Uakitite, VN, a New Mononitride Mineral from Uakit Iron Meteorite (IIAB) †

: Uakitite was observed in small troilite–daubr é elite ( ± schreibersite) inclusions (up to 100 µ m) and in large troilite–daubr é elite nodules (up to 1 cm) in Fe-Ni-metal (kamacite) of the Uakit iron meteorite (IIAB), Republic of Buryatia, Russia. Such associations in the Uakit meteorite seemed to form due to high-temperature ( > 1000 ◦ C) separation of Fe-Cr-rich sulﬁde liquid from Fe-metal melt. Most inclusions represent alternation of layers of troilite and daubr é elite, which may be a result of solid decay of an initial Fe-Cr-sulﬁde. These inclusions are partially resorbed and mainly located in ﬁssures of the meteorite, which is now ﬁlled with magnetite, and rarely other secondary minerals. Phase relations indicate that uakitite is one of the early minerals in these associations. It forms isometric (cubic) crystals (in daubr é elite) or rounded grains (in schreibersite). The size of uakitite grains is usually less than 5 µ m. It is associated with sulﬁdes (daubr é elite, troilite, grokhovskyite), schreibersite and magnetite. Carlsbergite CrN, a more abundant nitride in the Uakit meteorite, was not found in any assemblages with uakitite. Physical and optical properties of uakitite are quite similar to synthetic VN: yellow and transparent phase with metallic luster; Mohs hardness: 9–10; light gray color with a pinky tint in reﬂected light; density (calc.) = 6.128 g / cm 3 . Uakitite is structurally related to the osbornite group minerals: carlsbergite CrN and osbornite TiN. Structural data were obtained for three uakitite crystals using the electron backscatter di ﬀ raction (EBSD) technique. Fitting of the EBSD patterns for a synthetic VN model (cubic, Fm -3 m , a = 4.1328(3) Å; V = 70.588(9) Å 3 ; Z = 4) resulted in the parameter MAD = 0.14–0.37 ◦ (best-good ﬁt). Analytical data for uakitite ( n = 54, in wt. %) are: V, 71.33; Cr, 5.58; Fe, 1.56; N, 21.41; Ti, below detection limit ( < 0.005). The empirical formula (V 0.91 Cr 0.07 Fe 0.02 ) 1.00 N 1.00 indicates that chromium incorporates in the structure according to the scheme V 3 + → Cr 3 + (up to 7 mol. % of the carlsbergite end-member).

As a result of detailed mineralogical studies two new minerals, were identified in this meteorite: uakitite VN (IMA 2018-003) [26] and grokhovskyite CuCrS 2 (IMA 2019-065) [48]. In this paper, we provide a detailed description of uakitite. Some data on this mineral in the Uakit meteorite were reported in a few previous publications [24][25][26]48,49]. The name of the mineral is given after the meteorite. The holotype specimens of uakitite are on display in the meteorite collections of the CSGM IGM, SB RAS, Novosibirsk (registration numbers 52 and 52b, meteorite Uakit) and in the MBSC, SB RAS, Ulan-Ude (registration number Uakit-MBSC435/G84).

Analytical Methods
Polished fragments were used for optical examination of the Uakit meteorite in reflected light. The identification of all minerals was based on energy-dispersive spectra (EDS), backscattered electron (BSE) images and elemental mapping (EDS system), using a TESCAN MIRA 3MLU scanning electron microscope equipped with an INCA Energy 450 XMax 80 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) at the IGM, Novosibirsk, Russia, and a LEO-1430 scanning electron microscope equipped with an INCA Energy-300 EDS microanalysis system at the GI, Ulan-Ude, Russia. The instruments were operated at an accelerating voltage of 20 kV and a probe current of 1 nA in high-vacuum mode. EDS analyses of uakitite and other minerals were done at an accumulation time of 20-40 s. The following simple compounds and metals were used as reference standards for most of the elements: Ca 2 P 2 O 7 (P), Cr 2 O 3 (Cr), pyrite (S), Si 3 N 4 or BN (N), metallic Ti, Fe, Cu, Zn, Mn, Ni, V and others. Correction for matrix effects was done using the XPP algorithm, implemented in the software of the microanalysis system. Metallic Co served for quantitative optimization (normalization to probe current and energy calibration of the spectrometer). The overlapping of VKβ and CrKα was specially checked using the Cr 2 O 3 and metallic V standards.
Electron backscatter diffraction (EBSD) studies were provided for three grains of uakitite. Samples containing uakitite and intended for EBSD studies were subjected to polishing by BuehlerMasterMet2 non-crystallizing colloidal silica suspension (0.02 µm). EBSD measurements were carried out by means of an FE-SEM ZEISS SIGMA VP scanning electron microscope equipped with an Oxford Instruments Nordlys HKL EBSD detector, operated at 20 kV and 1.4 nA in focused beam mode with a 70 • tilted stage at Institute of Physics and Technology, Ural Federal University, Ekaterinburg, Russia. Structural identification of uakitite was performed by matching its EBSD patterns with the reference structural models using program FLAMENCO.

