Tamuraite, Ir 5 Fe 10 S 16 , a New Species of Platinum-Group Mineral from the Sisim Placer Zone, Eastern Sayans, Russia

: Tamuraite, ideally Ir 5 Fe 10 S 16 , occurs as discrete phases ( ≤ 20 µ m) in composite inclusions hosted by grains of osmium ( ≤ 0.5 mm across) rich in Ir, in association with other platinum-group minerals in the River Ko deposit of the Sisim Placer Zone, southern Krasnoyarskiy Kray, Russia. In droplet-like inclusions, tamuraite is typically intergrown with Rh-rich pentlandite and Ir-bearing members of the laurite–erlichmanite series (up to ~20 mol.% “IrS 2 ”). Tamuraite is gray to brownish gray in reﬂected light. It is opaque, with a metallic luster. Its bireﬂectance is very weak to absent. It is nonpleochroic to slightly pleochroic (grayish to light brown tints). It appears to be very weakly anisotropic. The calculated density is 6.30 g · cm − 3 . The results of six WDS analyses are Ir 29.30 (27.75–30.68), Rh 9.57 (8.46–10.71), Pt 1.85 (1.43–2.10), Ru 0.05 (0.02–0.07), Os 0.06 (0.03–0.13), Fe 13.09 (12.38–13.74), Ni 12.18 (11.78–13.12), Cu 6.30 (6.06–6.56), Co 0.06 (0.04–0.07), S 27.23 (26.14–27.89), for a total of 99.69 wt %. This composition corresponds to (Ir 2.87 Rh 1.75 Pt 0.18 Ru 0.01 Os 0.01 ) Σ 4.82 (Fe 4.41 Ni 3.90 Cu 1.87 Co 0.02 ) Σ 10.20 S 15.98 , calculated based on a total of 31 atoms per formula unit. The general formula is (Ir,Rh) 5 (Fe,Ni,Cu) 10 S 16 . Results of synchrotron micro-Laue diffraction studies indicate that tamuraite is trigonal. Its probable space group is R 3 m (#166), and the unit-cell parameters are a = 7.073(1) Å, c = 34.277(8) Å, V = 1485(1) Å 3 , and Z = 3. The c : a ratio is 4.8462. The strongest eight peaks in the X-ray diffraction pattern [ d in Å( hkl )(I)] are: 3.0106(216)(100), 1.7699(420)(71), 1.7583(2016)(65), 2.7994(205)(56), 2.9963(1010)(50), 5.7740(102)(45), 3.0534(201)(43) and 2.4948(208)(38). The crystal structure is derivative of pentlandite and related to that of oberthürite and torryweiserite. Tamuraite crystallized from a residual melt enriched in S, Fe, Ni, Cu, and Rh; these elements were incompatible in the Os–Ir alloy that nucleated in lode zones of chromitites in the Lysanskiy layered complex, Eastern Sayans, Russia. The name honors Nobumichi Tamura, senior scientist at the Advanced Light Source of the Lawrence Berkeley National Laboratory, Berkeley, California.


Introduction
Tamuraite, a new mineral species of ideal composition Ir 5 Fe 10 S 16 , was found in a placer along the River Ko, south of Krasnoyarsk, Russia (approximate location 54 • 47 N, 93 • 16 E). The heavy-mineral fraction recovered from the River Ko deposit and the placer in the neighboring River Seyba is mainly derived from ultramafic units of the Lysanskiy layered complex, Eastern Sayans, Russia [1][2][3]. The development of placers bearing platinum-group minerals (PGM) corresponds to the Sisim river system in the southern portion of Krasnoyarskiy Kray in South Central Siberia (Figure 1a,b). Our aims are to describe the mode of occurrence, associations and properties of tamuraite. The mineral and its name were approved (IMA 2020-098) by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association. Figure 1. Location of the River Ko placer deposit, which forms part of the Sisim Placer Zone, in the Krasnoyarskiy Kray of the Russian Federation (a). Geological sketch-map shows the distribution of the main massifs (after [4], and references therein) in the placer area, which is drained by the rivers Ko and Seyba, tributaries of the River Sisim (b).

