Unnamed Pt(Cu 0.67 Sn 0.33 ) from the Bolshoy Khailyk River, Western Sayans, Russia, and a Review of Related Compounds and Solid Solutions

: We describe a potentially new species of a platinum cupride–stannide mineral (PCSM) of composition Pt(Cu 0.67 Sn 0.33 ). It occurs in a placer deposit in the River Bolshoy Khailyk, southern Krasnoyarskiy kray, Russia. A synthetic equivalent of PCSM was obtained and characterized. The PCSM occurs as anhedral or subhedral grains up to 15 µ m × 30 µ m in association with various platinum-group minerals, Rh–Co-rich pentlandite and magnetite, all hosted by a placer grain of Cu– Au–Pt alloy. Synchrotron micro-Laue diffraction studies indicate that the PCSM mineral is tetragonal and belongs to the inferred space-group P 4/ mmm (#123). Its unit-cell parameters are a = 2.838 (3) Å, c = 3.650 (4) Å, and V = 29.40 (10) Å 3 , and Z = 1. The c:a ratio calculated from the unit-cell parameters is 1.286. These characteristics are in good agreement with those obtained for specimens of synthetic Pt(Cu 0.67 Sn 0.33 ). A review on related minerals and unnamed phases is provided to outline compositional variations and extents of solid solutions in the relevant systems PtNi–PtFe–PtCu, PdCu–PdHg–PdAu, PdHg–PtHg, and AuCu–PtCu. The PCSM-bearing mineralization appears to be related genetically with an ophiolitic source-rock of the Aktovrakskiy complex of the western Sayans. The unnamed phase likely crystallized from microvolumes of a highly fractionated melt rich in Cu and Sn. writing; sampling, writing. All authors have read and agreed to the published version of the


Introduction
The placer deposits of the River Bolshoy Khailyk, western Sayans, in the Ermakovskiy district, southern Krasnoyarskiy kray of Russia [1] are known for assemblages of platinumgroup minerals (PGM) and associated PGE-Au phases. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt. Bodies of serpentinite are fairly abundant in the drainage area. We focus here on a potentially new species of a platinum cupride-stannide mineral (PCSM) of composition Pt(Cu 0.67 Sn 0.33 ); we describe its properties and characteristics. This mineral is closely related to synthetic Pt(Cu 0.67 Sn 0.33 ), a phase recognized recently in the ternary system Pt-Cu-Sn [2]. Tatyanaite, Pt 9 Cu 3 Sn 4 , is another Figure 1. One of five domains of Pt(Cu0.67Sn0.33) encountered in a placer grain of Cu-Au-Pt alloy from the Bolshoy Khailyk placer. It is slightly darker than its host. The location of the EBSD spot is marked with a green cross symbol.

PCSM: Appearance, Physical, and Optical Properties
Grains of PCSM are opaque, with a metallic luster. It is metallic. The micro-indentation values of hardness measured on the synthetic analogue are in the range 94.8-100.8 kg/mm 2 , which corresponds to a Mohs hardness of ~2½. Cleavage, parting, and fractures were not observed. The density could not be measured owing to the small  33 ) encountered in a placer grain of Cu-Au-Pt alloy from the Bolshoy Khailyk placer. It is slightly darker than its host. The location of the EBSD spot is marked with a green cross symbol.

PCSM: Appearance, Physical, and Optical Properties
Grains of PCSM are opaque, with a metallic luster. It is metallic. The micro-indentation values of hardness measured on the synthetic analogue are in the range 94.8-100.8 kg/mm 2 , which corresponds to a Mohs hardness of~2 1 2 . Cleavage, parting, and fractures were not observed. The density could not be measured owing to the small grain size. The calculated density, 14.75 (5) g·cm −3 , is based on the empirical formula and unit-cell volume refined from the synchrotron microdiffraction data.
In reflected light, the color is yellowish cream; bireflectance, pleochroism, and internal reflections were not observed. The mineral is weakly anisotropic. The reflectance values obtained in air for the synthetic analogue, (Pt 0.97 Cu 0.03 ) Σ1.00 (Cu 0.67 Sn 0.33 ) Σ1.00 , are presented in Table 1 and Figure 2. grain size. The calculated density, 14.75 (5) g•cm -3 , is based on the empirical formula and unit-cell volume refined from the synchrotron microdiffraction data.

