The As4S6 molecule, although theoretically predicted and supposed to occur in amorphous arsenic sulfides, was missing in the reported structures of crystalline As chalcogenides; the thermodynamic stable phase for this stoichiometry, in fact, is that of the mineral orpiment, which shows a layered structural arrangement based on trigonal AsS3 pyramids. Here we report the first possible occurrence of the As4S6 molecule together with the other known As4Sn (n=3, 4, 5) molecules randomly replacing each other in the crystalline structure of a new monoclinic product obtained by the light-induced alteration of the mineral alacranite, As8S9. Our findings are based on single-crystal X-ray diffraction experiments. The fact that As4S6 formed in a crystalline light-induced alteration product could indicate a heretofore unknown role of this molecule in the photoinduced changes of the physico-chemical properties of both bulk glasses and thin films in the As–S system, widely studied in optics and optoelectronics.

Steno (1638-1686) operated in a historical context rich of discoveries and observations done by previous scientists such as Vannoccio Biringucci, Georg Bauer (Agricola), Johannes von Kepler, Robert Hooke, Christiaan Huyghens, Erasmus Bartholin, and others. Steno also had to fight against some irreducible dogmatic and “mythological” beliefs, such as the vis formativa and succus lapidescens, supported by e.g. Michele Mercati and Anselmo Boetius de Boot, respectively. In De solido intra solidum naturaliter contento dissertationis prodromus Steno deals with almost all aspects of Earth Sciences and not just "solid inclusions" as it might seem from the full title of the Prodromus. This contribution deals only with aspects related to crystallography and minerals in general. The most famous is highlighted by the sentence “non mutatis angulis” which is a clear reference to the fact that interfacial angles of quartz crystals do not change regardless of the size and the number of the faces. This observation was then generalized as a law for all minerals by Jean-Baptiste Romé de l’Isle a century later. Less well known but of great importance is Steno’s assertion that the crystals grow thanks to the addition of particles that come from an external fluid and are not “fed” from the inside like in vegetables; moreover, the speed of growth is not the same for all faces. For example, the faces of the “pyramid” in quartz can grow more or less rapidly than those of the prism (giving rise to either squat or elongated crystals). It can therefore be argued that Steno has greatly contributed to the concept of anisotropy in the solid state, typical of all crystals. Stenonite, Sr2Al(CO3)F5, is a new mineral dedicated to his memory about sixty years ago.

Bonazziite is a new mineral from Khaidarkan deposit, Kyrgyzstan and represents the natural analogue of the beta-form of the well known As4S4 compound. It occurs as rare crystals up to 100 mm across associated with realgar, sulfur, wakabayashilite, alacranite, non-stoichiometric As4S4+x sulfides and stibnite in a calcite matrix. In thick section, bonazziite is opaque with a resinous lustre and a dark-orange streak. It is brittle; the Vickers hardness (VHN15) is 70 kg/mm(2) (range: 60-76) (Mohs hardness of similar to 2(1/2)). In planepolarized incident light, bonazziite is strongly bireflectant and pleochroic from orange to light red. The mineral shows orange to red internal reflections. Between crossed polars, the mineral is strongly anisotropic with greyish to light-blue rotation tints. Reflectance percentages in air for R-min and R-max are 19.9, 22.2 (471.1 nm), 19.1, 21.3 (548.3 nm), 18.8, 19.7 (586.6 nm) and 17.8, 18.9 (652.3 nm), respectively. Bonazziite is monoclinic, space group C2/c, with a = 9.956(1), b = 9.308(1), c = 8.869(1) angstrom, b = 102.55(2)degrees and V = 802.3(2) angstrom(3), Z = 4. The crystal structure [R-1 = 0.0263 for 735 reflections with F-o > 4 sigma(F-o)] is based on the As4S4 cage-like molecule, in which each As atom links one As and two S atoms. The As4S4 molecule is identical to that found in the structure of realgar. The six strongest powder diffraction lines [d in angstrom (I/I-0) (hkl)] are: 5.74 (100) ((1) over bar 11); 4.10 (60) (021); 3.92 (50) ((1) over bar 12); 3.12 (60) (022, 310); 2.95 (50) (221, 202); 2.86 (80) ((2) over bar 22, (1) over bar 31). A mean of six electron microprobe analyses gave the formula As3.95S4.05, on the basis of eight atoms. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA No. 2013-141) and named for Paola Bonazzi, in recognition of her seminal contributions to the study of arsenic sulfides and their alteration induced by exposure to light.

