Structural role of tellurium in the minerals of the pearceitepolybasite group
Transcript of Structural role of tellurium in the minerals of the pearceitepolybasite group
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: [email protected]: 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.
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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).
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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
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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.
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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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