Compositional Data
Electron-microprobe analysis (  Note. Results of a total of five data points (n = 5), listed in weight %, that were acquired by means of WDS analysis.

Characterization of the Synthetic Analogue
The synthetic analogue Pt(Cu 0.67 Sn 0.33 ) was obtained [2] by heating stoichiometric mixtures of analytical grade powders of platinum (ChemPUR 99.95%), copper (ChemPUR 99.99%), and tin (MERCK 99%) in a molar proportion 3:2:1 (as inferred from Pt:Cu:Sn = 3:2:1 in the specimens from Bolshoy Khailyk). The mixtures were homogenized in an agate mortar and pressed into pellets. On the basis of differential scanning calorimetry (DSC) measurements, a heating rate of 6 K/min was selected for all syntheses. In one set of experiments, the furnace was switched off after holding the charge at the maximum temperature, and the pellets were cooled down. In a second set of experiments, the pellets were quenched to ambient temperature in less than one minute using compressed air. A total of 12 analyses (quantitative SEM/EDS) of different portions of the synthetic phase gave the following mean (and ranges): Pt 70.01 (69.2-70.8), , and Sn 14.53 (14.0-14.9), for a total of 100.9 wt.%, corresponding to (Pt 0.97 Cu 0.03 ) Σ1.0 (Cu 0.67 Sn 0.33 ) Σ1.0 (on the basis of Σatoms = 2 a.p.f.u.).
In addition, the phase Pt(Cu 0.67 Sn 0.33 ) was synthesized in an arc-melter (MAM-1, E. Bühler, GmbH, Hechingen) by melting the mixture of elements. Temperatures in the arc melter were above 2000 K. After the synthesis, the pellet rapidly reached ambient temperature [2].

Crystallography and Crystal Structure
The grains of PCSM are polycrystalline, as are those of the synthetic phase. Our attempts to extract a single crystal were unsuccessful, and even~15 micrometer-sized fragments turned out to be polycrystalline. Thus, a single-crystal study could not be carried out.
The X-ray diffraction pattern of PCSM is reported in Table 3. The mineral is tetragonal, and the inferred space group is P4/mmm (#123). The unit-cell parameters are a = 2.838(3) Å, c = 3.650(4) Å, V = 29.40(10) Å 3 , and Z = 1. The c:a ratio calculated from the unit-cell parameters is 1.286. Note. Results of synchrotron micro-Laue diffraction studies were indexed and analyzed using the software package XMAS v.6 [6]. The calculated values were obtained for the synthetic counterpart.  Note. Results of synchrotron micro-Laue diffraction studies were indexed and analyzed using the software package XMAS v.6 [6]. The calculated values were obtained for the synthetic counterpart.
The EBSD patterns of the PCSM (Figure 3a-d) are indexed satisfactorily on the basis of the P4/mmm structure obtained via micro-Laue synchrotron diffraction, with a mean angular deviation of 0.38°-0.45°. The structure of synthetic Pt(Cu0.67Sn0.33) was determined on the basis of powder-diffraction data [2]. The observed lattice parameters, the crystal structure, and the reliability factors are presented in Tables 4 and 5. Refinements of the site occupancies gave Pt(Cu0.59(5)Sn0.41(5)) as an approximate composition, which is in fairly good agreement with the Pt(Cu0.67Sn0.33) composition of the natural specimen. The crystal structure of the PCSM is shown in Figure 4. It is a tetragonal CuAu-type or L10-type structure, in which The structure of synthetic Pt(Cu 0.67 Sn 0.33 ) was determined on the basis of powderdiffraction data [2]. The observed lattice parameters, the crystal structure, and the reliability factors are presented in Tables 4 and 5. Refinements of the site occupancies gave Pt(Cu 0.59(5) Sn 0.41 (5) ) as an approximate composition, which is in fairly good agreement with the Pt(Cu 0.67 Sn 0.33 ) composition of the natural specimen. The crystal structure of the PCSM is shown in Figure 4. It is a tetragonal CuAu-type or L1 0 -type structure, in which Pt occupies the Wyckoff position 1a (0,0,0) and disordered Cu and Sn occupy the Wyckoff position 1d ( 1 2 , 1 2 , 1 2 ) in the space group P4/mmm (as obtained from the refined site-occupancy via Rietveld refinement of the synthetic analogue Pt(Cu 0.67 Sn 0.33 )) [2].
The cell parameters of the synthetic analogue of the PCSM are a = 2.82205(1) Å, c = 3.63637(2) Å, and V = 28.9599(2) Å 3 ; the space group is P4/mmm (Tables 4 and 5) [2]. These values are close to the parameters obtained for the PCSM specimen from Bolshoy Khailyk.   3 , from X-ray diffraction. [b] The DFT values are ⅓ of the supercell used in all the calculations. The angles of the supercell deviated by <0.1° from 90° after the optimization of the geometry. * After Juarez-Arellano et al., 2020 [2]. Syntheses S4 and S5 involved a first step at 523 K for five hours and a second step at 1023 K for 10 h.   3 , from X-ray diffraction. [b] The DFT values are 1 / 3 of the supercell used in all the calculations. The angles of the supercell deviated by <0.1 • from 90 • after the optimization of the geometry. * After Juarez-Arellano et al., 2020 [2]. Syntheses S4 and S5 involved a first step at 523 K for five hours and a second step at 1023 K for 10 h. Table 5. Crystal structure of synthetic Pt(Cu 0.67 Sn 0.33 ) on the basis of results of Rietveld refinement and reliability factors *.

