showing thrust contact at the base of the Styra Nappe and normal fault contact at its top (modified after Ring

and Glodny, 2010).

TE-RICH POLYBASITE

421

FIG. 3. Representative SEM-BSE images of Te-rich polybasite from the Kallianou deposit (concentrations in wt.% of

Te are given in the images).

422

L. BINDI ET AL.

content of pearceite–polybasite group minerals is

very low if selenium and/or tellurium are present.

X-ray crystallography

A Te-rich polybasite crystal was hand-picked

from a polished section, glued to a glass rod and

used for the room-temperature data collection

which was carried out on an Oxford Diffraction

Xcalibur 3 diffractometer, fitted with a Sapphire 2

CCD detector (see Table 1 for details) using

graphite-monochromatized MoKa radiation (l =

0.71069 A). Because of the typical ionic conduc-

tivity observed in these minerals and the probable

presence of twinning, a rather high sin(y)/l cutoff

and a full sphere exploration were considered.

Intensity integration and standard Lorentz-polar-

ization correction were performed with the

CrysAlis RED (Oxford Diffraction, 2006) soft-

ware package. The program ABSPACK in

CrysAlis RED (Oxford Diffraction, 2006) was

used for the absorption correction.

TABLE 1. Details pertaining to the single-crystal X-ray data collection and structure refinement of Te-richpolybasite.

Crystal dataSpace group P3m1 (#164)Cell parameters a 7.6122(5) (A)

c 12.0954(7) (A)V 606.98(7) (A3)

Z 1Crystal colour BlackCrystal shape BlockCrystal size 0.035 mm60.042 mm60.052 mm

Data collectionDiffractometer Oxford Diffraction Xcalibur 3Radiation type MoKa (l = 0.71069)Monochromator oriented graphite (002)Scan mode j/oTemperature (K) 298Detector to sample distance 5 cmNumber of frames 721Rotation width per frame (º) 0.15Measuring time (s) 100Maximum covered 2y (º) 69.88 (d = 0.65 A)Range of h, k, l –12 4 h 4 11, –12 4 k 4 11, –19 4 l 4 19Collected reflections 10587Rint before absorption correction 0.1362Rint after absorption correction 0.0457

RefinementRefinement coefficient F2

F(000) 937No. of reflections in refinement 1237No. of observed reflections 964No. of refined parameters 94Weighting scheme w =1 / [s2(I)+ (0.0446I)2 ]R{(obs) / R{(all) 0.0505/ 0.0783wR2{ (obs) / wR2{ (all) 0.0595 / 0.0617Secondary external coefficient{ 0.02(1)Difference Fourier (e–/A3) [–0.78, 1.01]

{ R = S||Fo|–|Fc|| / S|Fo|. wR2 = [ S w (|Fo|

2 – |Fc|2)2 / S w (|Fo|

4)]1/2;{ Isotropic secondary extinction – Type I � Gaussian distribution (Becker and Coppens, 1974).

TE-RICH POLYBASITE

423

The program JANA2006 (Petrıcek et al., 2006)

was used for the refinement of the structure which

was carried out in the space group P3m1 starting

from the atomic coordinates given by Bindi et al.

(2006a) for the crystal structure of pearceite-Tac.

The sites with partial substitution of S by Te were

easily identified (i.e. S1, S3 and S4). To mimic the

silver (Ag2 and Ag3) electron spreading along

diffusion paths and to model the Te distribution at

the S4 position, up to fourth-order non-harmonic

Gram-Charlier tensors were used for the Debye-

Waller description (Johnson and Levy, 1974;

Kuhs, 1984). Similarly, third- and fourth-order

tensors were introduced for the description of Ag2

and S3 partial substitutions, respectively.

Full site occupation (Sb/As, Ag2/Cu2, S1/Te1,

S3/Te3 and S4/Te4) was assured through

constraints and the overall charge balance was

ascertained. Adding a secondary extinction

coefficient (Becker and Coppens, 1974), the

final residual R = 0.0505 for 964 reflections [I >

2s(I)] and R = 0.0783 for all 1237 unique

reflections and 94 parameters was obtained.

Atomic parameters are reported in Tables 2 to 4.

Bond distances are reported in Table 5.

Unfortunately, the crystal used for the struc-

tural study was lost in an attempt to embed it in

epoxy to get electron microprobe data. However,

the final refined formula can be written as:

Ag14.46Cu1.54Sb1.58As0.42S9.67Te1.33, which is in

good agreement with those reported by Voudouris

et al. (2011), i.e. Ag14.6�15.2Cu1.1�2.0Sb1.6�1.9As0.1�0.4S9.2�9.5Te1.1�1.5.

Description of the structure

On the whole, the Te-rich polybasite structure

resembles that of pearceite (Bindi et al., 2006a). It

can be described as the succession, along the c

axis , of two pseudo-layer modules: a

[(Ag,Cu)6(Sb,As)2(S,Te)7]2� A module layer and

a [Ag9Cu(S,Te)2(S,Te)2]2+ B module layer

(Fig. 1).

