RMS DPI 2008-2-41-1

THE PEARCEITE- GROUP OF : AN OUTSTANDING EXAMPLE OF THE CLOSE LINK BETWEEN MINERALOGY AND THE MOST ADVANCED FIELDS OF CRYSTALLOGRAPHY

Bindi L.

Natural History Museum, Division of Mineralogy, Univ. Florence, Italy [email protected]

Introduction Sulfosalts belonging to the pearceite-polybasite group are relatively common in nature and were originally discovered in the 19th century, while the new names antimonpearceite and arsenpolybasite were introduced by Frondel in 1963 [1]. The names pearceite and polybasite are grandfathered whereas the names antimonpearceite and arsenpolybasite were approved by the IMA Commission. According to Frondel [1], these four minerals can be divided on structural basis into two series: the first one, formed by pearceite (Ag,Cu)16(As,Sb)2S11 and antimonpearceite (Ag,Cu)16(Sb,As)2S11, characterized by “small” unit-cell (labelled 111) and high Cu content, and, the second one, formed by polybasite (Ag,Cu)16(Sb,As)2S11 and arsenpolybasite (Ag,Cu)16(As,Sb)2S11, with double cell parameters (labelled 222) and low Cu content. Soon after Frondel’s study, Harris et al. [2] pointed out the existence of an intermediate type of unit-cell labelled 221 for a sample of polybasite from the Las Chispas deposit, Sonora, Mexico. They observed that the intermediate cell 221 was present in some areas of the sample analyzed and was closely associated with the small cell 111, which is present in other seemingly identical areas of the same sample. The same intermediate type of unit-cell (i.e., 221) was subsequently reported by Edenharter et al. [3] and by Minčeva-Stefanova et al. [4]. Phase relations for the system (Ag,Cu)16(As,Sb)2S11-(Ag,Cu)16(Sb,As)2S11 were investigated experimentally by Hall [5] who hypothesized that the variation of Cu content in different samples might play a key role in favouring chemical order-disorder phenomena and, therefore, to be the driving force in stabilizing the different unit-cells. From a chemical standpoint, the members of both series are generally pure containing only minor amounts of Bi, Pb, Zn, and Fe. However, Harris et al. [2] reported the occurrence of an antimonpearceite sample from the San Carlos mine, Guanajuato, Mexico, containing up to 8.7 wt% of Se but limited their study to the determination of the unit-cell parameters and chemical characterization only. Several authors [1,2,5,6,7] considered the members of the pearceite-polybasite group as monoclinic, space group C2/m, although dimensionally pseudo-hexagonal. On the other hand, Edenharter et al. [3] settled the Laue symmetry as trigonal -3m. Despite the fact that these

140 minerals are relatively common in nature, their had yet to be characterized.

Crystal structure of the members having the 111 unit-cell type The careful structural studies performed on minerals of the pearceite- polybasite group suggest that the reason for the lack of structural information for these complex minerals is probably related to the difficulty in describing the Ag or Cu electron density, which is influenced by the strong ionic conductivity of these minerals. It was shown that for such a description, a non-harmonic model based upon a development of the atomic displacement factor is the best approach to solving the structure [8-11]. The crystal structures of members of the pearceite-polybasite group that posses the 111 unit-cell type (i.e., pearceite [12] and antimonpearceite [13]) have been solved and refined at 300 K in the space group type P-3m1. The refinement of the structure leads to residual factors of R = 0.0464 (pearceite) and R = 0.0405 (antimonpearceite). Basically, their crystal structure can be described as a regular alternance of two types of layer stacked along [001]: layer “A”, with 2- the general composition [(Ag,Cu)6(As,Sb)2S7] , and layer “B” with the general 2+ composition [Ag9CuS4] . In this structure, atoms of As and Sb form isolated (As,Sb)S3 pyramids that are common in sulfosalts, cations link two atoms in a linear coordination, and cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. These positions correspond to low coordination (2, 3 and 4) sites, in agreement with the silver preference for such environments. The distribution of d10 silver ions was deduced by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split model. We also refined the crystal structure of a Se-rich variety of antimonpearceite (later approved by IMA as selenopolybasite, [(Ag,Cu)6(Sb,As)2(S,Se)7][Ag9Cu(S,Se)2Se2], [14]) up to R = 0.0489 [15]. At room temperature, it shows the same structural arrangement found in pure antimonpearceite but with one Se atom ordered in a specific structural position.