Genetic Implications
The PCSM grains are hosted by a composite grain of (Au,Pt)Cu alloy recovered in a remote placer deposit along the Bolshoy Khailyk river. Previously, a similar grain of (Au,Pt)Cu alloy was reported from a placer along River Zolotaya in the same area [11]. Similar grains of the (Au,Pt)Cu alloy have been documented at other localities: the Tulameen complex, Canada [12], the Sotajarvi area, Finland [13] and, in situ, in the Kondyor complex, Russian Far East [14]. As noted, the detrital grain hosting the PCSM grains also hosts several grains of various PGM, Co-(Rh)-rich pentlandite, and Cr-Mg-Mn-rich magnetite, among others. The observed system thus involves at least 17 elements (Cu, Au, Pt, Rh, Pd, Ir, Fe, Co, Ni, S, Sb, As, Sn, O, Cr, Mn, Mg), which occur, as major or minor constituents, in minerals of the PCSM-bearing grain. The large variety of participating elements clearly points to a natural origin of this specimen.
The Aktovrakskiy ophiolitic complex is considered to represent the lode source for the PCSM-bearing association. The notable extent of Ru enrichment in the associated Os-Ir-Ru alloy minerals is consistent with an ophiolitic source [1]. The PCSM-bearing assemblages presumably formed after the crystallization of chromian spinel (magnesiochromite) and Fo-enriched olivine. During the crystallization of the Os-Ir-Ru alloy phases, a local buildup of the incompatible Cu + Au, along with subordinate Pt, likely led to the crystallization of PCSM from globules of remaining melt.

Solid Solutions in the Ternary System PtNi-PtFe-PtCu
Natural series of solid solutions pertaining to this system were examined on the basis of 510 data points collected in the literature (Table 6; Figures 5 and 6). Nine sets of compositional data were evaluated, which are judged to be representative of various complexes located in different geological settings worldwide, including the Alaskan-Uralian-(Aldan)-type complexes (sets 1-3), layered intrusions (set 4), ophiolite-related deposits (set 5), an uncategorized chromitite (set 6), massive sulfide Cu-Ni ores (set 7), Ti-rich mineralization developed in alkaline ultramafic complexes (set 8), and different suites of placer deposits (set 9).    Table 6).  Table 6).  Table 6).
Values Pt + PGE and Σ(Fe + Cu + Ni + Sb + Hg) are in the ranges 0.7-1.2 and 0.8-1.3 a.p.f.u. for Σatoms = 2 a.p.f.u., respectively. The mean composition is notably stoichiometric, yielding the 1:1 proportion calculated for n = 510 data points. The observed variations imply that the excess atoms could enter both the Pt and base-metal sites.
The Alaskan-Uralian-(Aldan)-type complexes are most important sources of these alloy minerals (Figures 5 and 6). The major trend extends along the PtFe-PtCu join; numerous compositions are Cu-dominant. In contrast, the PtFe-PtNi series is much more limited, with relatively few alloy samples having a Ni-dominant compositions (#1, 12, 13, Table 7), reported from the Soldzhersky complex, Tuva, Russia, the Bushveld layered complex, South Africa, and from the Butyrinskoye deposit, Kytlym complex, Urals, Russia [38,52,57]. Interestingly, the PtNi-PtFe join is totally free of data points in spite of a large number of compositions examined from these complexes ( Figure 5). Thus, the Cu-for-Fe type of substitution is more common, whereas the Ni-for-Fe scheme likely requires special conditions of crystallization.