General Description of the Uakit Meteorite
The 3.96 kg mass of the meteorite is oval (10 × 10 × 7 cm). The exterior part is covered by thin crust of brown to yellow-brown secondary products (mainly, different Fe-rich hydroxides, Figure 1). Polished and then etched surfaces of the meteorite cut-offs show the presence of large (≈ 2 cm) kamacite crystals with evident Neumann lines; no Widmannstatten pattern is observed (Figure 2). Weathering and fusion crusts are less than 1 mm in the exterior. Sometimes they extend together along some fractures of the outer part, but their abundance is low (Figure 1). The shock stage is medium and mainly fixed by shifting of blocks in some schreibersite and carlsbergite crystals and by Neumann lines.
The  [25]. The Uakit iron meteorite is structurally and geochemically characterized to be a hexahedrite, IIAB group, with tendency to the IIA subgroup.
The presence of large sulfide nodules (up to 1 cm) of troilite-daubréelite composition is common in the Uakit meteorite ( Figure 1). Cohenite occurs mainly near the exterior and forms skeletal crystals up to 0.5 mm; sometimes it is observed on the boundary between kamacite grains. Ni-poorer taenite (<33 wt. % Ni), Ni-rich kamacite (>6.3 wt. % Ni) and Fe-rich nickelphosphide are most typical for the cohenite-containing associations.  Schreibersite (20-100 µm) and carlsbergite (1-10 µm) elongated crystals are sometimes oriented in one or more directions within large kamacite grains ( Figure 2). In addition to the above assemblages, the presence of rounded sulfide globules (mainly daubréelite + troilite, up to 100 µm), plessite isolations (taenite + kamacite + tetrataenite) and mono-or polymineralic inclusions are very common in kamacite (Figures 3-7). Sulfide globules with a "layered structure" (alternating layers of troilite and daubréelite as a possible result of solid decay of initial high-temperature Fe-Cr-sulfide) are more widespread than those without layering. Namely, in such associations two new minerals (uakitite, grokhovskyite), Fe-rich kalininite (first finding in meteorites), copper and sphalerite were identified in this meteorite ( Figure 3).

Morphology, Optical and Physical Properties of Uakitite
At present, uakitite is observed only in small troilite-daubréelite (±schreibersite) globules with "layered structure" hosted by kamacite and in large troilite-daubréelite nodules (Figures 1 and 4-8). It forms isometric (cubic) crystals (in daubréelite) or rounded grains (in schreibersite). The size of uakitite grains is commonly less than 5 µm (Figures 4-8); the largest detected crystal is 5 × 5 µm (Figures 4 and 5). Twinning was not observed. Uakitite from sulfide globules is associated with sulfides (daubréelite, troilite, grokhovskyite), schreibersite and magnetite. In large sulfide nodules, it is confined to their outer margins (Figure 7). In general, both globules and nodules exhibit varying degrees of partial resorption due to alteration processes. Carlsbergite CrN is a more abundant nitride in the Uakit meteorite, but it was not found yet in any assemblages with uakitite. In addition to polymineralic inclusions (Figure 3), carlsbergite also occurs in troilite-daubréelite globules without "layered structure", where its micron-sized crystals may decorate the boundary between globule and host kamacite [49]. The phase relationships in the sulfide associations indicate that uakitite is one of the early minerals (Figures 5 and 6) and crystallized under temperature higher than that for troilite-daubréelite solid decay (>1000 • C).
We were unable to obtain physical and optical properties of uakitite due to the very small sizes of the grains. So, in most cases, we have to refer to data for synthetic VN. It has a yellow color, white streak and metallic luster. The mineral is transparent, non-fluorescent and brittle. No cleavage and parting are observed. The hardness for synthetic VN is ≈ 9-10 (Mohs), microhardness: VHN load: 0.5-0.98 mN; range: 6.0-11.8 GPa [33,46,47]. The density (6.128 g/cm 3 ) for uakitite was calculated from unit-cell dimensions and results of EDS analyses. Under reflected light, uakitite is light gray with a pinky tint and does not show any internal reflections. Optical property data (transmitted light) for synthetic VN (at 0.5876 µm) are: refractive index n = 2.3031, N g = 1.4501, reflectance R = 0.43817 [31]. Uakitite is not soluble in H 2 O and weakly concentrated in HCl, HNO 3 and H 2 SO 4 . On BSE images, it resembles magnetite and daubréelite.