Geological Context, Materials and Methods
Hundreds of placers grains were collected and investigated at different times in the River Sisim Placer Zone [1][2][3]. Grains of tamuraite form part of roundish composite inclusions of sulfide minerals rich in platinum-group elements (PGE); the polymineralic inclusions are hosted by Os-Ir alloy grains. Many of the main massifs exposed in the area investigated and shown schematically in Figure 1b ( [4], and references therein) are composed dominantly of bodies of granite and granodiorite (namely Arzybeiskiy, Angulskiy, Derbinskiy and Kanzubinskiy, among others), peralkaline granite (Okunevskiy), members of the gabbro-diorite-granodiorite (Shindinskiy, Olkhovsko-Chibizhekskiy) and gabbromonzonite-syenite associations (e.g., Buedzhinskiy). None of these can be considered as the potential provenance of the PGM-bearing placer studied at River Ko. In the vicinity, two igneous complexes contain abundant ultramafic units: (1) the clinopyroxenite-gabbroanorthosite suite of the Kulibinskiy layered complex (i.e., Kuvaiskiy and Shirokologskiy massifs) and (2) the dunite-peridotite-gabbro series of the Lysanskiy layered complex (Figure 1b). The latter is especially pertinent; it is large, composed mainly of three blocks (Lysanskiy, Podlysanskiy and Kedranskiy,~0.5 km × 30 km in extent overall), and along with smaller fragmented bodies (Figure 1b). The complex displays tectonic contacts with the host rocks of the Bakhtinskaya suite of upper Proterozoic age; this suite comprises various basalts, andesitic basalts and volcanic tuff. The complex is formed of alternating sequences of serpentinite, wehrlite, lherzolite, harzburgite, clinopyroxenite, websterite, gabbronorite, troctolite, gabbro, and anorthosite. Some massifs of this complex are dominantly ultrabasic, and others contain mostly basic rocks. The complex hosts abundant Ti-(V) mineralization and podiform deposits of ilmenite-titanian magnetite ores. This complex was previously considered to represent the source rocks for the PGM-bearing placers associated with the Sisim zone [3].
Electron-microprobe analyses of tamuraite and the host grains of the alloy were conducted at McGill University using wavelength-dispersive spectrometry (WDS) using a JEOL JXA 8900 L instrument operated at 20 kV 20 nA, with a beam diameter set at 3 µm.
The following X-ray lines were used: CuLα, 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 were pure metals for the platinum-group elements, Fe, Cu, Co, and synthetic pentlandite for Ni and S. Values of standard deviations (σ) are ≤0.2 for Ir, Pt, and Cu, ≤0.1 for Ru, Os, Fe, and Ni, and ≤0.05 for Rh, Co and S.
Tiny grains of Rh-rich pentlandite and members of the laurite-erlichmanite series that coexist with tamuraite were analyzed via quantitative energy-dispersive spectrometry (EDS), at 20 kV and 1.6 nA, using a MIRA 3 LMU (Tescan Orsay Holding) scanning electron microscope equipped with an INCA Energy 450 XMax 80 (Oxford Instruments Nanoanalysis, UK) microanalysis energy-dispersive system at the Analytical Center for multi-elemental and isotope studies, SB RAS, Novosibirsk, Russia. The following X-ray lines were used: CuKα, NiKα, FeKα, CoKα, IrLα, RhLα, PtLα and SKα. Minimum detection limits (3σ criterion) are ≤0.2 wt % for S, Fe, Co, Ni, Cu; ≤0.4 wt % for Rh; ≤0.5-0.7 wt % for Ir. The analytical error for the main components did not exceed 1-2 relative% and satisfied the requirements for quantitative analysis. Specimens of certified standards were used: FeS 2 and pure metals. Other details of the analytical techniques applied were summarized in [5].
Synchrotron micro-Laue diffraction studies, followed by monochromator energy scans, were performed using the beamline 12.3.2 on the holotype specimens of tamuraite at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory, Berkeley, California, USA. The Laue diffraction patterns were collected with a PILATUS 1 M area detector using reflection geometry. The patterns were analyzed and indexed using XMAS v.6 following the procedure of Tamura (2014) [6].

