Twinning dislocations in haematite iron ore

Direct observation of twinning dislocations at rhombohedral twin interfaces in naturally deformed haematite ore is reported. An analysis of the coordination polyhedra produced by the operation of the observed twinning elements across {1012} composition planes leads to a simple description of the interface structure whereby Fo atoms occupy tetrahedrally coordinated sites. Simple displacive atomic movements allow for rapid propagation of the twinning dislocations. The structural results are also relevant to mechanical twinning of sapphire and other structures derived from hexagonal close-packing, including hexagonal metals and alloys.