Contrasting properties and behaviour of antiphase domains in pyroxenes

Antiphase domains in pigeonite arise from a displacive transformation whereas in omphacite they are due to cation ordering. Their properties and behaviour are quite distinct. Domains in pigeonite are elongate, coarsen by a homogeneous and rapid mechanism and have irregular boundaries which are susceptible to the segregation of cations. In omphacite they tend to be equiaxed with smoothly curving boundaries and they coarsen extremely slowly. A heterogeneous mode of coarsening has been postulated to explain some of the observed domain distributions. A brief review of antiphase textures in other minerals, in particular the type (b) and type (c) domains in calcic plagioclase, show many of these properties to be characteristic of the transformation by which they formed, i.e., displacive or order/disorder. It is suggested that displacive type antiphase boundaries have relatively low surface energies and that their migration requires only small atomic displacements locally. In contrast, the chemical interactions associated with mistakes in the cation ordering sequence at antiphase boundaries in an ordered phase give a relatively higher excess free energy. Furthermore their movement requires the exchange of cations between sites at the boundary and hence domain coarsening is slow. The factors which control the domain size in natural specimens have been evaluated qualitatively. For the purpose of comparing thermal histories only domains with a regular size distribution in homogeneous areas of crystal which are free of defects should be used.