Ferrotorryweiserite, Rh 5 Fe 10 S 16 , a New Mineral Species from the Sisim Placer Zone, Eastern Sayans, Russia, and the Torryweiserite–Ferrotorryweiserite Series

: Ferrotorryweiserite, Rh 5 Fe 10 S 16 , occurs as small grains ( ≤ 20 µ m) among droplet-like inclusions (up to 50 µ m in diameter) of platinum-group minerals (PGM), in association with oberthürite or Rh-bearing pentlandite, laurite, and a Pt-Pd-Fe alloy (likely isoferroplatinum and Fe-Pd-enriched platinum), hosted by placer grains of Os-Ir alloy ( ≤ 0.5 mm) in the River Ko deposit. The latter is a part of the Sisim placer zone, which is likely derived from ultramafic units of the Lysanskiy layered complex, southern Krasnoyarskiy kray, Russia. The mineral is opaque, gray to brownish gray in reflected light, very weakly bireflectant, not pleochroic to weakly pleochroic (grayish to light brown tints), and weakly anisotropic. The calculated density is 5.93 g · cm –3 . Mean results (and ranges) of four WDS analyses are: Ir 18.68 (15.55–21.96), Rh 18.34 (16.32–20.32), Pt 0.64 (0.19–1.14) K , Ti K , Al K , Cr , , Fe K , K , K K , Cu K , Ni K , , L , Rh L , Pt L , and S K Minimum detection limits (2 σ level) are ≤ 0.2 wt.% for ≤ 0.4 wt.% for Rh; and ≤ 0.5–0.7 wt.% for Certiﬁed , , 2 , 2 3 , and , along orthoclase. analytical for the


Introduction
Ferrotorryweiserite, Rh 5 Fe 10 S 16 , is a new mineral species discovered in the River Ko suite, which represents part of the Sisim placer zone, south of Krasnoyarsk, Krasnoyarskiy kray, Russia (approximate location 54 • 45 N, 93 • 09 E). The mineral and its name were approved (IMA 2021-055) by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association [1]. Our aims are to describe its properties and to report, with reference to literature sources, on the probable existence of a complete solid-solution series between ferrotorryweiserite and torryweiserite, Rh 5 Ni 10 S 16 , discovered in the Marathon Cu-Pd deposit, Coldwell complex, Ontario, Canada [2][3][4]. Ferrotorryweiserite is also related to tamuraite, Ir 5 Fe 10 S 16 , and kuvaevite, Ir 5 Ni 10 S 16 [5,6], which are other members of the torryweiserite family that also occur as inclusions in grains of Os-Ir alloy in heavy-mineral fractions of platinum-group minerals (PGM) in the placer zone of the Sisim river and its tributaries, rivers Ko and Seyba [7,8]. An evaluation of the economic potential of these placers had not yet been completed. The placer grains of PGM were likely derived from ultramafic units of the Lysanskiy layered complex, Eastern Sayans, south-central Siberia [9]. In relation to the series of ferrotorryweiserite-torryweiserite, we document an unusual occurrence of the association olivine-albite (Fo 73-76 and Ab 82-86 ) developed in the core of a globular inclusion hosted by a grain of iridium from the River Ko. Additional occurrences of members of the ferrotorryweiserite-torryweiserite series are expected in a variety of ore deposits associated with ultramafic-mafic complexes that are known to contain Rh-and Ir-based species of PGM, cf. [10,11].

