Background information

As noted, melonite and its synthetic analogue normally contain <0.4 apfu Bi (Barkov et al., Reference Barkov, Shvedov, Flemming, Vymazalová and Martin2017a, and references therein). Maucherite generally contains between 18 and 265 ppm Pd (Gervilla et al., Reference Gervilla, Cabri, Kojonen, Sie, Papunen and Hach-Alí2000). In experiments, the extent of Pd-enrichment increases with decreasing temperature, to attain 5.5 at.% Pd at 450°C (Gervilla et al., Reference Gervilla, Makovicky, Makovicky and Rose-Hansen1994). Elevated contents, up to 1.5–1.8 wt.% Pd, are known in grains of maucherite from the Sudbury, Canada and Noril'sk, Russia complexes (Cabri and Laflamme, Reference Cabri and Laflamme1976; Gritsenko and Spiridonov, Reference Gritsenko and Spiridonov2008). In addition, maucherite is one of the principal arsenides deposited from melts rich in As under experimental and natural conditions (Prichard et al., Reference Prichard, Fisher, McDonald, Knight, Sharp and Williams2013). Arsenic-rich melts segregated experimentally via immiscibility from sulfide melts are capable of efficiently scavenging the initially dissolved PGE, especially Pd (Fleet et al., Reference Fleet, Chryssoulis, Stone and Weisener1993; Makovicky et al., Reference Makovicky, Makovicky and Rose-Hansen1992). Elevated contents of Cu (and S), up to ~5 wt.% each, were documented in grains of maucherite from metamorphosed Cu–Ni ores of the Noril'sk complex, Russia (Gritsenko and Spiridonov, Reference Gritsenko and Spiridonov2008).

Geological setting and ore-mineral associations

Both ognitite and Co-rich maucherite occur in dunite in zones of disseminated Cu–Ni–PGE sulfide mineralisation in the Ognit (or Medek) dunite–wehrlite complex of Neoproterozoic age, located at the southern margin of the Siberian Craton, Irkutskaya (Irkutsk) oblast, Russia (Figs 1a, b). The Ognit complex belongs to a suite of dunite–peridotite–pyroxenite complexes, namely Shumikha, Kingash, Golumbei, Tartai, Ognit, Zhelos, Tokty-Oi and Malyi Zadoi, associated with the Yoko–Dovyren complex in the Baikal–Patom zone. These complexes formed in a continental margin setting in the East Siberian metallogenic province (e.g. Mekhonoshin et al., Reference Mekhonoshin, Tolstykh, Podlipsky, Kolotilina, Vishnevsky and Benedyuk2013; Tolstykh et al., Reference Tolstykh, Polyakova, Izokh, Podlipsky, Mekhonoshin, Orsoev and Kolotilina2014). They give a radiometric age in the range 731–710 Ma, which is close to the range reported for the Franklin Province in northwestern Canada, 725–710 Ma, and is thus consistent with the inferred breakup of Rodinia in Neoproterozoic time (Gladkochub et al., Reference Gladkochub, Wingate, Pisarevsky, Donskaya, Mazukabzov, Ponomarchuk and Stanevich2006; Ernst et al., Reference Ernst, Hamilton and Soderlung2012; Tolstykh et al., Reference Tolstykh, Polyakova, Izokh, Podlipsky, Mekhonoshin, Orsoev and Kolotilina2014).

Fig. 1. (a) The location of the Ognit complex in the Russian Federation and (b) geological map of the Ognit complex, Eastern Sayans, Russia, based on various sources (Mekhonoshin et al., Reference Mekhonoshin, Tolstykh, Podlipsky, Kolotilina, Vishnevsky and Benedyuk2013, Reference Mekhonoshin, Kolotilina and Doroshkov2018, and references therein; Oleshkevich et al., pers. comm.). The sample location is shown for ognitite (labelled Ogn) and Co-rich maucherite (Mch).

