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).