Abstract Cupropearceite, [(Cu3.51Ag2.50Fe0.01)Σ6.02(As1.72Sb0.24)Σ1.96S7][Ag9CuS4], and cupropolybasite, [(Cu3.82Ag2.42Zn0.02Pb0.01)S6.27(Sb1.19As0.73)S1.92S7][Ag9CuS4], both exhibit fast-ion conduction at very low temperatures. The structural relationship between the various phases is not fully understood as yet and is addressed in this study. Samples of these materials were studied by means of synchrotron radiation at room temperature and transmission electron microscopy at room temperature and low temperature (both liquid N2 and liquid He) to have a better understanding of the stabilization of the fast-ion conducting form at low and ultra-low temperature in these minerals. The study at room temperature did not evidence any doubling of unit-cell parameters with respect to the basic Tac unit cell, of the type typically observed for minerals of the pearceite-polybasite group. On the other hand, relatively strong and well-defined satellite reflections relating to the pseudo-hexagonal arrangement of the Ag+ ions at G ± ~1.39(1) <110>* positions of the reciprocal space, where G represents the average structure Bragg reflections, were clearly observed. Although this seems to suggest that the Ag+ ion distribution can adequately be described by a two-dimensional displacive modulation of the average P3m1 structure (Tac polytype) with the incommensurate modulation wave vectors of the satellite reflections q1 = ~0.39(1) (aF* + bF*) and q2 = ~0.39(1)(aF* - bF*), where the subscript F indicates the framework substructure, the structure observed is better described as a composite modulated structure because of the intensity asymmetry of the satellite reflections. Low-temperature TEM investigations show that the satellites are still present at both 90 and 4.2 K, with a remarkable temperature-dependent shift in their positions giving rise to a variation of the coefficient a of the modulation vectors from 0.39 at room temperature, trough ~0.40 at 90 K to ~0.5 at 4.2 K. Thus, the incommensurate modulation, strengthened by the very low temperature, approaches almost the a ~0.5 value, indicative of a commensurate modulation. The 4.2 K structure could thus be a low-temperature commensurate superstructure (“lock-in phase”), observed for the first time in the minerals of the pearceite-polybasite group

The mineral fettelite, [Ag6As2S7][Ag10HgAs2S8], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag6As2S7] and a second module layer (labeled B) with composition [Ag10HgAs2S8]. Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chañarcillo, Copiapó Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P3m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.

The mineral fettelite, [Ag6As2S7] [Ag10HgAs2S8], has been recently structurally characterized. On the whole, the structure can be described as a regular succession of two module layers stacked along the c-axis: a first module layer (labeled A) with composition [Ag6As2S7](2-) and a second module layer (labeled B) with composition [Ag10HgAs2S8](2+). Here we report an integrated high-temperature single-crystal X-ray diffraction (HT-SCXRD), differential scanning calorimetry (DSC), and complex impedance spectroscopy (CIS) study on a sample of fettelite from Chanarcillo, Copiapo Province, Chile. DSC and conductivity measurements pointed out that fettelite shows a ionic-transition at about 380 K. HT-SCXRD experiments confirmed the phase transition toward a disordered phase having a trigonal symmetry with the a and b unit-cell parameters halved. In the HT-structure, the disorder is located in the B layer where the Ag-Hg cations are found in various sites corresponding to the most pronounced probability density function locations of diffusion-like paths. This indicates that at least two polytypes could exist for fettelite, the ordered, monoclinic RT-structure (space group C2), and a fast ion conducting, trigonal, disordered HT-form (space group P (3) over bar m1) with a and b parameters halved. The two unit-cell types (corresponding to two different polytypes) could be also found in nature. Slightly different chemical compositions for different fettelite samples (e.g., different Ag/Hg ratios) could play a crucial role as driving forces for different unit-cell stabilizations.

SessionsC575 suggested to occur in [100].From consider of the interaction between the dodecahedral and tetrahedral cations, it is inferred that a part of V 5+ occupies the 48f site (x, 0, 0.25), off-centered along [100], and the remainder stays at the 24d site (0.375, 0, 0.25), the average position.To confirm the presence of the V static disorder, the refinement based on this split-atom model was conducted at 96 K by applying isotropic ADPs (U iso ) only in V 5+ at the 24d and 48f sites under the constraint of U iso (24d) = U iso (48f).The electron density distribution was better fitted by this refinement, and the isotropic ADP of V 5+ [U iso = 0.00283(16) Å 2 ] resulted in the smaller value than that [U eq = 0.00457(3) Å 2 ] from the refinement on the normal model, assigning V 5+ only to the 24d site.Moreover, the resulting V positional parameter at the 48f site is x = 0.3835(3), significantly deviating from the 24d site.The displacement quantity from the 24d site to this 48f position is 0.107(4) Å, agreeing approximately with √ static [= 0.079(1) Å] of V 5+ .Thus, we conclude that the peculiar atomic displacement behavior of V 5+ is due to its static disorder along [100].