Chemical Composition of Uakitite
The presence of elevated Cr and Fe and absence of Ti are characteristic features of uakitite. Its empirical formula (n = 54) based on two ions is V 0.91 Cr 0.07 Fe 0.02 N 1.00 (Table 2). In general, the variations in all components are negligible for individual uakitite grains. It is supported by elemental maps for some grains (Figure 8). The ideal formula for uakitite is VN, which requires V 78.43, N 21.57 and total 100.00 wt. % (Table 2). Uakitite is structurally related to carlsbergite CrN and osbornite TiN [1,3,[7][8][9][10]18]. The essential impurity of chromium incorporates in uakitite according to the scheme V 3+ → Cr 3+ , up to 7 mol. % of the carlsbergite end-member (Table 2, Figure 9). The isomorphic scheme for insignificant Fe (1.2-2.1 wt. %) is unclear. Two variants are possible: V 3+ → Fe 3+ and 2V 3+ → V 4+ + Fe 2+ (Figure 9). In contrast to uakitite, the concentration of V in carlsbergite is less than 0.2 wt. %; Fe is up to 4.5 wt. %; Ti was not detected (Table 2, Figure 9

Crystal Structural Data for Uakitite
It was difficult to obtain single-crystal and X-ray powder diffraction data for uakitite because of its very small crystal size (<5 µm), and its mineral structure was resolved by EBSD method. Before considering the structure of uakitite, it is important to consider the data for the V-N system and the structure of the synthetic VN phase.

Crystal Structural Data for Uakitite
It was difficult to obtain single-crystal and X-ray powder diffraction data for uakitite because of its very small crystal size (<5 µm), and its mineral structure was resolved by EBSD method. Before considering the structure of uakitite, it is important to consider the data for the V-N system and the structure of the synthetic VN phase.

EBSD Data for Uakitite
Single-crystal X-ray studies could not be carried out because of the small crystal size of uakitite. Structural data were obtained using the EBSD technique ( Figure 12) and fitted to the following structural model of synthetic VN (space group Fm-3m (225); a = 4.1328(3) Å; V = 70.588(9) Å 3 ; Z = 4) [62]. The EBSD patterns for three uakitite crystals were obtained at working distances of 15-20 mm. Fitting of the EBSD patterns for a VN model with the cell parameters given below resulted in the parameter MAD = 0.14-0.37 • (best-good fit). EBSD studies showed full structural identity between uakitite and its synthetic analog VN (NaCl-type). Uakitite is structurally related to the osbornite group which also includes carlsbergite CrN and osbornite TiN [1,3,[7][8][9][10]18].

Diffraction Data for Uakitite
Because uakitite occurs only in small concentrations, X-ray powder diffraction data were not collected. The theoretical powder diffraction pattern was calculated using the structural data of the synthetic analog [62] and the empirical formula of uakitite (Table 2). Data are given in Table 3. Calculated structure data for uakitite are presented in Supplementary Materials (Cif file). MoKα1 = 0.70932 Å, Bregg-Brentano geometry, fixed slit, no anomalous dispersion, I > 1; data were calculated using PowderCell 2.4 [83]. The strongest diffraction lines are given in bold.

Discussion and Concluding Remarks
The detailed mineralogical and petrographic studies for the Uakit iron meteorite gave a possibility to describe the chemical composition and some structural affinities for a new mineral, uakitite VN, which belongs to the osbornite group. Phase relations indicate that uakitite is one of the early minerals in the troilite-daubréelite associations. These sulfide associations in the Uakit meteorite seemed form due to high-temperature (>1000 °C) separation of Fe-Cr-rich sulfide liquid from Fe-metal melt. We do not exclude that crystallization of uakitite was under high temperature (≈ 1000 °C) from the sulfide melt, but was not below 650 ± 50 °C according to the system Cr-Fe-S [84].

Discussion and Concluding Remarks
The detailed mineralogical and petrographic studies for the Uakit iron meteorite gave a possibility to describe the chemical composition and some structural affinities for a new mineral, uakitite VN, which belongs to the osbornite group. Phase relations indicate that uakitite is one of the early minerals in the troilite-daubréelite associations. These sulfide associations in the Uakit meteorite seemed form due to high-temperature (>1000 • C) separation of Fe-Cr-rich sulfide liquid from Fe-metal melt. We do not exclude that crystallization of uakitite was under high temperature (≈ 1000 • C) from the sulfide melt, but was not below 650 ± 50 • C according to the system Cr-Fe-S [84]. In general, conditions for high accumulation of V as VN are not yet clear. Bulk compositions of whole meteorite and kamacite (ICP-MS and LA-ICP-MS) indicate very low vanadium concentrations 0.04-0.52 ppm [25]. Probably sulfide-metal liquid immiscibility is the main factor for the partitioning of chalcophile V (and also Cr) in sulfide melt.
Some rare and exotic minerals occur as very minute grains (size: <1-20 µm and smaller). It creates a lot of problems in their identification and detailed description; especially in regards to new mineral species (composition, unit-cell data and crystal structure). However, modern analytical methods permit the study of such small objects. In addition to the classic analytical methods, the application of the TEM, EBSD and other techniques allow for improved studies of micron-sized minerals. In the last decades, these technologies are successfully used for detailed identification of new minerals in both meteorites and terrestrial rocks, especially when their synthetic analogs are known (for example [26,48,[85][86][87][88][89] and many other works).