Ellinaite, CaCr2O4, a new natural post-spinel oxide from Hatrurim Basin, Israel, and Juína kimberlite field, Brazil

Ellinaite, a natural analog of the post-spinel phase β-CaCr2O4, was discovered at the Hatrurim Basin, Hatrurim pyrometamorphic formation (the Mottled Zone), Israel, and in an inclusion within the super-deep diamond collected at the placer of the Sorriso River, Juína kimberlite field, Brazil. Ellinaite at the Hatrurim Basin is confined to a reduced rankinite–gehlenite paralava, where it occurs as subhedral grains up to 30 μm in association with gehlenite, rankinite and pyrrhotite or forms the rims overgrowing zoned chromite–magnesiochromite. The empirical formula of the Hatrurim sample is (Ca0.960Fe 0.016Na0.012Mg0.003)0.992(Cr1.731V 3+ 0.183Ti 3+ 0.068Al0.023Ti 4+ 0.003)2.008O4. The mineral crystallizes in the orthorhombic system, space group Pnma, unit-cell parameters refined from X-ray single-crystal data: a 8.868(9), b 2.885(3), c 10.355(11) Å, V 264.9(5) Å3 and Z = 4. The crystal structure of ellinaite from the Hatrurim Basin has been solved and refined to R1 = 0.0588 based on 388 independent observed reflections. Ellinaite in the Juína diamond occurs within the micron-sized polyphase inclusion in association with ferropericlase, magnesioferrite, orthorhombic MgCr2O4, unidentified iron carbide and graphite. Its empirical formula is Ca1.07(Cr1.71Fe 0.06V0.06Ti0.03Al0.03Mg0.02Mn0.02)61.93O4. The unit-cell parameters obtained from HRTEM data are as follows: space group Pnma, a 9.017, b 2.874 Å, c 10.170 Å, V 263.55 Å3, Z = 4. Ellinaite belongs to a group of natural tunnel-structured oxides of the general formula AB2O4, the so-called post-spinel minerals: marokite CaMn2O4, xieite FeCr2O4, harmunite CaFe2O4, wernerkrauseite CaFe3+ 2 Mn O6, chenmingite FeCr2O4, maohokite MgFe2O4 and tschaunerite Fe(FeTi)O4. The mineral from both occurrences seems to be crystallized under highly reduced conditions at high temperatures (>1000 C), but under different pressure: near-surface (Hatrurim Basin) and lower mantle (Juína diamond). Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. 728 V. V. Sharygin et al.: Ellinaite, CaCr2O4, a new post-spinel oxide

This work covers all data obtained for a new mineral ellinaite, a natural analog of β-CaCr 2 O 4 , coming from two diverse localities: rankinite-gehlenite paralava at Hatrurim Basin, Hatrurim pyrometamorphic formation (Mottled Zone), Israel (holotype, Sharygin et al., 2019a), and a micron-sized inclusion in a diamond from the Sorriso River, Brazil (co-type; Kaminsky et al., 2015). The mineral was approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) as a new mineral species in December 2019, IMA 2019-091 (Sharygin et al., 2020). After the IMA registration, the same mineral has also been identified in varicolored spurrite marbles at Tulul Al Hammam, Daba-Siwaqa, Hatrurim pyrometamorphic formation, central Jordan (Galuskina et al., 2021a).
An individual grain of ellinaite from rankinite-gehlenite paralava, Hatrurim Basin, Israel (sample MP-2013-6, used for many studies), was deposited in the collections of the A.E. Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5439/1 (holotype). Another grain of ellinaite from this Hatrurim paralava (in polished thin section) is in the collections of the Central Siberian Geological Museum at V.S. Sobolev Institute of Geology and Mineralogy (IGM), Siberian Branch of the RAS, Novosibirsk, Russia (catalogue number VII-102/1, holotype). The co-type sample of ellinaite from Córigo Sorriso, Mato Grosso State, Brazil (TEM foil from diamond), is located in the scientific collection of F.V. Kaminsky (V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia).