We have characterized a portion of cotype billingsleyite, Ag 7 (As,Sb)S 6 , a rare As 5+ -bearing sulfosalt from the silver ores of the North Lily mine, East Tintic district, Utah, USA, by single-crystal X-ray diffraction and electron-microprobe analysis. We found billingsleyite to be structurally identical to synthetic Ag 7 AsS 6 . It is cubic, space group P 2 1 3, with a cell parameter a = 10.4760(8) A, V = 1149.7(2) A 3 , and Z = 4. Electron-microprobe analyses gave the following formula: (Ag 6.94 Cu 0.04 Fe 0.01 ) ∑6.99 (As 0.87 Sb 0.13 ) ∑1.00 S 6.01 . The crystal structure has been solved and refined to R = 1.64%. It consists of (As 5+ ,Sb 5+ )S 4 tetrahedra and Ag polyhedra (2-, 3- and 4-fold coordinated) forming a three-dimensional network. We present structural relationships with other natural and synthetic thioarsenates and thioantimonates.

It is known that dioctahedral 2:1 phyllosilicates are topotactically dehydroxylated by heating.This dehydroxylation induces cation migration in the octahedral sheet in several phyllosilicates [1].For instance, the cisvacant (cv) illite transforms to a structure with cation distribution close to the trans-vacant (tv) structure.On the other hand, it was reported that superstructures within a 2:1 layer are formed in celadonite, trans-vacant (Fe 3+ , Mg)-rich dioctahedral mica, by dehydroxylation [2].The present paper reports the origin and detail of these superstructures by using HRTEM [3].When celadonite was annealed around 650°C, a primitive a-b super-cell formed.HRTEM images indicate that the superstructure is owing to a long-range cation ordering in the octahedral sheet, in agreement with the suggestion by Muller et al. [2].However, the image simulation using the site occupancy they proposed could not completely reproduce the experimental contrast.A new model to explain the contrast is under consideration.Celadonite annealed at ~800°C shows extra reflections which correspond to a 3a-b super-cell in the hk0 electron diffraction pattern [2].Cross-sectional HRTEM images along the X i directions are almost similar to that of natural celadonite.However, the images along the Y i directions are completely different from that for normal micas.First of all, the 3a periodicity is owing to a long-period ordering of the (Fe 3+ , Mg) cations.The most interesting structural feature is that the contrasts corresponding to the two tetrahedral sheets in a 2:1 layer are not staggered at all.Probably cation migration induced lateral shift of ±a/3 for the T(tetrahedral sheet)-K(potassium)-T unit.A plausible model to explain the experimental contrasts is shown in Fig. 1.In the model, two third of the spaces surrounded by the two facing hexagonal rings of tetrahedra accommodate three (Fe 3+ , Mg) cations and one third of the spaces is empty.(Fe 3+ , Mg) cations are coordinated by six or five oxygen atoms, forming a trigonal prisms or a square pyramid, respectively.

The crystal structure of the rare mineral fettelite was solved using intensity data collected from a twinned crystal from Chanarcillo, Copiapo Province, Chile. This study revealed that, in spite of the strong hexagonal pseudosymmetry, the structure is monoclinic (space group C2) with a= 26.0388(10), b = 15.065](8), c = 15.536](8) angstrom, beta = 90.48(1)degrees, and V= 6094.2(5) angstrom(3). The refinement of an anisotropic model led to an R index of 0.0656 for 7143 observed reflections [I > 2 sigma(I)] and 0.0759 for all 17447 independent reflections. Fettelite is intimately twinned with six twin domains. The structure consists of the stacking of two module layers along [001]: an A module layer with composition [Ag6As2S7](2-) and a B module layer with composition [Ag10HgAs2S8](2+). The As atoms form isolated AsS3 pyramids typical of sulfosalts, Hg links two Sulfur atoms in linear coordination, and Ag occupies sites with coordination ranging from quasi linear to almost tetrahedral. The A module layer found for fettelite is identical to that described for the minerals belonging to the pearceite-polybasite group. On the basis of information gained from this characterization the crystal chemical formula was revised according to the structural results, yielding [Ag(6)AS(2)S(7)][Ag10HgAs2S8] (Z = 8).