In the A module layer, Ag atoms are

triangularly coordinated by S atoms in a quasi

planar environment. As Ag is partially substituted

by Cu (Ag2 position) and S by Te (mainly at the

S3 and S4 positions), we have used a Gram-

Charlier description of the Debye-Waller factor in

the structure refinement. For the same reason, the

calculated distances are only average distances

and are not, therefore, useful pieces of informa-

tion. The (Sb,As) atoms are also in a threefold

coordination, but in a trigonal pyramidal config-

uration. The [AgS3] and [SbS3] subunits are

linked together through corners to constitute the

A module layer.

In the B module layer, the silver d10 cations are

distributed along 2D diffusion paths, in a structure

skeleton made of face-sharing tetrahedra (as in

argyrodite type ionic-conductor compounds;

Boucher et al., 1993) around the Cu atom (see

TABLE 2. Wyckoff positions, site occupation factors, fractional atomic coordinates, and equivalent isotropicdisplacement parameters (A2) for the selected Te-rich polybasite crystal.

Atom Wyckoff s.o.f. x y z Uiso

Ag1 6i 0.68(2) 0.2856(7) 0.1428(3) 0.3895(5) 0.0543(9)Ag2 6i 0.23(3) 0.350(3) 0.175(1) 0.3624(8) 0.068(2)Cu2 6i 0.09 0.350 0.175 0.3624 0.068Ag3 12j 0.294(6) 0.245(2) 0.3644(2) 0.1194(3) 0.094(7)Ag4 12j 0.456(6) 0.3520(8) 0.314(1) 0.1183(2) 0.087(3)Sb 2d 0.79(2) 0.3333 0.6667 0.40556(5) 0.0236(2)As 2d 0.21 0.3333 0.6667 0.40556 0.0236Cu 1a 1.000 0 0 0 0.0322(5)S1 2c 0.91(1) 0 0 0.1820(2) 0.0314(6)Te1 2c 0.09 0 0 0.1820 0.0314S2 6i 1.000 0.0147(2) 0.50738(8) 0.3076(2) 0.0395(5)S3 1b 0.83(2) 0 0 0.5 0.088(4)Te3 1b 0.17 0 0 0.5 0.088S4 2d 0.51(2) 0.6667 0.3333 0.0181(2) 0.049(5)Te4 2d 0.49 0.6667 0.3333 0.0181 0.049

424

L. BINDI ET AL.

Fig. 4). It is worth noting that the modes (maxima

of density) observed in the diffusion paths do not

correspond to the Ag refined positions and that

there are no atomic positions in the connections,

the Gram-Charlier expansion of the Debye-Waller

factor providing the connecting density. For this

TABLE 3. Anisotropic displacement parameters, Uij (A2), for the selected Te-rich polybasite crystal.

Atom U11 U22 U33 U12 U13 U23

Sb0.0244(2) 0.0244(2) 0.0218(3) 0.0122(1) 0 0As

Ag1 0.060(1) 0.0505(7) 0.052(2) 0.0297(6) 0.0032(7) 0.0016(5)Ag2

0.095(4) 0.051(1) 0.080(4) 0.052(2) 0.050(5) 0.025(3)Cu2Ag3 0.21(2) 0.039(1) 0.055(2) –0.020(4) –0.048(5) 0.0050(8)Ag4 0.080(3) 0.289(6) 0.070(3) 0.067(3) 0.005(2) –0.023(3)Cu 0.0394(6) 0.0394(6) 0.0176(7) 0.0197(3) 0 0S1

0.0369(8) 0.0369(8) 0.0204(9) 0.0185(4) 0 0Te1S2 0.0218(6) 0.0393(6) 0.0498(9) 0.0109(3) –0.0039(6) –0.0019(3)S3

0.088(3) 0.088(3) 0.052(7) 0.076(2) 0 0Te3S4

0.0462(6) 0.0462(6) 0.0489(8) 0.0231(3) 0 0Te4

FIG. 4. Non-harmonic joint probability density isosurface of silver for Te-rich polybasite at room temperature. Sulfur

and copper atoms have an arbitrary size. Level of the 3D map: 0.05 A�3. The figure illustrates the silver diffusion in

the ab plane among the various S/Te tetrahedral sites.

TE-RICH POLYBASITE

425

reason, the refined positions should not be used to

calculate distances (although meaningful

distances could be obtained with mode positions).