Materials and Methods
Hundreds of grains of Os-Ir alloy minerals and inclusions were examined from placer suites in the Sisim zone. In some of these grains from the River Ko, ferrotorryweiserite occurs as composite inclusions in other PGM hosted by osmium enriched in Ir or, less commonly and inversely, by iridium enriched in Os.
Electron-microprobe analyses of ferrotorryweiserite and iridium were done at McGill University by means of wavelength spectrometry (WDS) using a JEOL JXA 8900L facility operated at 20 kV, 20 nA, with a beam diameter set at 3 µm. The following X-ray lines were used: CuKα, NiKα, FeKα, CoKα, IrLα, RhLα, PtLα, PdLβ, OsMα, RuLα, and SKα. Counting times were 20 s on the peak. The Phi-Rho-Z method of corrections was applied. The peak-overlap corrections included Fe → Co, Os → Ir, Ru → Pd, Ir → Pt, and Ru → Rh corrections. The standards used are pure metals for the platinum-group elements (PGE), Fe, Cu, Co, and pentlandite for Ni and S. Values of standard deviations (σ) are ≤0.2 wt.% for Ir, Pt, and Cu, ≤0.1 wt.% for Ru, Os, Fe, and Ni, and ≤0.05 wt.% for Rh, Co, and S.

Appearance, Properties, and Morphology
Ferrotorryweiserite occurs as small grains (≤20 µm), which are portions of composite, droplet-like inclusions (up to 50-70 µm in diameter) of PGM, mainly oberthürite or pentlandite enriched in Rh, laurite, Pt-(Pd)-Fe alloy (isoferroplatinum or Fe-Pd-rich platinum), braggite-vysotskite, vasilite, and chalcopyrite, hosted by placer grains of Os-or Ir-dominant Os-Ir alloys (≤0.5 mm across) (Figures 1 and 2a,b). In general, the PGM assemblage is dominated by Os-Ir-Ru alloys, where osmium is a major species, and iridium is subordinate. Rutheniridosmine is rare. The compositional field is limited to the Ru-poor portion of the system Os-Ir-Ru by the line Ru:Ir = 1 in the entire Sisim-Ko-Seyba placer area [9]. In addition, ferrotorryweiserite has also been observed in the Marathon deposit, Coldwell Complex, Ontario, Canada, in association with torryweiserite and oberthürite, Rh 3 (Ni , Fe) 32 S 32 [3]. Ferrotorryweiserite develops as anhedral grains; no evidence of twinning was observed. The c:a ratio calculated from the unit-cell parameters is 4.8502.
In reflected light, ferrotorryweiserite is gray to brownish gray. Bireflectance is very weak to absent. The mineral is not pleochroic or weakly pleochroic (gray to light brown tints). It is weakly anisotropic (gray to light yellow tints). Internal reflections were not observed; neither reflectance values nor streak could be examined because of the small grain sizes. The mineral is opaque; its luster is metallic. Fluorescence and tenacity were not determined, and hardness values (Mohs or micro-indentation) could not be measured owing to the small grain size. No cleavage or fracture were observed. The density also could not be measured. The calculated value of density, 5.93 g·cm −3 , is based on the unit-cell volume (1484 Å 3 ) derived from diffraction measurements by means of synchrotron radiation and the empirical formula.

Mineral Association and Compositions
The silicate core, mantled by the rim-like aggregate of grains of ferrotorryweiseritetorryweiserite and other PGM (Figures 1 and 2b,c), is composed mainly of microgranular olivine, Fo 73-76 , with a subordinate quantity of sodic plagioclase enriched in albite: Ab 82-86 (  Various species of PGM and base-metal sulfides were recognized in association with ferrotorryweiserite: braggite-vysotskite, vasilite, laurite, oberthürite, or Rh-rich pentlandite, Pt-(Pd)-Fe alloys, and chalcopyrite (Table 2). Our compositions are consistent with pentlandite, corresponding to the general formula Rh(Ni 4 Fe 4 )S 8 or (Rh, Co, Pd)(Ni 4 Fe 4 )S 8 , in which one atom per formula unit of Rh + Pd + Co likely enters a distinct site.  19) were recalculated based on a total of 100 at %. A minor amount of S (1.34 wt.%), present in analysis #15, is ascribed to interference from the associated sulfide. The slightly lower totals observed in some of the analyses are attributed to an insufficient volume of the grains.