Solid Solutions in the Ternary System PtNi-PtFe-PtCu
Elevated amounts of Pd and Ir are typical of PtFe alloys (Figures 7 and 8), as they are in other species of Pt-Fe minerals, i.e., Fe-bearing platinum and isoferroplatinum, cf. [6]. Levels of Pd attain 0.3 Pd a.p.f.u. (#1, 4 in Table 7) [38,55]. A value greater than 0.35 Ir a.p.f.u. (Figure 8), if it corresponds to a single phase, may imply the existence of an Ir-dominant member in this series. Examples of other members of the ternary system are poorer in Ir (Table 7).  Table 6).
Values Pt + PGE and Σ(Fe + Cu + Ni + Sb + Hg) are in the ranges 0.7-1.2 and 0.8-1.3 a.p.f.u. for Σatoms = 2 a.p.f.u., respectively. The mean composition is notably stoichiometric, yielding the 1:1 proportion calculated for n = 510 data points. The observed variations imply that the excess atoms could enter both the Pt and base-metal sites.
The Alaskan-Uralian-(Aldan)-type complexes are most important sources of these alloy minerals (Figures 5 and 6). The major trend extends along the PtFe-PtCu join; numerous compositions are Cu-dominant. In contrast, the PtFe-PtNi series is much more limited, with relatively few alloy samples having a Ni-dominant compositions (#1, 12, 13, Table 7), reported from the Soldzhersky complex, Tuva, Russia, the Bushveld layered complex, South Africa, and from the Butyrinskoye deposit, Kytlym complex, Urals, Russia [38,52,57]. Interestingly, the PtNi-PtFe join is totally free of data points in spite of a large number of compositions examined from these complexes ( Figure 5). Thus, the Cufor-Fe type of substitution is more common, whereas the Ni-for-Fe scheme likely requires special conditions of crystallization.
The maximum extent of Cu enrichment occurs in the phase Pt 1.10 (Cu 0.65 Fe 0.26 ) Σ0.91 analyzed in the River Pustaya placer, Kamchatka, Russia [44]. The same level of Cu is attained in the unnamed Pt(Cu 0.67 Sn 0.33 ) at Bolshoy Khailyk.

Solid Solutions in the Ternary System PtNi-PtFe-PtCu
Elevated amounts of Pd and Ir are typical of PtFe alloys (Figures 7 and 8), as they are in other species of Pt-Fe minerals, i.e., Fe-bearing platinum and isoferroplatinum, cf. [6]. Levels of Pd attain 0.3 Pd a.p.f.u. (#1, 4 in Table 7) [38,55]. A value greater than 0.35 Ir a.p.f.u. (Figure 8), if it corresponds to a single phase, may imply the existence of an Ir-dominant member in this series. Examples of other members of the ternary system are poorer in Ir (Table 7).  Table 6 and expressed in terms of atoms per formula unit.  Table 6 and expressed in terms of atoms per formula unit.   Table 6 and expressed in terms of atoms per formula unit.  Table 6 and expressed in terms of atoms per formula unit.  Table 6 and expressed in terms of atoms per formula unit.   Table 6 and expressed in terms of atoms per formula unit. The maximum levels of Sb and Hg (#14, 15, Table 7) are similar: 0.15 and 0.17 a.p.f.u., respectively [17,38]. The incorporation of Hg is unusual for a Pt-Fe alloy mineral, though it is consistent with the compositions of potarite, PdHg, synthetic PtHg or NiHg, also having the AuCu-type structure [64,65].
As noted by Fleet et al. (2002) [23], the auriferous variety of potarite displays a notable deviation from the ideal atomic proportions toward Pd 3 Hg 2 . A similar departure also is reported for the tulameenite series, members of which can be somewhat nonstoichiometric: (Pt,PGE) 1+x (Fe,Cu,Ni) 1-x , where 0 < x < 0.1 [62].

