Abstract

Electron-diffraction studies indicate that the deviation from spinel stoichiometry in “ferritchromit” from the podiform chromitite deposit at Heng-Chun in southern Taiwan can be reasonably explained by the topotaxial intergrowth of an RO phase (rock-salt structure) in an R3O4 spinel, where R = Fe, Cr, Mg, and Al. The absence of creep-induced defect microstructures in “ferritchromit” and its deviation from spinel stoichiometry indicate that it was formed by metasomatism subsequent to the deformation of the Cr-rich spinel core. The topotaxial oxides RO + R3O4 were later oxidized to RO + R3O4 + R2O3 (corundum structure). The depletion of Al and Mg accompanied by enrichment of Cr and Fe in “ferritchromit” is related to the formation of interleaved layers of trioctahedral chlorite and serpentine in the matrix.

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