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

Ellinaite, a natural analog of the post-spinel phase beta-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, Juina 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 mu m 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+)(2.008)O-4. 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) angstrom, V 264.9(5) angstrom 3 and Z=4. The crystal structure of ellinaite from the Hatrurim Basin has been solved and refined to R-1=0.0588 based on 388 independent observed reflections. Ellinaite in the Juina diamond occurs within the micron-sized polyphase inclusion in association with ferropericlase, magnesioferrite, orthorhombic MgCr2O4, unidentified iron carbide and graphite. Its empirical formula is Ca-1.07(Cr1.71Fe0.063+V0.06Ti0.03Al0.03Mg0.02Mn0.02)(Sigma 1.93)O-4. The unit-cell parameters obtained from HRTEM data are as follows: space group Pnma, a 9.017, b 2.874 angstrom, c 10.170 angstrom, V 263.55 angstrom(3), Z=4. Ellinaite belongs to a group of natural tunnel-structured oxides of the general formula AB(2)O(4), the so-called post-spinel minerals: marokite CaMn2O4, xieite FeCr2O4, harmunite CaFe2O4, wernerkrauseite CaFe23+Mn4+O6, chenmingite FeCr2O4, maohokite MgFe2O4 and tschaunerite Fe(FeTi)O-4. The mineral from both occurrences seems to be crystallized under highly reduced conditions at high temperatures (>1000 degrees C), but under different pressure: near-surface (Hatrurim Basin) and lower mantle (Juina diamond).