The PdHg-PtHg Series
In addition, potarite displays a considerable extent of solid solution with PtHg, also having an AuCu-type structure ( [65] and references therein). The existence of a new and Ptdominant member is implied by compositions reported from vein-like pegmatitic ores of the Butyrinskoye (Butyrin) deposit, Kytlym complex, Urals, Russia [66]. Indeed, one of these compositions is notably Pt-rich, with a Pt/Pd ratio of 0.9 (#16, Table 8). Nineteen data points provided by the authors gave values of the atomic ratio (Pd + Pt)/(Hg + Cu + Fe + Ni + Sb) ranging 0.9 to 1.2, with a mean of 1.0.

The PdHg-PtHg Series
In addition, potarite displays a considerable extent of solid solution with PtHg, also having an AuCu-type structure ( [65] and references therein). The existence of a new and Pt-dominant member is implied by compositions reported from vein-like pegmatitic ores of the Butyrinskoye (Butyrin) deposit, Kytlym complex, Urals, Russia [66]. Indeed, one of these compositions is notably Pt-rich, with a Pt/Pd ratio of 0.9 (#16, Table 8). Nineteen data points provided by the authors gave values of the atomic ratio (Pd + Pt)/(Hg + Cu + Fe + Ni + Sb) ranging 0.9 to 1.2, with a mean of 1.0.
The revision proposed by [18] involves a different setting of the cell (e.g., 3.891 ≈ √ 2 * 2.7477; #1, 2, Table 9). The powder XRD pattern simulated on the basis of the structure data of [18] is identical to the powder data reported by [4]. The different setting is also provided for tetra-auricupride, AuCu, with a revision of space group to P4/mmm; the C4/mmm symmetry proposed previously is a multiple cell of P4/mmm (#11, 12, Table 9). This revision is consistent with characteristics of the AuCu(I) phase, P4/mmm, a = 2.785-2.810 Å and c = 3.671-3.712 Å [15].

Concluding Comments and Principles of Future Classification
The unnamed species of PGM investigated at Bolshoy Khailyk is analogous, both compositionally and structurally, to synthetic Pt(Cu 0.67 Sn 0.33 ) obtained and characterized by Juarez-Arellano et al. [2]. It represents a member of a large family of isostructural members that have similar unit-cell parameters and conform to the space group P4/mmm. These species and their variants are composed of several participating elements (Pt, Pd, Ir, Au) vs. (Fe, Cu, Ni, Sn, Sb, Hg, Au), some of which (e.g., Au) can probably occupy more than a single site in the structure. Considerable extents of mutual solid-solution exist among the inferred end-members in these series. Consequently, new members can reasonably be expected in accordance with the 50% rule.
The intermetallic compounds or alloys related to tetraferroplatinum and tulameenite can be better grouped (R. Miyawaki, written commun.; Figures 11 and 12) on the basis of the degree of order of metals in terms of Fm3m (#225), Pm3m (#221), P4/mmm (#123) 'tP4', C4/mmm (a multiple cell of the smaller P4/mmm), 'tP2', among other possibilities. It is thus necessary to clarify the degree of order of the metal atoms in these minerals in order to establish in each case the true space-group of the unit cell. If the crystal structures of the polymorphs have essentially the same topology, differing only in terms of a structural distortion or in the degree of order of some of the atoms comprising the structure, such polymorphs are not regarded as separate species [74]. Thus, on the basis of the literature data on valid mineral species making up the potential group(s), the species can be classified into two types. (1) ABC 2 type, with an ordered distribution of metal atoms in the tetragonal system, space group P4/mmm, 'tP4'. The members are tulameenite Pt 2 CuFe, P4/mmm, a = 3.89, and c = 3.58 Å [3], and ferronickelplatinum Pt 2 FeNi, P4/mmm, a = 3.871, and c = 3.635 Å [26]. (2) AB type, with a disordered distribution of metal atoms in the tetragonal system (P4/mmm), 'tP2'. The members are tetraferroplatinum PtFe, P4/mmm, a = 2.724, c = 3.702 Å [25], tetra-auricupride CuAu, P4/mmm, a = 2.81, c = 3.72 Å [24], and unnamed Pt(Cu 0.67 Sn 0.33 ), P4/mmm, a = 2.838, and c = 3.650 Å (this study), among others.

Figure 11.
A general scheme proposed for ABC 2 -type compounds on the basis of an ordered distribution of metal atoms in the 'tP4' structure.The colored spheres represent the A (blue), B (green) and C (magenta) atoms.