We report data on the composition and crystal structure of the most Ag-rich (15.63 apfu) natural polybasite yet discovered. It shows the -M2a2b2c polytype. The crystal studied was found in a sample (mineralogical collection of the Royal Ontario Museum) from Gowganda, Timiskaming District. Ontario, Canada. Electron microprobe analysis yields the formula [Ag(6)(Sb(1.78)As(0.18))(Sigma=1.96)S(7)] [Ag(9)(Ag(0.63)Cu(0.43))(Sigma=1.06)S(4)]. Lattice parameters are a = 26.2625(4), b = 15.1623(5), c =24.1061(6) angstrom, beta = 90.045(5)degrees, V= 9599.0(4) angstrom(3). The structure was refined in the space group C2/c to R = 0.0581 using 7725 observed reflections [I > 2 sigma(I)]. The refinement shows that one of the three structural positions of the B module layer usually occupied by Cu is dominated by Ag. Crystal-chemical characteristics are compared with published data on the other members of the pearceite-polybasite group. Some remarks concerning nomenclature are also given.

The macaulayite-burckhardtite family [1] of layered silicates is discussed along in terms of the modular concept.The characterization of a new mineral britvinite [Pb 7 (OH) 3 F(BO 3 ) 2 (CO 3 )][Mg 4.5 (OH) 3 (Si 5 O 14 )] [2,3] allowed considering this mineral group as a polysomatic series with a TOT-"pyrophyllite" block as one end-member.Along c axis of the unit cells these three-layered blocks alternate with lamellar modules of different size and composition, such as oxide, oxide-carbonate, oxide-carbonate-sulfate, ect.The group involves among britvinite the minerals macaulayite, lourencewalsite, burckhardtite, kegelite, surite, ferrisurite, and niksergievite.In the absence of data of precision structure investigations, the preliminary inferences regarding their structures were made using the results of chemical, spectroscopic, and X-ray powder diffraction analyses and, in a number of cases, microdiffraction experiments.Tetrahedral fragments of the britvinite crystal structure built from 12-membered rings are topologically identical to tetrahedral nets in zeophyllite Ca 13 (F,OH) 10 [Si 5 O 14 ] 2 .

pyrargyrite, the band positions in stephanite spectra are shifted to higher vibrational regions than those in pyrargyrite

Selenopolybasite, ideally [(Ag,Cu)(6)(Sb,As)(2)(S,Se)(7)][Ag9Cu(S,Se)(2)Se-2], is a new mineral species from the De Lamar mine, Owyhee County, Idaho, USA. It occurs as black anhedral to subhedral grains up to 400 mu m across, closely associated with naumannite, covellite, pyrite and calcite. Selenopolybasite is opaque with a metallic luster and possesses a black streak. It is brittle, and neither fracture nor cleavage was observed; the Vickers microhardness (VHN100) is 131 kg/mm(2) (range 125-141) (corresponding Mohs hardness: 3-3 1/2). The density could not be measured owing to the small grain-size. The calculated density is 6.548 g/cm(3) (on the basis of the empirical formula). In plane-polarized reflected light, selenopolybasite is light grey, weakly to moderately bireflectant, and weakly pleochroic from grey to a violet-blue grey. Between crossed polars, it is anisotropic, with no characteristic rotation-tints and no internal reflections. Reflectance values (R-min and R-max) for the four standard COM wavelengths are 32.8, 34.1 (471.1 nm), 31.0, 32.9 (548.3 nm), 30.2, 31.8 (586.6 nm), and 29.3, 30.0% (652.3 nm), respectively. Selenopolybasite is trigonal, space group P (3) over bar m1, with unit-cell parameters: a 7.5950(4), c 12.0731(6) angstrom, V 603.12(5) angstrom(3), c : a 1.5896, Z = 1. The strongest eight X-ray powder-diffraction lines [d in angstrom(I/I-0)(hkl)] are: 3.173](48)(201), 3.0183(84)(004), 2.8880(48)(022), 2.8880(100)(202), 2.5466(23)(023), 2.3629(34)(114), 2.2237(28)(024) and 1.8987(31)(220). Average results of 10 electron-microprobe analyses gave the chemical formula [(Ag5.67Cu0.20Bi0.01Pb0.01Zn0.01Fe0.03)(Sigma 5.93)(Sb1.86As0.19)(Sigma 2.05)(S6.68Se0.34)(Sigma 7.02)] [Ag9Cu(S1.79Se0.21)(Sigma 2.00)Se-2], on the basis of 29 atoms and according to results of the structure refinement. The name chosen, selenopolybasite, recalls its status as the Se-dominant analogue of polybasite. Moreover, as it shows the I I I unit-cell type, the full name for the polytype is selenopolybasite-Tac, the Se-dominant analogue of polybasite-Tac.