Crystal structure of the members having the 221 unit-cell type The crystal structures of members of the pearceite-polybasite group that posses the 221 unit-cell type (i.e. arsenpolybasite-221 [13] and polybasite-221 [16]) have been solved and refined in the space group type P321, with twinning (first-degree twin, 3'2 / m'1 polychromatic point group, [17] and references therein). At the beginning, the data collections (full diffraction sphere) were carried out at room temperature. In the P321 space group type, the refinements smoothly converged toward residual R ≈ 0.07 values (for both arsenpolybasite- and polybasite-221), however, large positive residues (up to 4 e-/Е3) were 2- observed almost on top of all heavy atoms of the [(Ag,Cu)6Sb2S7] A pseudo 2+ layer and in the vicinity of Ag atoms of the [Ag9CuS4] B pseudo layer. Those

141 residues could be taken into account by introducing a stacking fault, that is, the presence of a minor component (ca 10%) related to the cell content by an inversion centre (introduced in Jana2000 with rigid bodies), but not by the introduction of an inversion twin law. However, although without any significant residues left in the difference Fourier synthesis maps, the correlations were still very high and did not allow the refinement convergence (even with a very small damping factor). New data collections were then performed on the same crystals and the same diffractometer at 120K. The refinements largely improved with a residual R value of ≈ 0.04 and without the presence of significant positive residues in the difference Fourier synthesis. The refinement of the structure leads to residual factors of R = 0.0531 (arsenpolybasite-221) and R = 0.0359 (polybasite-221). The structure of the 221 members is obtained from the polybasite-222 structure (see below) by selecting half its cell content along the c axis, that is either the A-B (or A'-B') double module layer or the A/2-B- A'/2 (or A'/2-B'-A/2) double module layer, since A and A' approximately correspond to each other by a Ѕ translation along the c axis. The selected double module layer approximately fulfils a trigonal symmetry, space group type P321, except for one sulfur atom which appears as disordered (out of the threefold axis) in the 221- structure. The coordination of the atoms is very similar to that for members having the 111 unit-cell type. The reason for doubling the a parameter is linked to the ordering of silver.

Crystal structure of the members having the 222 unit-cell type The crystal structures of members of the pearceite-polybasite group that posses the 222 unit-cell type (i.e., arsenpolybasite-222 [13] and polybasite-222 [16]) were solved and refined at 300 K in the space group type C2/c, with a second-degree twin with the polychromatic point group: (6) ⎛ 6()6 2()2,2 2()2 ⎞ (p) ⎜ ⎟ K WB = ⎜ ()2 ()2,2 ()2 ⎟ ⎝ m m m ⎠ ([17] and references therein). The refinement of the structure leads to residual factors of R = 0.0700 (arsenpolybasite-222) and R = 0.0500 (polybasite- 222). The structure of the 222 members can be described as a succession along 2- the c axis of two module layers: the [(Ag,Cu)6Sb2S7] A (or A') module layer 2+ and the [Ag9CuS4] B (or B') module layer (A/B and A'/B' being related by a c glide mirror symmetry operation). The coordination of the atoms is very similar to that for members having the 111 unit-cell type. The reason for doubling the unit-cell parameters is linked to the ordering of silver.

Conductivity and thermal experiments We have also carried out an integrated XREF, DSC, CIS, and EPMA study on all the members of the pearceite-polybasite group [18]. DSC and conductivity measurements pointed out that the 222 members show ionic-transitions at 340 K (arsenpolybasite-222) and 350 K (polybasite-222), whereas the 221 members have transitions at lower temperature, that is, 310-330 K (arsenpolybasite-221) 142 and 335 K (polybasite-221). For the 111 members (pearceite and antimonpearceite), the transition occurs below room temperature at 273 K. In- situ single-crystal X-ray diffraction experiments showed that these minerals present the same high temperature structure and are observed at room temperature either in their high temperature (HT) fast ion conductivity form or in one of the low temperature (LT) fully ordered (222), partially ordered (221) or still disordered (111) forms, with transition temperatures slightly above or below room temperature. The pearceite-polybasite group of minerals can be thus considered as a homogeneous series with the same aristotype, fast ion conducting form at high temperature. It seems that the higher the Cu content, the more likely is the structure to be of the 221 type. As is generally the case, the higher the disorder, the higher the apparent symmetry. With a low substitution of Cu for Ag, the is monoclinic with a double c period (i.e., 222 structures). However, the AB (A'B') double layer module has a pseudo trigonal symmetry, which explains the twinning and the apparent hexagonal cell. By increasing the extent of the substitution of Cu for Ag, the cell doubling along the c direction is lost, and the average symmetry becomes trigonal (i.e., 221 structures). A further increase of the disorder gives rise to a folding of the cell along the a and b directions, as found in the 111 structures. Our findings support the idea of Hall [5], who suggested that the variation of Cu content in different samples might be the driving force of different unit-cell stabilizations.

Changes to the existing nomenclature Several authors [2,19,20] pointed out problems associated with the classification scheme of Frondel [1]. Harris et al. [2], for example, noted that the doubled dimensions, manifested as weak intermediate layer lines on rotation photographs, represented less-than-fundamental differences; the basic structural unit and the external crystal form are the same for all the members of the pearceite-polybasite group. Therefore, these authors [2] proposed to maintain the original classification with the addition of a symbol to signify the type of the cell. Pearceite (1-1-1) has As > Sb and unit-cell parameters of the basic structural unit (i.e., a ~ 13, b ~ 7.5, c ~ 12 Е, β ~ 90°), polybasite (2-2-2) has Sb > As and unit-cell parameters doubled with respect to the basic structural unit (i.e., a ~ 26, b ~ 15, c ~ 24 Е, β ~ 90°), whereas polybasite (2-2-1) has a and b doubled with respect to the basic structural unit (i.e., a ~ 26, b ~ 15, c ~ 12 Е, β ~ 90°). More recently, Guinier et al. [20], in a report for the International Union of Crystallography dealing with the nomenclature of polytype structures, proposed a new polytype symbolism for minerals belonging to the pearceite-polybasite group. Since these minerals had been previously described as exhibiting a strongly pseudohexagonal symmetry, Guinier et al. [20] suggested adding a hyphenated italic symbol, such as ψH where the Greek letter ψ is the abbreviation of pseudo-, to the root names (i.e., pearceite and polybasite). Thus,