Unfortunately, no new data for ellinaite in the Brazilian diamond have been obtained since the IMA registration of this mineral (Sharygin et al., 2020). As a result, the text below and figures provided here for the Brazilian ellinaite represent a compilation of data adapted from Kaminsky et al. (2015).
Within the polyphase inclusion ellinaite is associated with ferropericlase (+magnesioferrite), orthorhombic MgCr 2 O 4 , iron carbide and graphite (Figs. 1 and S1 in the Supplement). It occurs as a 2 × 1 µm subhedral grain. The EDX TEM spectrum for ellinaite ( Fig. S1) was obtained by applying the TIA™ software package and using the k AB factors from the software. The quantitative results were normalized to the total sum of 100 %. The approximate uncertainties are ±3 % for concentrations in the range 30 at %-50 at %, ±6 %-12 % for concentrations between 5 at %-25 at %, ±12 %-25 % for concentrations between 1 at %-5 at % and ±25 %-100 % for concentrations below 1 at %. The unit-cell parameters were calculated from HRTEM data (see Table 5 in Kaminsky et al., 2015) using high-resolution images and fast Fourier transform (FFT) software packages (Fig. S1) and gave the following results: orthorhombic symmetry, space group Pnma (#62), a = 9.017 Å, b = 2.874 Å, c = 10.170 Å, V = 263.55 Å 3 and Z = 4. Identification of ellinaite and related minerals in the Hatrurim rankinite-gehlenite paralavas was based on energy-dispersive spectra (EDS), back-scattered 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. EDS analyses of minerals were operated at an accelerating voltage of 20 kV and a probe current of 1 nA in high-vacuum mode and at an accumulation time of 20-40 s. Synthetic compounds, pure metals and minerals were used as reference standards for the elements SiO 2 (Si and O), Al 2 O 3 (Al), diopside (Mg and Ca), albite (Na), orthoclase (K), Ca 2 P 2 O 7 (P), BaF 2 (Ba and F), Cr 2 O 3 (Cr), CsRe 2 Cl 6 (Cl), LaPO 4 (La), CePO 4 (Ce), SrF 2 (Sr), metallic Nb, Ti, Ta, Zr, Fe, Mn, Zn and V. Matrix effects were corrected 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).
Reflectance data for ellinaite were obtained using a microscope spectrophotometer LOMO MSP-R equipped with spectrophotometric attachment PEI "R928" (Hamamatsu, Japan) at the South Urals Federal Research Center of Mineralogy and Geoecology, Miass, Russia. Measurements were done in the air using a ×40 objective with numerical aperture of 0.65: photometric diaphragm -0.3 mm, size of analyzing area -0.007 mm, diffraction grating -600 grooves per millimeter, spectral interval -6 nm, voltage for PEI -450 V and standard -elemental silicon. Measurements were provided for the 400-700 nm range.
Single-crystal data were collected by means of a Bruker Kappa APEX DUO CCD diffractometer using MoKα radiation. The crystal structure of ellinaite has been solved by the dual space method and refined to R 1 = 0.059 using the SHELX-2014 set of programs (Scheldrick, 2015) via the Olex2 v.1.2.8 graphical user interface (Dolomanov et al., 2009). Further details of data collection and structure refinement can be retrieved from the CIF attached to the Supplement.
The Raman spectra for ellinaite were recorded on a LabRAM HR 800 mm (HORIBA Scientific Ltd.) spectrometer equipped with a 1024 pixel LN/CCD detector and coupled to an Olympus BX40 confocal microscope (×100 objective) at the IGM. A semiconductor laser emitting at 514.5 nm with a nominal output power of 50 mW was used for excitation. In each case, 20 spectra were recorded for 20 s each at a hole diameter of 200 µm and a resolution of 0.5 cm −1 and then integrated. Most spectra were recorded between 100 and 1400 cm −1 . The monochromator was calibrated using the 520.7 cm −1 Raman line of elemental Si.