The S4 atom in the middle of the B module layer,

along with the Cu atom, is the most enriched Te

position [S0.51(1)Te0.49], whereas the S1 atom

linearly coordinating Cu has an occupation of

S0.91(1)Te0.09. In spite of the small amount of

tellurium at the S1 position, the Cu-S1/Te1

distance [2.201(2) A] is longer than both the

Cu-S1/Se1 ’ distance in selenopolybasite

[2.199(2) A � Evain et al., 2006b] and the Cu-

S1 distance in pearceite [2.161(3) A � Bindi et

al., 2006a]. In any case, most of the metal–anion

bond distances for the crystal studied here are

very similar to those in selenopolybasite. This is

due to the fact that a mixed species S0.5Te0.5would show a mean ionic radius of 2.03 A

(Shannon, 1976), a value fairly in agreement with

the ionic radius of pure Se (2.21 A; Shannon,

1976).

Nomenclature remarks

The crystal structure of the Te-rich polybasite is

similar to all compounds of the pearceite/

polybasite group in their higher temperature

form (i.e. 111 pearceite-type structure). The real

difference is the presence of partially substituted

S/Te sites, but these sites are the same where the

Se-for-S substitution occurs in selenopolybasite

(Evain et al., 2006b; Bindi et al., 2007d). In

selenopolybasite, however, Se occupies entirely

the S4 position and this validates it as mineral

species. On the other hand, no Te-dominant sites

are present in the polybasite crystal studied here

and, for this reason, it does not warrant a new

TABLE 4. Higher-order displacement parameters{{ forthe selected Te-rich polybasite crystal.

Ag2 Ag3 S4

C111 0.087(9) 0.019(5) -0.0038(8)C112 0.043(5) 0.105(7) -0.0019(4)C113 0.024(3) –0.004(1) 0.0033(4)C122 0.015(3) 0.20(1) 0.0019(4)C123 0.012(2) –0.007(2) 0.0017(2)C133 0.012(2) –0.0020(5) 0C222 0.001(5) 0.05(4) 0.0038(8)C223 0.007(1) 0.001(4) 0.0033(4)C233 0.006(1) 0.0003(9) 0C333 0.002(1) –0.0022(5) 0.0002(3)

Ag3 S3

D1111 0.050(7) –0.35(2)D1112 0.095(9) –0.173(8)D1113 –0.004(2) –0.026(4)D1122 0.137(13) –0.173(8)D1123 –0.004(2) –0.013(2)D1133 0.0011(5) –0.001(2)D1222 0.14(2) –0.173(8)D1223 –0.002(3) 0.013(2)D1233 0.0011(7) –0.0003(8)D1333 –0.0002(3) 0D2222 –0.35(5) –0.35(2)D2223 0.076(8) 0.026(4)D2233 –0.011(2) –0.001(2)D2333 –0.0010(5) 0D3333 0.0014(4) 0.001(1)

{ Third-order tensor elements Cijk are multiplied by 103;{ Fourth-order tensor elements Dijkl are multiplied by104.

TABLE 5. Main interatomic distances (A) for the selected Te-rich polybasite crystal.

Sb –S2 2.412(2) Cu –S1 2.201(2) Ag1 –S2 2.606(6)–S2 2.412(2) –S1 2.201(2) –S2 2.606(6)–S2 2.412(2) <Cu –S> 2.201 –S3 2.309(5)

<Sb –S> 2.412 <Ag1–S> 2.507Ag2 –S2 2.296(8) Ag3 –S1 2.564(8) Ag4 –S4 2.622(8)

–S2 2.296(8) –S4{ 2.639(9) –S2 2.642(9)–S3{ 2.845(9) –S2 2.805(6) –S1 2.661(6)

<Ag2–S> 2.479 –Cu 2.843(9) –Cu 2.922(9)<Ag3–S> 2.669 <Ag4–S> 2.643

{ the Ag2–S3 and the Ag3–S4 distances correspond to the most probable distance calculated fromthe modes (maxima) of jpdf (joint probability density function) maps.

426

L. BINDI ET AL.

name. Nevertheless, given the large concentra-

tions of Te in S4 (0.49 occupancy; Table 2), it is

very probable that ‘telluropolybasite’ exists in

nature.

Bindi et al. (2007d) noted that relatively high

Se contents are usually associated with low Cu

contents and to disordered trigonal structures (Tac

polytype). This is quite surprising as the copper

content has always been considered as the main

factor inducing structural disorder, and therefore

as the driving force giving rise to a folding of the

cell along the three axes producing the Tac

polytype. Nontheless, it seems that the Se-for-S

substitution induces a structural disorder strong

enough to increase and stabilize the long-range

order symmetry, even in presence of low Cu

contents. The same can be observed in the

presence of Te. Indeed, the total Cu content of

the crystal studied here derived from the structure

refinement is 1.54 a.p.f.u., corresponding to

4.15 wt.% Cu. Thus, it appears reasonable to

suppose that the fully ordered polytype (i.e.

�M2a2b2c) for the Se- and Te-endmembers is

unlikely to be found in nature.

Acknowledgements

The paper benefited from reviews by Andrew

Christy and an anonymous reviewer. Associate

Editor Stuart Mills is thanked for his efficient

handling of the manuscript. This work was

funded, in part, by a grant through the National

Science Foundation (Award #1047671) to PGS.

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