Mode of Occurrence and Associated Minerals
The grains of tamuraite (≤20 µm; Figure 2a Table 1), which corresponds to the mineral osmium [7]. Native Ir is also observed, but in subordinate quantities. Alloy grains of rutheniridosmine composition are rare. The composition of the alloys in the entire Sisim-Ko-Seyba placer area is limited to the Ru-poor portion of the system Os-Ir-Ru by the line Ru:Ir = 1 [3].   The pentlandite in the droplet-shaped inclusions is invariably enriched in Rh, with minor amounts of Ir, Pd, other PGE, Cu and Co. Interestingly, the observed content of Rh is relatively constant and close to~1 apfu (Table 2). This feature is reminiscent of the synthesis of the pentlandite-type phase Rh(Ni 4 Fe 4 )S 8 at 700 • C, along with other Fe 4 Ni 4 MS 8 compounds (M = Ru, Pd) [8]. Rhodium is not associated with Co in compositions of the grains analyzed in the River Ko suite, in contrast to specimens of Rh-Co-bearing pentlandite from the River Bolshoy Khailyk deposit, in western Sayans, Russia, in which Rh and Co enter solid solution and substitute for Fe, not Ni, via a coupled mechanism of substitution: Rh 3+ + Co 3+ + → 3 Fe 2+ [9]. Several occurrences of PGE-rich pentlandite have been reported ( [10,11] and references therein). The members of the laurite-erlichmanite series in the tamuraite-bearing globules display notable compositional variations (Table 3, Figure 4a-c). The covariation trends observed are similar to those already reported for the Sisim placers [3]. A relative enrichment in Ir (up to 0.2 apfu) is recorded. The Ir-Os correlation is positive, with a value of the correlation coefficient R = 0.83, based on 31 data points (quantitative SEM/EDS data; Figure 4c). The inverse Ru-Os correlation (Figure 4a) is accompanied by a negative Ru-Ir correlation (R = −0.90; Figure 4b). These findings are consistent with a scheme previously inferred: (Os 2+ + 2Ir 3+ + ) → 4Ru 2+ [3], or with (Os 2+ + Ir 2+ ) → 2Ru 2+ , an alternative that seems less likely. Other mechanisms controlling incorporating Ir into the laurite-erlichmanite solid solution involve a sulfarsenide component [12,13]. The limited nature of the [Ir x (Ru,Os) 1-x ]S 2 solid solutions is well recognized in nature. A continuous and complete series of solid solutions exist between the RuS 2 and OsS 2 end-members (the laurite-erlichmanite series), whereas the "IrS 2 " content is typically less than 20 mol.% [14]. The maximum extent of Ir appears to be documented in erlichmanite at Rudnaya, western Sayans, Russia: (Os 0.59 Ir 0.35 Ru 0.09 ) Σ1.03 (S 1.94 As 0.03 ) Σ1.97 [13]. The inferred limit is likely a result of vacancy-type defects and related complications arising from incorporating Ir [12].

Physical Properties of Tamuraite
The new mineral is gray to brownish gray in reflected light. It appears gray next to grains of Rh-rich pentlandite, which exhibits sharp and discrete boundaries with tamuraite (Figure 2a-d). The mineral is opaque with a metallic luster. Its bireflectance is very weak to absent. It is nonpleocroic to slightly pleochroic (grayish to light brown tints). It

Physical Properties of Tamuraite
The new mineral is gray to brownish gray in reflected light. It appears gray next to grains of Rh-rich pentlandite, which exhibits sharp and discrete boundaries with tamuraite (Figure 2a-d). The mineral is opaque with a metallic luster. Its bireflectance is very weak to absent. It is nonpleocroic to slightly pleochroic (grayish to light brown tints). It appears to be very weakly anisotropic (gray to light yellow tints). Reflectance values could not be measured because of insufficient grain sizes. Its streak was not observed, and the powder could not be extracted owing to the tiny grain size. The fluorescence, tenacity, magnetic properties and micro-indentation hardness also were not evaluated owing to the small grain size. The cleavage, parting, fracture and internal reflections were not observed. Density could not be measured owing to the small grain size.
The calculated value of density is 6.30 g·cm −3 ; it is based on the unit-cell volume inferred from diffraction measurements and the empirical formula obtained for tamuraite in this study.

