Abstract
Abstract. 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 (Ca0.960Fe0.0162+Na0.012Mg0.003)0.992(Cr1.731V0.1833+Ti0.0683+Al0.023Ti0.0034+)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(Cr1.71Fe0.063+V0.06Ti0.03Al0.03Mg0.02Mn0.02)Σ1.93O4. 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 CaFe23+Mn4+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).
Highlights
The double oxide CaCr2O4 is known as an important component of composite materials explored in metallurgy and as ceramic materials (Róg et al, 2007)
The unit-cell parameters were calculated from HRTEM data 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 Å3 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 Institute of Geology and Mineralogy (IGM), Novosibirsk, Russia
Summary
The double oxide CaCr2O4 is known as an important component of composite materials explored in metallurgy and as ceramic materials (Róg et al, 2007). The β-polymorph belongs to a series of double oxides having the parent structure of CaFe2O4 (CF), which, altogether with CaTi32+O4 (CT) and CaMn32+O4 (CM), are gathered under the group name of post-spinel phases (Hill et al, 1956; Bright et al, 1958; Hörkner and Müller Buschbaum, 1976; Irifune et al, 1991; Kirby et al, 1996; Damay et al, 2010; Xue et al, 2021) These oxides are regarded as structural models for the high-pressure (HP) oxyspinels stable at the conditions of Earth’s deep mantle (Chen et al, 2003a; Zhai et al, 2016, and references ). This work covers all data obtained for a new mineral ellinaite, a natural analog of β-CaCr2O4, coming from two diverse localities: rankinite–gehlenite paralava at Hatrurim Basin, Hatrurim pyrometamorphic formation (Mottled Zone), Israel (holotype, Sharygin, 2019; Sharygin et al, 2019a), and a micron-sized inclusion in a diamond from the Sorriso River, Brazil (co-type; Kaminsky et al, 2015).
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