The Ognit complex is a relatively small ultramafic body ≤1.5 km long at the contact of granodiorite and gneiss. It is mostly composed of dunite, wehrlite, and olivine pyroxenite. The core-like zone is essentially dunitic; elements of large-scale layering are recognised in the complex (Fig. 1b). A notable feature is the development of a pronounced zone of metasomatic alteration, especially at the southeastern contact.

The ultramafic rocks of the Ognit complex are variably serpentinised; they are composed of a serpentine-group mineral, clinochlore and calcic amphiboles (actinolite and tschermakite). Chromite, including skeletal grains of low-Al compositions (Figs 2a, 3, Table 1), and magnetite are common accessories. The ore zones consist of sulfides (≤3 modal%), mainly pentlandite and chalcopyrite, subordinate troilite and cubanite, rare heazlewoodite, pyrite, bornite, mackinawite, secondary chalcocite, covellite, violarite, valleriite and, occasionally, sphalerite and molybdenite. Less common/rare species of ore minerals include orcelite, maucherite, cobaltite–gersdorffite, altaite, hessite, Se-bearing galena, shandite, parkerite, native silver and Ag–Au alloy (Ag-dominant), auricupride, awaruite, native copper, graphite and native bismuth (Shvedov and Barkov, Reference Shvedov and Barkov2017).

Fig. 2. A back-scattered electron (BSE) image (a) and reflected light microphotograph (b) showing the zoned grain of maucherite (Mch)–orcelite (Orc), associated with skeletal grains of chromite (Chr), hosted by fine intergrowths of serpentine (Srp) and clinochlore; Mag is secondary magnetite.

Fig. 3. Compositions of skeletal grains of chromite and associated magnetite zones (shown by blue and brown symbols, respectively) from the Ognit complex in terms of the Cr–Fe³⁺–Al plot (atomic %). The positions of Fe–Ti and Cr–Al trends and the miscibility gap are from Barnes and Roeder (Reference Barnes and Roeder2001).

Table 1. Compositions of skeletal grains of chromite from the Ognit complex, Eastern Sayans.

Notes: 10(R) refers to a rim zone of skeletal grain of chromite; *FeO(tot): all Fe is expressed as FeO. FeO and Fe₂O₃ are calculated on the basis of stoichiometry and charge balance.

Mg# is 100Mg/(Mg + Fe²⁺+Mn); Cr# is 100Cr/(Cr + Al), and Fe³⁺# is 100Fe³⁺/(Fe³⁺ +Cr + Al).

‘–’ = below the detection limit

Two associations of platinum-group minerals (PGM) exist in the Ni–Cu–PGE zones at Ognit. The first includes species rich in Ir–Ru–Os: Ru–Os disulfides (laurite–erlichmanite series), Ru–Os diarsenides (anduoite–omeiite) and sulfarsenides (platarsite–osarsite–ruarsite). These PGM, crystallising relatively early, exhibit a close affinity to primary grains of base-metal sulfides. The second association is late, more diverse, and mainly consists of Pd-rich PGM: palladium bismuthotellurides and antimonides, such as phases of the type Pd(Bi,Te,Sb)_1+x, merenskyite, michenerite, mertieite-II and stibiopalladinite, naldrettite, polarite, urvantsevite, froodite, palladogermanide (Pd₂Ge: cf. IMA2016-086) and unnamed Pd_2+xCu_1–x(Sb,Sn) (Shvedov and Barkov, Reference Shvedov and Barkov2017).

