The observation of a direct phase transformation from haematite a-Fe2O3 (antiferromagnetic) to maghemite y-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 a-Fe2O3 particles. All crystallographic and magnetic measurements show existence of a phase transformation a -" y. 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 y -> a.
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