Summary: Partial replacement of original igneous augite crystals by omphacite during blueschist metamorphism of a dolerite from the Mihalliççik area of north-west Turkey has been studied by transmission electron microscopy. The replacement occurred topotactically, apparently by ion exchange with a fluid phase. which left the basic pyroxene structure unchanged. Cation ordering in the omphacite caused a symmetry change from C-face centred to primitive with the formation of fine-scale antiphase domains. Selected-area diffraction provides evidence for P2 and P2/c space groups for the ordered omphacite though the best ordered areas show a tendency towards P2/n (reflections violating the n-glide are very weak) and also contain fine, wavy, disordered precipitates approximately parallel to (too).

It is suggested that the replacement temperature was below the cation-ordering temperature and that the omphacite grew in a metastable, disordered state. Subsequent ordering occurred under irreversible conditions via a series of intermediate structures. The ordering sequence may illustrate the relationship between different ordering schemes in other blueschist pyroxenes.