Chemical Composition and Formula of Ferrotorryweiserite
Mean results of electron-microprobe analyses (WDS) of four grains of ferrotorryweiserite, each based on four data-points (n = 4; Table 3), correspond to the following formula based on a total of 31 atoms per formula unit (apfu) by analogy and according to the structure of the Ni-dominant analog, i.e., torryweiserite [3]  Compositional variations documented in the ferrotorryweiserite-torryweiserite series in the River Ko placer area are presented in Table 4. Compositions of grains of ferrotorryweiserite from other complexes and ore deposits are listed in Table 5.

Crystallography and Results of Synchrotron Micro-Laue Diffraction Study
A standard single-crystal study could not be conducted because of the small size of the grains. Thus, we have performed a synchrotron X-ray study, which also is a single-crystal diffraction technique, of ferrotorryweiserite grains analyzed by Laue microdiffraction at the 12.3.2 beam line of the Advanced Light Source (ALS). The Laue diffraction patterns were collected using a PILATUS 1M area detector operated in reflection geometry. The patterns were indexed and analyzed using XMAS v.6 (Tamura, 2014) [12]. Monochromator energy scans were used to determine the unit-cell parameters and to evaluate the basic symmetry features of ferrotorryweiserite.
The X-ray powder-diffraction pattern derived for ferrotorryweiserite from results of synchrotron micro-Laue diffraction study (based on principles described in [12]) is listed in Table 6 together with the observed pattern for torryweiserite (IMA2020-048) from the Marathon deposit, Coldwell complex, Ontario, Canada [3]. The characteristics of ferrotorryweiserite are compared with those of related species in Table 7. Based on these results, it is inferred that ferrotorryweiserite is isostructural with torryweiserite. By analogy with the structural motif and assignments presented for torryweiserite by McDonald et al., 2021 [3], the crystal structure of ferrotorryweiserite is composed of three distinct layers of poly-hedra stacked along [001]. The first is a layer of Rh1S 6 octahedra sharing edges; it has octahedral voids, such as are found in dioctahedral micas. The second is a mixed layer composed of Rh2S 6 octahedra, Fe1S 4 , and Fe2S 4 tetrahedra arranged in a pinwheel fashion, with Rh2S 6 at the center. The third layer consists of a double sheet of Fe3S 4 tetrahedra that share edges along [001] to form six-membered Fe3S 4 rings. Thus, the structure is related to that of synthetic Cu 4 Sn 7 S 16 [16]. It is described in detail and illustrated in the article on torryweiserite [3].

Name and Type Material
The name ferrotorryweiserite reflects its Fe-dominant composition as an analog of torryweiserite.

