EXISTENCE OF A DIRECT PHASE-TRANSFORMATION FROM HEMATITE TO MAGHEMITE

The observation of a direct phase transformation from haematite alpha- Fe2O3 (antiferromagnetic) to maghemite gamma-Fe2O3 (ferrimagnetic) by the mechanical action of grinding is reported. A simple grinding proce ss is used; it is characterized by the presence of a shearing componen t exerted on the alpha-Fe2O3 particles. All crystallographic and magne tic measurements show existence of a phase transformation alpha --> ga mma. The interpretation is based on a mechanism using two crystallogra phic concepts: the orientation relationship between the two phases, an d movement of the oxygen planes due to a periodic shearing sequence. T he results are also consistent with the phenomena related to the inver se transformation gamma --> alpha.