Electron backscatter diffraction (EBSD) studies were provided for one grain of ellinaite. The sample containing ellinaite and intended for EBSD studies was subjected to polishing by BuehlerMasterMet2 non-crystallizing colloidal silica suspension (0.02 µm). EBSD measurements were carried out by means of a FE-SEM ZEISS SIGMA VP scanning electron microscope equipped with an HKL Technology Nordlys HKL EBSD, operated at 20 kV and 1.4 nA in focused beam mode with a 70 • tilted stage at the NANOTECH Center, Ural Federal University, Ekaterinburg, Russia. Structural identification of ellinaite was performed by matching its EBSD patterns with the reference structural models using the program FLAMENCO. The structural data for synthetic β-CaCr 2 O 4 (Damay et al., 2010) and α-CaCr 2 O 4 (Pausch and Müller Buschbaum, 1974) were used for the Kikuchi pattern simulation and comparison.

General information for ellinaite-bearing paralava at Hatrurim Basin, Israel
Ellinaite was observed in one of the rankinite-gehlenite paralavas, which were first found in 2011 in the southern part of the Hatrurim Basin, the largest combustion metamorphism complex of the Hatrurim Formation (also known as the "Mottled Zone") within Israel (Gross, 1977;Burg et al., 1992;Vapnik et al., 2007). The paralava with ellinaite (sample MP-2013-6) was found as a boulder in the dry Zohav wadi (nahal Zohav), near the Arad -Dead Sea  (Britvin et al., 2015(Britvin et al., , 2017(Britvin et al., -d, 2020a(Britvin et al., -c, 2021a). The rock is moderately to highly weathered and brecciated and has the appearance of a "pseudoconglomerate", in which individual fragments are "cemented" and cross-cut by a carbonate-gypsum-Ca-hydrosilicate matrix. In general the individual fragments are altered in different degrees and contain abundant vesicles now partially filled with Ca hydrosilicates. The alteration degree is indicated by color change (from dark to white) from the center to rim (Fig. 2). The dark parts of some fragments are less weathered and consist of gehlenite, rankinite, larnite, flamite, cuspidine, Cr-rich pyrrhotite, fluorapatite and Si-Cr-V-rich perovskite as essential minerals. Minor and accessory phases are represented by chromite-magnesiochromite, wüstitemagnesiowüstite, wollastonite, Ti-rich cuspidine, ellinaite, Ba-rich djerfisherite minerals, magnetite, unidentified Ca-Fe silicate Ca 4 Fe 2 (SiO 4 ) 3 and oxysulfide close to CaFe 3 S 2 O 5 , the (Ni,Fe) 2 As phase, pentlandite, native iron, cohenite, schreibersite Fe 3 P, and barringerite Fe 2 P (e.g., Britvin et al., 2015Britvin et al., , 2017. Crystallization of the paralava likely occurred under highly reduced conditions that are determined by the chemistry of the essential minerals: Ca silicates and perovskite are very poor in Fe, whereas sulfides (pyrrhotite, Ba-rich phases), chromite, wüstite, native iron and phosphides are the main carriers of Fe. A list of mineral phases found in the rankinite-gehlenite paralava with ellinaite (sample MP-2013-6) and chemical composition of essential minerals are given in Tables S1 and S2. The metal-phosphide mineralization is mainly localized along the boundary of rock fragments with the carbonate-gypsum-Ca-hydrosilicate matrix, rarely around vesicles in the paralava. It forms rounded isolations varying in size from 10 µm to 1-2 mm and modal composition. Namely, in such associations the new phosphide minerals (murashkoite FeP, transjordanite Ni 2 P, zuktamrurite FeP 2 , negevite NiP 2 , halamishite Ni 5 P 4 , polekhovskyite MoNiP 2 , nazarovite Ni 12 P 5 ) were first found in the Halamish wadi, Hatrurim Basin (Britvin et al., 2015(Britvin et al., , 2017(Britvin et al., -d, 2020a. The following metal-phosphide assemblages with Cr-V-rich pyrrhotite are most common in the Zohav paralava: kamacite, steadite (Fe-P eutectics, mainly kamacite + schreibersite Fe 3 P) + kamacite, barringerite Fe 2 P + steadite, schreibersite + barringerite, barringerite + murashkoite and murashkoite + transjordanite. Other phosphide phases, cohenite, graphite, copper, Mo-rich phases, unidentified phosphates and daubreelite sometimes occur in such assemblages. The formation of paralava and metal-phosphide mineralization appears to be unrelated in that the metal-phosphide assemblage was probably formed in the solidified paralava from reduced gases (Britvin et al., 2015(Britvin et al., , 2017. In addition to the metal phosphide and sulfide association, the new Fe-Ni-rich phosphates were found in the carbonategypsum-Ca-hydrosilicate matrix as alteration products of phosphides (phosphocyclite-(Fe) Fe 2+

Morphology and optical and physical properties of ellinaite from Hatrurim Basin
Ellinaite is very rare mineral in the Hatrurim Basin, and only two grains were observed in a dark unaltered part of the rankinite-gehlenite paralava (Fig. 2). It forms subhedral opaque grains in association with gehlenite, rankinite and pyrrhotite (size -30 × 20 µm, ellinaite-1 grain, Figs. 3a, 4 and S2) or is intergrown with chromite-magnesiochromite (size -10 × 10 µm, ellinaite-2 grain, Figs. 3b and S3). In reflected light and in BSE images this phase resembles perovskite and chromite, which are more abundant in the studied paralava. Mineral relations indicate that ellinaite crystallized after gehlenite, rankinite and chromite, but before pyrrhotite (Fig. 3). The color of ellinaite is black, and the color of the powdered mineral is black. It is brittle and has submetallic luster. Its hardness is ≈ 4.5-5 (Mohs), which is close to neighboring chromite (Fig. 3b). No cleavage and parting were observed; fracture is uneven. Microhardness and density were not measured directly because of the small grain size of ellinaite. Density calculated from unit-cell    (Table S3). Negative correlations of Cr with Ti and V (Fig. 5) strongly suggest the occurrence of limited isomorphous substitutions according to possible schemes: Cr 3+ ↔ (V 3+ ,Ti 3+ ) and  (Kaminsky et al., 2015). In the Jordanian case we do not see any contradictions since Fe-oxide minerals (brownmillerite, srebrodolskite, hematite) and garnet-supergroup minerals (mainly andradite and priscillagrewite-(Y)) in spurrite marble are dominated by Fe 3+ (Galuskina et al., 2021a, b). 7 Structural data for ellinaite from Hatrurim Basin

Raman spectroscopy
The Raman spectra for the Hatrurim ellinaite show the complete band set common to synthetic β-CaCr 2 O 4 (Zhai et al., 2016). However, in addition to the strong 596 cm −1 band, the Hatrurim phase indicates the strong and broad band at 690 cm −1 with respect to the synthetic compound (Fig. 6). This seems to be related to crystallographic orientation of the natural sample. The appearance and disappearance of some bands and their shifting and intensity change in the dependency on orientation have been shown for the Raman spectra of synthetic CaFe 2 O 4 (Kolev et al., 2003). The vibration modes of the Raman spectrum of ellinaite may be interpreted by analogy with the Raman spectra of harmunite and wernerkrauseite and related synthetic compounds (Kolev et al., 2003;Galuskina et al., 2014;Galuskin et al., 2016;Zhai et al., 2016). Observed Raman bands are (cm −1 ) ∼ 1300 (combination first-order phonons A g in the 680-700 range); 689, 598 and 556 (A g -stretching modes); 470 and 441 (B 2g and A g -tilting and bending modes); 387, 337 and 324 (A g , B 1g and B 3g -rotating modes); 243 and 204 (A g ); 140 (A g and B 2g ); and 123 and 107 (A g ) (Ca-related vibrations). In general, Raman spectra of ellinaite from Hatrurim Basin are similar to those of ellinaite from Jordan (Galuskina et al., 2021a).

