A Raman spectroscopy study of the oxidation processes in synthetic chromite FeCr2O4

A crystal of synthetic chromite FeCr2O4 has been annealed in air at 700 °C for 50 days at room pressure in order to study physical-chemical changes. After the annealing treatment, Scanning Electron Microscopy (SEM) images of the polished surface of the sample showed areas of different composition. Detailed Raman mapping revealed that the annealed chromite undergoes an oxidation process, proceeding inwards from the outer surface and leading to the transformation of chromite to magnetite, and ultimately to hematite. The oxidation process also leads to the formation of trellis-like lines, arguably via stress-related mechanisms associated with the phase transformation and consequent volume changes. The present study confirms the possibility of non-stoichiometry or structural defects causing slight structural asymmetries and the formation of new modes, resulting in a valuable support in characterizing both natural and artificial material. The potential of Raman spectroscopy in the identification and spatial resolution of mineralogical phases, is also highlighted.