Genetic Implications and the Ferrotorryweiserite-Torryweiserite Series
The inferred terrane affinities and mineral associations indicate that the placer PGM grains with inclusions of the ferrotorryweiserite-torryweiserite solid solution are related to zones of chromitite within ultramafic units of the Lysanskiy layered complex of duniteperidotite-gabbro. Indeed, detrital grains of chromian spinel (up to 16.6 wt.% MgO in magnesiochromite) are associated with PGM grains in the Sisim placer zone.
This complex is the likely primary source for the PGM-bearing placers for the entire Sisim placer zone. The direct intergrowths of PGM with an REE mineral, monazite-(Ce), documented at Sisim [9], resemble examples of atypical mineralization reported from the Oktyabrsky deposit, Norilsk complex, Russia [19]. Heterogeneous micro-inclusions rich in Ti, documented in the placer grains of PGM, are consistent with the inferred provenance related to the Lysanskiy complex, which is known to contain reserves of titanium [9]. The Ti-enriched composition of augite (Table 1) is notably consistent.
Members of the ferrotorryweiserite-torryweiserite series associated with the tamuraitekuvaevite solid solution likely formed at an advanced stage of crystallization because of the buildup in sulfur in droplets of incompatible residual melt enriched in Ni, Fe, Cu, Rh, and lithophile elements during the formation of the alloys in lode zones of chromitites in the Lysanskiy layered complex, Eastern Sayans.
The extent of Fe-for-Ni substitution is determinative for members of the inferred ferrotorryweiserite-torryweiserite series (Figure 3a,b), as is reflected in data for related PGM from the River Ko placer, Eastern Sayans, Russia (this study), the Marathon deposit, Coldwell complex, Ontario, Canada [3], the Tulameen Alaskan-type complex, British Columbia, Canada [13], the Yubdo placer deposit in Ethiopia [14,20], and the Miass placer zone, southern Urals, Russia [15].
A plot of Fe vs. Ni (Figure 3a) shows a negative correlation (R = −0.73), calculated for Fe vs. Ni and based on a total of 39 data points available for members of the series, which contain substantial Cu. The Rh vs. Ir correlation is inverse (R = −0.97 for n = 39: Figure 3b). Note that correlations of Fe vs. Rh, Fe vs. Ir, Ni vs. Rh, and Ni vs. Ir all are statistically insignificant. Thus, the observed substitutions of Fe vs. Ni and Rh vs. Ir operate independently in the structure, and the PGE occupy crystallographic sites different from those of Ni+Fe. This is corroborated by crystal-chemical considerations reported for the type specimen of torryweiserite from the Marathon deposit, Coldwell complex, Ontario, Canada [3].

The Olivine-Plagioclase Inclusion and Its Significance
We describe the occurrence of a highly unusual core in a globular inclusion hosted by the Ir-Os alloy (Figures 1 and 2b-d). It is composed of microgranular olivine, subordinate amounts of sodic plagioclase, and skeletal or lamellar grains of titaniferous augite. These silicates are rimmed by an intermediate member of the ferrotorryweiserite-torryweiserite solid solution, oberthürite or Rh-enriched pentlandite, laurite, vasilite, braggite-vysotskite, and other species.
There is a remote similarity with parageneses of olivine and plagioclase known in inclusions in carbonaceous chondrites, i.e., POIs [21] or, e.g., with the olivine-anorthite melt inclusions present in allivalite of the low-K tholeiitic island-arc series of the Kuril-Kamchatka island, Russia [22]. On the other hand, note that the association in the globule hosted by iridium differs drastically by its high enrichment in Na (Ab 82-86 ), which clearly cannot have resulted from an equilibrium crystallization with olivine of the Fo 73-76 compo-sition. Thus, we infer metastable conditions of crystallization with effects of overcooling, which are consistent with the development of skeletal crystals of titaniferous augite in the core (Figure 2d).
Interestingly, the sulfide phases were deposited along the periphery and around the silicate core, thus indicating a highly efficient differentiation and fractionation of sulfur and ore components, PGE, Cu-Ni-Fe, in the late portion of residual melt crystallizing after the silicate globule. Laurite, inferred to be a result of magmatic crystallization, e.g., [23], occurs in the intimate association with palladium species, such as braggite-vysotskite (Figure 2b), which formed under submagmatic conditions, cf. [24]. These observations are consistent with the hydrothermal origin of laurite, as inferred in the Imandra complex, Russia [25], and could well imply a similarly low-temperature crystallization of laurite, ferrotorryweiserite, and associated species in the ore assemblage deposited in the rim assemblage.
The overall variations in compositions of the ferrotorryweiserite-torryweiserite series (Figure 3a,b) can reflect Ni/Fe and Rh/Ir ratios that vary importantly among droplets of fractionated melt or portions of fluid in various ore deposits. In addition, sulfur fugacities can also be important to control the Ni/Fe ratio, as is known in pentlandite, e.g., [26]. In the Marathon deposit, an increase in oxygen fugacity likely played a major role in the formation of torryweiserite-ferrotorryweiserite via the progressive oxidation of precursor grains of pentlandite and intermediate oberthürite [3].