Chemical Composition of Tamuraite and the Tamuraite-Kuvaevite Series
Results of WDS analyses of tamuraite are listed in  5 (Fe,Ni,Cu) 10 S 16 . The formula is recalculated based on 31 apfu by analogy with the Rh-(Ni)-dominant analog, i.e., torryweiserite (IMA2020-048) and with the Ni-dominant analog, i.e., kuvaevite (IMA2020-043) [15,16]. The total content of metals in the formula unit is 15.03 apfu. A minor amount of a Fe-site cation must be present at the Ir site to achieve the 1:2 stoichiometric proportion. The ideal formula of tamuraite is Ir 5 Fe 10 S 16 , which requires Ir 47.28, Fe 27.48, and S 25.24, a total of 100 wt %. Note: Results of WDS analyses obtained using a JEOL JXA 8900L microprobe. * Composition (in wt %) of two grains of tamuraite investigated by synchrotron micro-Laue diffraction. Mean (#3) is the mean of six datasets.
The extent of Fe-for-Ni substitution is important, and tamuraite forms a well-established series of solid solutions with kuvaevite, ideally Ir 5 Ni 10 S 16 , as is indicated by the WDS data from the River Ko suite ( Figure 5). Tamuraite is the Fe-dominant member, and kuvaevite (IMA2020-043) [15] is the Ni-dominant member of the series; this inference is consistent with our results of the synchrotron-based microdiffraction studies. In addition, note that the type tamuraite has a composition that is the richest in Fe and is close to the end member in the inferred solid-solution series ( Figure 5).
Tamuraite is considered isostructural with torryweiserite (Table 6). It is thus composed of three distinct layers of polyhedra that are stacked along [001]. By analogy with the structural motif and assignments worked out for torryweiserite [16], the first is a layer of Ir1S 6 octahedra sharing edges; it has octahedral voids, such as are found in dioctahedral micas. The second is a mixed layer composed of Ir2S 6 octahedra, Fe1S 4 and Fe2S 4 tetrahedra arranged in a pinwheel fashion, with Ir2S 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. The structure will be described in greater detail in the article on torryweiserite [16].

Etymology and Conservation
The mineral is named after Dr. Nobumichi Tamura (born 1966), senior scientist at the Advanced Light Source of the Lawrence Berkeley National Laboratory, Berkeley, California, to recognize his innovative investigations of minerals and materials by synchrotron microdiffraction. He is also recognized as one of the leading paleoartists, illustrators of the prehistoric denizens roaming the Earth.
The holotype specimen of tamuraite is deposited (catalog number: III-102/2) at the Central Siberian Geological Museum, Sobolev Institute of Geology and Mineralogy, Novosibirsk, Russia.
Tamuraite is distinct from oberthürite (Rh 3 Ni 32 S 32 ), both compositionally and structurally; oberthürite is a member of the pentlandite group (with a = 10.066(5) Å) from the Marathon deposit, Coldwell complex, Ontario, Canada [16,18]. Tamuraite also differs, both compositionally and structurally, from ferhodsite [(Fe,Rh,Ir,Ni,Cu,Co,Pt) 9-x S 8 ] described at Nizhniy Tagil, Urals. The latter exhibits an Rh-enriched composition, with the dominance of Rh over Ir and other PGE, and has a tetragonal unit-cell (P42/n or P4/nmm) with parameters a = 10.009(5) and c = 9.840(8) Å [19]. Members of the tamuraitekuvaevite series are related to Ir 5 (Ni,Fe,Cu) 10  Two separate series, i.e., the PGE-bearing pentlandite series and the ferhodsite series, were recently recognized in a representative set of global occurrences [2]. Based on our micro-Laue data and the latest structural results [16], we suggest that the latter series be renamed the torryweiserite series.

Provenance and Origin of Tamuraite
Our new results obtained for the tamuraite-bearing associations are consistent with a derivation from the Lysanskiy complex (Figure 1b) [3,4,21]. We contend that tamuraite and members of the tamuraite-kuvaevite-torryweiserite solid-solution series formed at an advanced stage of crystallization due to accumulation and buildup in sulfur in droplets of a residual melt enriched in S, Fe, Ni, Cu, and Rh. These elements were all essentially incompatible during the formation of the Os-Ir alloy in lode zones of chromitites (or Cr-Ti-rich zones) in the Lysanskiy layered complex. In the globules of melt trapped in the alloy host, tamuraite and laurite-erlichmanite, both enriched in Ir, likely crystallized before the Rh-rich pentlandite poor in Ir. This sequence is consistent with evolutionary paths established for the Ni-Fe-(Co)-PGE sulfarsenide droplets, which developed with a core-to-rim decrease in their values of the Ir# index, i.e., 100 Ir/(Ir + Rh) [12,22]. Data Availability Statement: Additional data are available from the corresponding author upon a reasonable request or query.