Rutile Exsolution in Fe3+-chromites: A Case Study from Layered Granulitic Complex of Sittampundi, S. India

The presence of Fe³⁺ at both tetrahedral (A) and octahedral (B) sites (-45% of total iron). from Mossbauer spectroscopic data of Sittampundi chromite, strongly suggests that high fO₂(10^-7.30 to 10^-7.34) conditions were prevalent during its formation. This Cr-spinel was formed with enough of Ti and Al (at its B-site) in its initial structure, attained while in the primary melt at high P-T field (>10.6 kbar, >950°C). The high fO₂ in the crystallizing environment did not allow iron to remain as "free" Fe²⁺ ion to be stabilized into ilmenite structure. In chromite lattice, Ti⁴⁺ gets stabilized at Bsite only by coupled occupancy with bivalent cations, strongly with Fe²⁺. Inadequacy of Fe²⁺ content to charge-balance Ti⁴⁺ at B-site due to further oxidation of iron to Fe³⁺ caused an expulsion of Ti⁴⁺ from the chromite lattice. The released free Ti⁴⁺ ion readily combines with oxygen alone to form rutile (or its ilk) which comes out as acicular exsolution bodies within chromite, often following its crystallographic planes [along (111)].