Ferritchromit and its opto-chemical behaviour

Abstract

The compositions of the highly reflective zones (e.g., rim, patches, fractures, etc.) in chromites from two Indian chromite deposits (Sukinda and Nuggihalli schist belt) put them in the class of ‘ferritchromit’ and its ilk. EPMA study shows depletion of Mg, Al and enrichment of Cr, Fe from the host chromites to ferritchromit. The weakly anisotropic ‘ferritchromit’, however, hardly show any exsolution relationship with its host. High Fe³⁺/Fe²⁺ ratios in ‘ferritchromit’ determined from EPMA study are also found to have positive correlation with high Fe³⁺/Fe²⁺ values, as obtained from ⁵⁷Fe Mössbauer spectroscopic study, of the bulk samples; all are suggestive of oxidation as the process causing ferritchromitization at any stage after the crystallization of chromites. The process of chemical transformation essentially involves the expulsion of Al, Mg and enrichment of Cr, Fe in these zones. The causative force arises out primarily from release of electron during oxidation and relaxation of the inbuilt lattice strain and transition from a perfectly cubic to a lower symmetry. The progress of work on this problem of ferritchromitization has been reassessed in the light of the present observations on samples from widely separated tectono-stratigraphic horizons of India.