EBSD study
Before single-crystal X-ray study, we attempted to obtain the EBSD data for ellinaite. Two structural patterns of synthetic CaCr 2 O 4 were used for EBSD comparison studies: α-CaCr 2 O 4 (Toth et al., 2011) and β-CaCr 2 O 4 (Damay et al., 2010). We determined a good diffraction pattern, but, unfortunately, it was not indexed in both comparison structural models. This appears to have been due to unsuccessful orientation of the ellinaite crystal, although the data obtained indicated an orthorhombic symmetry. The forced Kikuchi pattern using the β-CaCr 2 O 4 model (Damay et al., 2010) outlined strong mean angular deviation for some diffraction lines (Fig. S4).

Crystal structure of ellinaite
The crystal structure of ellinaite has been solved by the dual space method and refined to R 1 = 0.059 using the SHELX-2014 set of programs (Scheldrick, 2015) via Olex2 v.1.2.8 graphical user interface (Dolomanov et al., 2009). The data collection and structure refinement details are summarized in Table 4. Fractional atomic coordinates, displacement parameters and selected bond lengths for the crystal structure of ellinaite are given in Tables 5 and 6. In general, the crystal structure of ellinaite may be represented as a framework built up of two types of the distorted [Cr 3+ O 6 ] octahedra (Cr1 and Cr2), sharing common edges and corners and containing the system of channels (tunnels) propagated along the b axis. The channels are occupied by calcium ions (Fig. 7). The framework is composed of two types of the distorted [Cr 3+ O 6 ] octahedra (Cr1 and Cr2), which form double chains, connected by common oxygen corners. The distorted octahedra [Cr 3+ O 6 ] in the chains have different average distances: Cr1-O = 1.950(3) Å and Cr2-O = 1.962(9) Å (Table 6). Owing to the specific connection of the chains, each tunnel has a cross section of a distorted trigonal prism (Fig. 7). 0.080 The ellinaite-1 crystal was used.
The overview of ellinaite from the three localities assumes different PT-X-f O 2 conditions for the mineral and its host rocks. The structural data for ellinaite strongly indicate the β-CaCr 2 O 4 modification and no evidence of the precursor α-CaCr 2 O 4 . According to the phase diagram CaO-Cr 2 O 3 (Degterov and Pelton, 1996;Shabanova et al., 2019), the phase transition between α-CaCr 2 O 4 and β-CaCr 2 O 4 (ellinaite) is near 1600 • C (Fig. S5). The mineralogy of the diamond-hosted inclusion with ellinaite from Brazil ( Fig. 1) indicates lower-mantle crystallization conditions and low f O 2 due to the presence of iron carbide and/or carbonitride (Kaminsky et al., 2015). Ellinaite-containing rocks of the Hatrurim Formation are characterized by near-surface pressure conditions and different T-f O 2 (this work; Galuskina et al., 2021a, b). Mineral relations in the Hatrurim Basin rankinite-gehlenite paralava indicate that gehlenite, rankinite and larnite are the earliest phases crystallizing from Ca-rich silicate paralava melt (Fig. 3). Other minerals occupy interstitial spaces between the above phases and were clearly formed after them. Ellinaite crystallized after chromite but before pyrrhotite. According to experimental and calculated data for the system CaO-SiO 2 -Al 2 O 3 (Mao et al., 2006;Haccuria et al., 2015), gehlenite+rankinite+larnite is the liquidus association (peritectic) at 1317-1343 • C, and rankinite occupies a very narrow field at temperatures near 1308 • C. The formation of pyrrhotite as the latest phase was probably near 1000 • C according to Kosyakov et al. (1996). Silicate-melt inclusions in schorlomite-rankinite-melilite paralava from the Hatrurim Basin indicate that rankinite crystallized at T >1160 • C (Sharygin et al., 2006). The presence of Cr-rich pyrrhotite, wüstite, native iron, cohenite and phosphides (Table S1) in the Hatrurim rankinite-gehlenite paralava indicates highly reduced conditions. Thus, local melting of the calcareous sedimentary protolith, and crystallization of resultant rankinite-gehlenite paralava under reduced conditions with the formation of holotype ellinaite, is suggested to have occurred between 1000-1343 • C.
