Natural occurrence and synthesis of two new postspinel polymorphs of chromite.

A high-pressure polymorph of chromite, the first natural sample with the calcium ferrite structure, has been discovered in the shock veins of the Suizhou meteorite. Synchrotron x-ray diffraction analyses reveal an orthorhombic CaFe2O4-type (CF) structure. The unit-cell parameters are a = 8.954(7) A, b = 2.986(2) A, c = 9.891(7) A, V = 264.5(4) A3 (Z = 4) with space group Pnma. The new phase has a density of 5.62 g/cm3, which is 9.4% denser than chromite-spinel. We performed laser-heated diamond anvil cell experiments to establish that chromite-spinel transforms to CF at 12.5 GPa and then to the recently discovered CaTi2O4-type (CT) structure above 20 GPa. With the ubiquitous presence of chromite, the CF and CT phases may be among the important index minerals for natural transition sequence and pressure and temperature conditions in mantle rocks, shock-metamorphosed terrestrial rocks, and meteorites.