Natural CaTi2O4-structured FeCr2O4 polymorph in the Suizhou meteorite and its significance in mantle mineralogy

The first natural occurrence of a high-pressure polymorph of chromite has been discovered in the shock-metamorphosed Suizhou meteorite. The composition of this high-pressure polymorph is identical to that of the precursor chromite. The Raman spectrum of this polymorph is distinct from that of chromite. Synchrotron X-ray diffraction analysis revealed that this polymorph has an orthorhombic CaTi2O4-type structure. The cell parameters are: a = 9.462(6) Angstrom, b = 9.562(9) Angstrom, c = 2.916(1) Angstrom, V = 263.8(4) Angstrom(3) (Z=4), space group = Bbmm, and the density = 5.63 g/cm(3) (the numbers in parentheses are standard deviations on the last significant digits). This polymorph is 11.5% denser than chromite. The P-T conditions for the phase transformation from chromite to the CaTi2O4-structured polymorph are estimated at 20-23 GPa and 1800 to 2000 degreesC, respectively. This dense CaTi2O4- structured FeCr2O4 phase could be a host phase for Cr, Al, Fe, Mg and Mn and other metallic elements in the deep Earth. Copyright (C) 2003 Elsevier Ltd.