The present paper reports changes to the existing nomenclature for minerals belonging to the pearceite-polybasite group. Thirty-one samples of minerals in this group from different localities, with variable chemical composition, and showing the 111, 22 1, and 222 unit-cell types, were studied by means of X-ray single-crystal diffraction and electron microprobe. The unit-cell parameters were modeled using a multiple regression method as a function of the Ag, Sb, and Se contents. The determination of the crystal structures for all the members of the group permits them to be considered as a family of polytypes and for all members to be named pearceite or polybasite. The main reason for doubling the unit-cell parameters is linked to the ordering of silver. The distinction between pearceite and polybasite is easily done with an electron microprobe analysis (As/Sb ratio). A hyphenated italic suffix indicating the crystal system and the cell-type symbol should be added, if crystallographic data are available. Given this designation, the old names antimonpearceite and arsenpolybasite are abandoned here and the old names pearceite and polybasite, previously defined on a structural basis (i.e., 111 and 222), are redefined on a chemical basis. The old name pearceite will be replaced by pearceite-Tac, antimonpearceite by polybasite-Tac, arsenpolybasite-221 by pearceite-T2ac, arsenpolybasite-222 by pearceite-M2a2b2c, polybasite-221 by polybasite-T2ac, and polybasite-222 by polybasite-M2a2b2c. Since all polytypes are composed of two different layers stacked along [001]: layer A, with general composition [(Ag,Cu)(6)(As,Sb)(2)S-7](2-), and layer B, with general composition [Ag9CuS4](2+), the chemical formulae of pearceite and polybasite should be written as [Ag9CuS4][(Ag,Cu)(6)(As,Sb)(2)S-7] and [Ag9CuS4][(Ag,Cu)(6)(Sb,As)(2)S-7], respectively, instead of (Ag,Cu)(16)(As,Sb)(2)S-11 and (Ag,Cu)(16)(Sb,As)(2)S-11, as is currently accepted. The new nomenclature rules were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association.

The crystal structures of antimonpearceite, arsenpolybasite-222 and arsenpolybasite-221 have been solved and refined from single-crystal X-ray-diffraction datasets. Antimonpearceite crystallizes in the trigonal space-group P 3 m 1, with a 7.4805(5), c 11.8836(13) A, V 575.89(8) A 3 and Z = 1. The refinement of the structure leads to R = 0.0352 for 1095 independent observed reflections [ I /σ( I ) ≥ 2] and 98 parameters. Arsenpolybasite-222 crystallizes in the monoclinic space-group C 2/ c , with a 26.036(2), b 15.0319(13), c 24.042(3) A, β 90.000(13)° (pseudohexagonal cell with the a = √3 b orthohexagonal relation), V 9409.5(15) A 3 and Z = 16. A second-degree twinning by metric merohedry gives rise to an apparent trigonal symmetry, with the unit-cell parameters (hexagonal cell) a 15.0319(13) and c 24.042(3) A. The refinement of the structure leads to R = 0.0716 for 22008 independent observed reflections [ I /σ( I ) ≥ 2] and 552 parameters. Arsenpolybasite-221 crystallizes in the space group P 321, with a 14.9746(17), c 11.9982(6) A, V 2330.0(4) A 3 and Z = 4. The refinement of the structure, including a mirror-twin operation (first-degree twin, 3′2/ m ′1 polychromatic point-group), leads to R = 0.0434 for 4639 independent observed reflections [ I /σ( I ) ≥ 2] and 184 parameters. All the structures consist of the stacking of [(Ag,Cu) 6 (As,Sb) 2 S 7 ] 2− and [Ag 9 CuS 4 ] 2+ module layers along [001]; (As,Sb) forms isolated (As,Sb)S 3 pyramids typically occurring in sulfosalts, copper links two sulfur atoms in a linear coordination, and silver occupies sites with coordination ranging from quasilinear to almost tetrahedral. The substitution of Cu for Ag in the [(Ag,Cu) 6 (As,Sb) 2 S 7 ] 2− module layer becomes greater in going from the 111 structure, through the 221, to the 222 structure. The determination of the crystal structure for all the members of the group leads us to consider them as a family of polytypes.

