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 process is used; it is characterized by the presence of a shearing component exerted on the alpha-Fe2O3 particles. All crystallographic and magnetic measurements show existence of a phase transformation alpha --> gamma. The interpretation is based on a mechanism using two crystallographic concepts: the orientation relationship between the two phases, and movement of the oxygen planes due to a periodic shearing sequence. The results are also consistent with the phenomena related to the inverse transformation gamma --> alpha.