With respect to the ellinaite-bearing marbles in the Tulul Al Hammam area of Jordan, the presence of spurrite and calcite implies maximum metamorphic temperatures for these rocks of <950 • C (Sokol et al., 2005); Khoury et al. (2016) and Sokol et al. (2020) have subsequently determined values of 800-850 • C. On the other hand, Galuskina et al. (2021a, b) consider a value of ∼ 1000 • C as the maximum temperature for the ellinaite-bearing marbles and suggest the possibility that these rocks may be retrograde products of earlier pyrometamorphic crystallization with the formation of high-temperature Ca silicates and lime. The crystallization of the Tulul Al Hammam spurrite marbles with ellinaite occurred under oxidized conditions as evidenced by the presence of Fe-rich minerals (brownmillerite, srebrodolskite, hematite, andradite and priscillagrewite-(Y)), in which iron is largely present as Fe 3+ (Galuskina et al., 2021a, b).
The compositional and structural data of ellinaite from three known localities show that it is stable under a wide range of PT-X-f O 2 conditions, i.e., from the Earth's surface to the lower mantle. Limited solid solutions in ellinaite such as CaCr 3+  Data availability. Crystallographic data for ellinaite (CIF) are available in the Supplement. This part also additionally contains four tables (Tables S1 to S4) and five figures (Figs. S1 to S5).
Author contributions. VVS provided Raman spectroscopy and SEM and microprobe analysis for the Hatrurim ellinaite and wrote the paper, with help from all coauthors. SNB performed single-crystal X-ray studies and refinement of the crystal structure for the Hatrurim ellinaite. FVK and RW conducted HRTEM and interpreted the results for the Brazilian ellinaite. ENN provided microprobe analyses and their complete processing for the Hatrurim ellinaite. GAY helped with EBSD study of the Hatrurim ellinaite. KAN identified optical properties of ellinaite. MNM provided fieldwork and collected phosphide-containing rocks at Hatrurim Basin, Israel.
Competing interests. The contact author has declared that neither they nor their co-authors have any competing interests.
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Acknowledgements. Victor V. Sharygin would like to thank Mikhail V. Khlestov and Nikolai S. Karmanov (IGM, Novosibirsk, Russia) for their technical assistance during SEM studies. Special thanks for Yevgeny Vapnik (Ben-Gurion University, Israel) for providing field trips at Hatrurim Basin (Israel). The authors are grateful to the Centre for X-ray diffraction studies at the St. Petersburg State University for the access to instrumental and computational resources. We are highly appreciative of the comments, suggestions and careful editing of reviewers (Rodney Grapes, Nikita V. Chukanov).
Financial support. Raman spectroscopy and EBSD investigations for the Hatrurim ellinaite were done on state assignment of IGM SB RAS (IX.125.2) and the Initiative Project of Ministry of Science and Higher Education of the Russian Federation (Act 211 of the Government of the Russian Federation (grant agreement no. 02.A03.21.0006)). SEM and microprobe studies for the Hatrurim ellinaite were supported by the Russian Science Foundation (grant no. 17-17-01056p). Crystallographic studies of the Hatrurim ellinaite were provided by the Russian Science Foundation (grant no. 18-17-00079).
Review statement. This paper was edited by Cristiano Ferraris and reviewed by Rodney Grapes and Nikita V. Chukanov.