Cubic (or pseudo-cubic) perovskite ABO 3 can be non-stoichiometric because of the presence of A-site or oxygen-site vacancies.During studies on K 2 O-MnO-V 2 O 5 and Na 2 O-MnO-V 2 O 5 ternary systems, we have identified the new KMnVO 4 and Na 3 MnV 2 O 7.5 compounds.Crystallographic studies have confirmed the composition of the phases and established that the structures correspond to new types of ordered anionic defect cubic perovskites with cationic ordering on the A and B sites.The compound formulas can be expressed as (K 2 Mn)[MnV 2 Ο 8 and (Na 3 )[MnV 2 Ο 7.5 .The atoms between brackets and square brackets occupy the A and B sites, respectively.The structures of KMnVO 4 and Na 3 MnV 2 O 7.5 have been refined from single crystal X-ray diffraction in the P21/n and C2/c space groups, with the unit cell parameters a = 12.081(2) Å, b = 5.645(7) Å, c = 23.804(12)Å and b = 103.45(2)°anda = 5.6190 Å, b = 17.1010Å, c = 7.4090 Å, and β = 89.93°,respectively.In both structures, the B site cations Mn 2+ and V 5+ are six and four coordinated, respectively.The originality in the KMnVO 4 structure is that the Mn 2+ cations occupy both the A and B sites.

The crystal structure of a Se-rich antimonpearceite has been solved and refined by means of X-ray diffraction data collected at temperatures above (room temperature) and below (120 K) an ionic conductivity-induced phase transition. Both structure arrangements consist of the stacking of [(Ag,Cu)(6)(Sb,As)(2)(S,Se)(7)](2-) A (A') and [Ag(9)Cu(S,Se)(2)Se(2)](2+) B (B') module layers in which Sb forms isolated SbS(3) pyramids typically occurring in sulfosalts; copper links two S atoms in a linear coordination, and silver occupies sites with coordination ranging from quasi-linear to almost tetrahedral. In the ionic-conducting form, at room temperature, the silver d(10) ions are found in the B (B') module layer along two-dimensional diffusion paths and their electron densities described by means of a combination of a Gram-Charlier development of the atomic displacement factors and a split-atom model. The structure resembles that of pearceite, except for the presence of both specific (Se) and mixed (S, Se) sites. In the low-temperature ;ordered' phase at 120 K the silver d(10) ions of the B (B') module layer are located in well defined sites with mixed S-Se coordination ranging from quasi-linear to almost tetrahedral. The structure is then similar to that of 222-pearceite but with major differences, specifically its cell metric, symmetry and local arrangement in the B (B') module layer.

The crystal structure of the mineral pearceite, (Ag,Cu)16(As,Sb)2S11, has been solved and refined at 300, 120 and 15 K. At room temperature pearceite crystallizes with trigonal symmetry, space group P3m1; the refinement of the structure leads to a residual factor of R = 0.0464 for 1109 independent observed reflections and 92 variables. The crystal structure consists of sheets stacked along the c axis. The As atoms form isolated (As,Sb)S3 pyramids, which typically occur in sulfosalts, copper cations link two S atoms in a linear coordination, and the silver cations are found in a fully occupied position and in various sites corresponding to the most pronounced probability density function locations (modes) of diffusion-like paths. These positions correspond to low-coordination (2, 3 and 4) sites, in agreement with the preference of silver for such environments. d10 silver-ion distribution has been determined by means of a combination of a Gram-Charlier description of the atomic displacement factors and a split-atom model. To analyse the crystal chemical behaviour of the silver cations as a function of temperature, a structural study was carried out at 120 K (R = 0.0450). The refinement indicates that the mineral exhibits the same structural arrangement as the room-temperature structure (space group P3m1) and shows that the silver cations are still highly disordered. In order to investigate a possible ordering scheme for the silver cations, a data collection at ultra-low temperature (15 K) was performed. The structural skeleton was found to be similar to that of the room-temperature and 120 K atomic structures, but the best solution was achieved with a fully split-atom model of five silver positions, giving an R value of 0.0449 for 651 observed reflections and 78 parameters. Although the silver cation densities condense into better defined modes, the joint probability density function still exhibits a strong overlapping of neighbouring sites.