Analytical methods

Compositions of ognitite were established via wavelength-dispersive spectrometry (WDS) using a JEOL JXA-8900L electron microprobe, at 20 kV, 50 nA, and with a 5-μm beam size at McGill University, Montreal, Canada. The following lines and standards were used: Kα and CoNiAs (for Ni), Kα and metallic iron (Fe), Kα and CuFeS₂ (Cu), Kα and CoNiAs (Co), Lα and Pd₃HgTe₃ or metallic Pd (Pd), Mβ and metallic Pt (Pt), Lα and PbTe (Te), Mα and Bi₂Te₃ (Bi), Mα and PbTe (Pb), Kα and Bi₂S₃ (S), Lβ and CoNiAs (As), and Lβ and pure Sb (Sb). The PRZ correction was applied. The counting times were 150 s for As, 60 s for Pd, Pt, Co, Sb and Pb, and 20 s for Ni, Fe, Cu, Te, Bi and S. Minimum detection limits (wt.%) are: 0.01 (S and Co), 0.02 (Fe), 0.03 (Ni), 0.04 (Cu, Pd, Pt, Pb and As), 0.05 (Sb), 0.06 (Te) and 0.08 (Bi).

The Co-rich maucherite and associated minerals (chromite, clinochlore and serpentine) were analysed with a JEOL JXA-8200 microprobe at the E.F. Stumpfl laboratory, University of Leoben, Austria. The analytical conditions (WDS with a ≤2 µm beam) used for the zoned maucherite grain were: 20 kV, 10 nA, and 20 s counting times for the peak and 10 s for the backgrounds. The following lines and standards were used: NiKα (pentlandite), FeKα, SKα (pyrite), AsLα, CoKα (skutterudite), BiMα, TeLα (synthetic Bi₂Te₃), and SbLα and SbLβ (Sb₂Te₃). The detection limits expressed in ppm are: 100 for As and S, 150 for Ni, Fe and 600 for Co, Sb, Bi and Te. Levels of Sb, Bi and Te were below the detection limit.

Scanning-electron microscopy (SEM) and energy-dispersive analysis (EDS) were also employed, using a MIRA 3 LMU (Tescan Ltd.) system combined with INCA Energy 450+ XMax 80 (Oxford Instruments Ltd), at the Institute of Geology and Mineralogy, Novosibirsk, Russia. The reflectance measurements on ognitite were carried out using a WTiC standard at the Natural History Museum, London, UK.

Single-crystal X-ray diffraction data were collected on the ognitite specimen using an Oxford Diffraction Xcalibur diffractometer equipped with an Oxford Diffraction CCD detector, with graphite-monochromatised MoKα radiation (λ = 0.71073 Å).

The powder X-ray diffraction data were obtained on the same fragment of the ognitite grain used for the single-crystal study with an Oxford Diffraction Xcalibur PX Ultra diffractometer fitted with a 165 mm diagonal Onyx CCD detector and using copper radiation [CuKα (λ = 1.54184 Å)]. The working conditions were 40 kV and 40 nA with 3 hr of exposure; the detector-to-sample distance was 7 cm. The program CrysAlis RED (Oxford Diffraction, 2006) was used to convert the observed diffraction rings to a conventional powder-diffraction pattern. The least-squares refinement gave the unit-cell values listed below.

We also provide results of synchrotron X-ray Laue microdiffraction measurements at beam line 12.3.2 of the Advanced Light Source (ALS), Berkeley, California, USA. A polychromatic X-ray beam (5–24 keV) was focused to approximately 2 µm × 2 µm (full width at half maximum) with Kirkpatrick–Baez mirrors. Laue diffraction patterns were collected with a PILATUS 1M area detector in reflection geometry, with the sample inclined 45° to the primary beam, and the area detector was set to 90°. The Laue diffraction patterns were collected with raster scanning across the entire sample area. The patterns were indexed and analysed using XMAS v.6 (Tamura, Reference Tamura, Barabash and Ice2014). An energy scan (10–20 keV, 10 eV step size) was performed on single reflections to determine the lattice parameters.

Occurrence, physical and optical properties, and composition of ognitite

Ognitite forms a single homogeneous grain (80 µm × 30–40 µm) hosted by chalcopyrite; it is anhedral, irregularly shaped, and occurs next to a small grain of native bismuth (Fig. 4). The new species is metallic and opaque; its colour and streak are metallic black (powder). The value of its density (calculated) is 8.75 g/cm³.