143 pearceite would be written as pearceite-ψHabc and arsenpolybasite as pearceite- ψH2a2b2c. Despite Guinier et al.’s proposal, systematic mineralogy books such as Strunz and Nickel [21] and Mandarino and Back [22] still maintain the classification scheme of Frondel [1] for minerals of the pearceite-polybasite group. This is in large part due to the fact that no complete crystal structure characterization of this group has yet been published in the literature. We have reported that minerals in the pearceite-polybasite group exhibit three unit-cell types (i.e., 111, 222, and 221) that correspond to three separate structures (space group types P-3m1, P321, and C2/c for 111, 221, and 222, respectively). Hence, it is evident that only careful crystallographic studies permit the characterization of unit-cell type. Moreover, as Harris et al. [2] pointed out, unit-cell types may be different in various parts of the same sample. Therefore, without precise X-ray data the most practical nomenclature is to use only two names, pearceite and polybasite, since their compositions are easily obtained by microprobe analysis (As/Sb ratio). If crystallographic data are available (it is sufficient to determine unit-cell parameters without carrying out a complete crystal structure refinement), a hyphenated italic suffix, indicating the crystal system with the cell-type symbol, should be added to the names pearceite or polybasite. With the introduction of these new nomenclature rules we also avoid the problem of requiring two new names for species possessing the 221 unit-cell type. In short, the old names antimonpearceite and arsenpolybasite are abandoned and the former names pearceite and polybasite, previously defined on structural basis (i.e., 111 and 222), are redefined on chemical basis. The crystal structures of all members of the pearceite-polybasite group allow them to be considered as a family of polytypes, since they are built up by layers stacked along the c axis of nearly identical structure and composition. In addition to the revision in mineral names it is also recommended that the formulae of these minerals be changed. All crystal structures of pearceite-polybasite group 2- minerals consist of two different layers: layer A [(Ag,Cu)6(As,Sb)2S7] , and 2+ layer B [Ag9CuS4] . Therefore, chemical formulae should be written as [Ag9CuS4][(Ag,Cu)6(As,Sb)2S7] and [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7] instead of (Ag,Cu)16(As,Sb)2S11 and (Ag,Cu)16(Sb,As)2S11 for pearceite and polybasite, respectively. The new nomenclature rules were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association and published by Bindi et al. [23].

Two new members of the group: cupropearceite and cupropolybasite We have just seen that the pearceite-polybasite group of minerals, general IMA-approved formula [M6T2S7][Ag9CuS4] with M = Ag, Cu and T = As, Sb, show a crystal structure which can be described as the succession, along the c 2- axis, of two pseudo-layer modules: a [M6T2S7] A module layer and a 2+ [Ag9CuS4] B module layer. Cu is present in one structural position of the B 144 module layer and replaces Ag in the only fully occupied M position of the A module layer. When the Cu content is > 4.00 a.p.f.u., the structural position of the A module layer becomes Cu-dominant and, consequently, the mineral deserves its own name. Bindi et al. [24] have reported the crystal-chemical characterization of two Cu-rich members exhibiting the 111 unit-cell type (corresponding to the polytype -Tac). One sample [space group P-3m1, a 7.3218(8), c 11.8877(13) Е, V 551.90(10) Е3, Z = 1] having As > Sb and the structural position of the A module layer dominated by copper, has been named cupropearceite and the other sample [space group P-3m1, a 7.3277(3), c 11.7752(6) Е, V 547.56(8) Е3, Z = 1] having Sb > As has been named cupropolybasite. Both the new minerals and mineral names have been approved by IMA-CNMNC.

TEM studies A careful electron diffraction investigation of pearceite-Tac and polybasite- + 9+ Tac samples in the Ag fast ion conducting state [25] revealed that the [Ag9] sub-structure in these mineral solid electrolyte systems is mutually 2- 7- incommensurable with respect to the remaining [(Ag,Cu)6(As,Sb)2S7] [CuS4] framework sub-structure. It is shown how the mutual incommensurability of the two component sub-structures is compatible with crystal chemical commonsense, with previously reported average structure refinements of these phases as well as providing a simple explanation for their Ag+ fast ion conductivity at such low temperatures. In addition, Ag+/Cu+ ion ordering (and 2- 2- associated displacive shifts of the centreing S ions) in the S (Ag,Cu)6 octahedral sites is shown to provide a mechanism for low temperature superstructure ordering.

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