Fig. 4. Reflected light microphotograph (a) showing the ognitite grain (Ogn), which is hosted by chalcopyrite (Ccp), and is associated with native bismuth, hessite (Hs), altaite (Alt), and also with late veinlets of magnetite (Mag). (b) BSE image showing ognitite (Ogn) and associated grains, complementary to those shown in the optical photograph in (a). Type specimen (#3292/I).

Under reflected light, ognitite is creamy white. It is slightly to distinctly bireflectant, with a pleochroism from creamy white to slightly lighter creamy white. Ognitite is slightly to distinctly anisotropic. Internal reflections were not observed. Reflectance values of ognitite (WTiC standard, in air) are listed in Table 2.

Table 2. Reflectance values for ognitite.

Notes: These values were measured in air; the standard used is WTiC.

The values required by the Commission on Ore Mineralogy are given in bold.

Representative compositions of ognitite are given in Table 3. Other elements (Co, Pt, Sb, Pb, As and S) were also sought, but not detected. The mean composition (and ranges for seven point analyses, n = 7) are: Ni 17.05 (16.91–17.15), Fe 0.07 (0.02–0.16), Cu 0.14 (0.08–0.26), Pd 0.14 (0.09–0.16), Te 32.53 (31.79–33.67), Bi 49.64 (48.27–50.31), total 99.57 (99.01–100.07) wt.%. Minor Pd is thus present. Results of two independent methods of microprobe analyses (WDS and SEM/EDS) are in excellent agreement; they yield essentially the same empirical formula: (Ni_1.11Cu_0.008Fe_0.005Pd_0.005)_Σ1.13Bi_0.90Te_0.97, calculated on the basis of a total of 3 apfu. A minor extent of Ni-for-Bi substitution is thus implied. Examples of Ni–Bi substitution are known in some compounds: LaNi_4.7−xBi_xAl_0.3 with x = 0.1, 0.2 or 0.3 (Yilmaz et al., Reference Yilmaz, Kilicaslan, Atanur, Hong and Uzun2012) and Bi_2−xNi_xSr₂CaCu₂O_y with x = 0, 0.05, 0.1 or 0.2 (Özkurt, Reference Özkurt2012). In addition, a mean electron number <83 at the Bi site of ognitite is consistent with the inferred presence of a lighter element (presumably Ni) at the 1b position. The ideal formula of ognitite is NiBiTe, which requires Ni 14.85, Bi 52.87 and Te 32.28, for a total of 100 wt.%.

Table 3. Compositions of ognitite from the Ognit complex.

Note: Analyses #1–7 are results of WDS analyses listed in weight %. Composition #8 is mean result of the seven point analyses (WDS). Analysis #9 pertains to results of an SEM/EDS analysis. Other elements (Co, Pt, Sb, Pb, As and S) were sought (WDS), but not detected. The bottom part of the table shows the compositions recalculated on the basis of three atoms per formula unit.

An initial study of the ognitite grain was undertaken using synchrotron micro-Laue diffraction followed by a monochromator energy scan, results of which indicate that is a single phase, apparently untwinned, with a = 3.925(5) Å and c = 5.381(7) Å. These patterns were indexed and analysed using XMAS v.6 (Tamura, Reference Tamura, Barabash and Ice2014).

Ognitite is trigonal; its space group is P3m1. The unit-cell parameters derived from the single-crystal X-ray diffraction data are: a = 3.928(1) Å, c = 5.385(1) Å and V = 71.95(4) ų, with Z = 1. The c:a ratio calculated from the unit-cell parameters is 1.37.

The powder X-ray diffraction data (Table 4), obtained on the same fragment as used for the single-crystal study, are: a = 3.9332(4) Å, c = 5.3920(6) Å and V = 72.24(1) ų.

Table 4. Measured and calculated X-ray powder diffraction data (d in Å) for ognitite*.

*Calculated diffraction pattern obtained with the atom coordinates reported in Table 6 (only reflections with I _rel ≥ 1 are listed). The strongest diffraction lines are given in bold.

Crystal structure of ognitite

A small fragment (0.040 mm × 0.050 mm × 0.060 mm) was handpicked from the ognitite grain (Fig. 4) and examined with an Oxford Diffraction Xcalibur single-crystal diffractometer equipped with an Oxford Diffraction CCD detector, with graphite-monochromatised MoKα radiation (λ = 0.71073 Å). The collected data were integrated and corrected for standard Lorentz and polarization factors with the CrysAlis RED package (Oxford Diffraction, 2006). The program ABSPACK in CrysAlis RED (Oxford Diffraction, 2006) was used for the absorption correction. Details of the selected crystal, data collection, and refinement are given in Table 5. The calculated value of the R-factor is R ₁ = 0.0276 for 81 reflections with F _o > 4σ(F _o).

Table 5. Data and experimental details for the selected ognitite crystal.

Although the statistical tests (|E ²–1| = 0.756) suggested an acentric space-group, the crystal structure was initially refined starting from the atomic coordinates of melonite, NiTe₂ (Peacock and Thompson, Reference Peacock and Thompson1946) and using the program SHELXL-97 (Sheldrick, Reference Sheldrick2008). Given the observed larger unit-cell volume of ognitite (i.e. 71.95 ų) compared to melonite (i.e. 66.93 ų; Peacock and Thompson, Reference Peacock and Thompson1946), the site-occupancy factor at the crystallographic sites was allowed to vary (Ni vs. structural vacancy at the 1a site, and Bi vs. Te at the 2d site) using scattering curves for neutral atoms taken from the International Tables for Crystallography (Wilson, Reference Wilson1992). After several cycles of anisotropic refinement, a final R ₁ = 0.0795 was achieved, but the atomic displacement parameter at the 2d position was abnormally high and asymmetric, thus suggesting a split into two positions. The introduction of two partially occupied (50%) positions (one partially occupied by Bi and one partially occupied by Te) led the R ₁ index to 0.0608. At this stage, given the indication coming from the normalised structure-factors, we decided to try to solve the structure in the non-centrosymmetric space-group P3m1, which allows the separation of the 2d position into two positions, 1b and 1c. The new refinement (R ₁ = 0.0276) showed that Bi and Te are completely ordered at 1b and 1c, respectively, with Bi probably being replaced by 0.03 Ni (according to the refined mean number of electrons at the site). The Te–Ni and Bi–Ni bond distances vary accordingly: the Te–Ni distance (2.575 Å) is identical to that observed in melonite (2.575 Å; Peacock and Thompson, Reference Peacock and Thompson1946), whereas the Bi–Ni distance (2.696 Å) is in agreement with those observed in parkerite, Ni₃Bi₂S₂ (2.64–2.86 Å; Fleet, Reference Fleet1973b). Ognitite exhibits a brucite-type structure (Fig. 5). The edge-sharing NiTe₃Bi₃ octahedra form sheets parallel to (0001); the sheets are linked by van der Walls interactions.

Fig. 5. The crystal structure of ognitite. (a) Ball and stick representation: red, white and grey spheres refer to Ni, Te and Bi, respectively; (b) polyhedral representation. The unit-cell and the orientation of the figures are outlined.

Atom coordinates, site occupancies, and equivalent isotropic displacement parameters are given in Table 6. Selected bond-distances are given in Table 7. A crystallographic information file has been deposited as supplementary material (see below)

Table 6. Atoms, site occupancy factors (s.o.f.), fractional atomic coordinates and anisotropic displacement parameters (Ų) for the ognitite crystal extracted.

Table 7. Bond distances (in Å) in the structure of ognitite.

Ognitite is thus related to melonite; however, it differs compositionally (i.e. the marked Bi-enrichment) and structurally, because atoms of Bi and Te are ordered at two specific sites, leading to the loss of the centre of symmetry in the crystal structure, corresponding to a variant of the CdI₂-2H structure-type. An additional characterisation of kitkaite (NiTeSe; Häkli et al., Reference Häkli, Vuorelainen and Sahama1965) is probably required to check whether it also belongs to that structure-type.

Co-rich maucherite and zoning in Ni–(Co) arsenides

As noted, the anomalously Co-rich variant of maucherite occurs in the core of a zoned grain (Figs 2b, 6a,b). The rim, composed of orcelite, is essentially devoid of Co (<0.1 wt.%), in contrast to the core zone, which contains ~8 to 10 wt.% Co (or 1.7–2.0 Co apfu: Table 8, #1–6). At least in some cases, the quantitative SEM/EDS method is preferable in analysing narrow zones or tiny phases. Indeed, the SEM/EDS composition (#6, Table 8) is somewhat closer to the ideal stoichiometry; nevertheless, the WDS results obtained (Table 8, #1–5) are consistent.

Fig. 6. BSE image (a) of the zoned grain of Co-rich maucherite (core: Mch) and orcelite (rim: Orc) hosted by serpentine. (b) False-colour X-ray map showing the distribution of Co in the zoned grain of Ni–(Co) arsenides.

Table 8. Compositions of Co-rich maucherite and associated orcelite in a zoned grain from the Ognit complex.

Note: Results of WDS (#1–5) and SEM/EDS (#6, 7) analyses are listed in the top part of the table in wt.%. Analyses #1–6 pertain to maucherite. Analysis #7 pertains to orcelite. Atomic proportions, are based on a total of 19 atoms per formula unit (apfu) for maucherite, and on two apfu S for orcelite.

‘–’ = not detected.

Our results of the synchrotron micro-Laue diffraction followed by a monochromator energy scan yielded the following cell parameters for the Co-rich maucherite at Ognit: a = 6.85(2) and c = 21.83(5) Å. Thus, these data agree well with those known for the ‘normal’ phase of maucherite (Co-free; Fleet, Reference Fleet1973a).

The orcelite rim corresponds to Ni₅As₂ (Table 8, #7). The structure of orcelite [Ni_5–xAs₂; x = 0.25] represents a distorted variant of the Pd₅Sb₂ structure (Bindi et al., Reference Bindi, Tredoux, Zaccarini, Miller and Garuti2014).

As noted, the Co-rich maucherite–orcelite crystal is hosted by a matrix of fine intergrowths of serpentine and clinochlore (Fig. 2). Five grains of serpentine (WDS data) yielded the composition (Mg_2.69–2.74Fe_0.09–0.11Al_0.02–0.08Cr_0–0.02Mn_<0.01Ni_<0.01)_{Σ2.83–2.90}Si_2.03–2.08O₅(OH)₄ (calculated for O = 7 apfu). The Mg# index [100Mg/(Mg + Fe²⁺ + Mn)] is high: ~97.

Compositions of six grains of clinochlore analysed (WDS) in this sample are uniformly rich in Mg, and in Cr, with 2.93–3.21 wt.% Cr₂O₃, and correspond to (Mg_4.57–4.70Fe_0.21–0.24Mn_<0.01Ni_≤0.01)_{Σ4.80–4.96}Al_1.0(Si_3.00–3.09Al_0.73–0.80Cr_0.22–0.25Ti_≤0.01)_{Σ4.01–4.10}O₁₀(OH)₈ (for O = 14 apfu). Their values of the Mg# index are correspondingly high: up to ~96.

The skeletal grains of chromite associated with the zoned grain of Co-rich maucherite–orcelite (Fig. 2a) are weakly Mg-rich, with Mg# in the range 17–22, and are low in aluminium (≤6.5 wt.% Al₂O₃); they display very high values of Cr# [100Cr/(Cr + Al + Fe³⁺]: 85–92 (Table 1). In addition, elevated levels of Ti (0.7–1.4 wt.% TiO₂), V (≤0.3 wt.% V₂O₃), Mn (0.9–1.0 wt.% MnO), and Zn (0.5–0.7 wt.% ZnO) are present. The chromite grains have a narrow rim of virtually pure magnetite (Fig. 3, Table 1, #10).