Abstract
Pigeonites in coarse grained dolerites from the Whin Sill have been examined by transmission electron microscopy. The observed micro-structures involve coarse, heterogeneous (001) augite lamellae and antiphase boundaries (APB's), some of which have nucleated fine (001) plates of augite. APB's approximately parallel to (211) and \((\bar 211)\) are Ca enriched and those approximately parallel to (120) are not (or at least are less so), the former changing their orientation according to the local Ca content of the pigeonite and providing suitable sites for the heterogeneous nucleation of augite. Simple structural models show how (211) and \((\bar 211)\) APB's provide more suitable sites for Ca2+ ions that those parallel to (120).
The development of the antiphase domains is discussed. Concentrating Ca2+ ions onto a restricted number of sites is not favoured by entropy and therefore possibly does not occur at high temperatures. The final antiphase domain scale is a complex function of the Ca content and cooling rate of the pigeonite.
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Carpenter, M.A. Nucleation of augite at antiphase boundaries in pigeonite. Phys Chem Minerals 2, 237–251 (1978). https://doi.org/10.1007/BF00308176
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DOI: https://doi.org/10.1007/BF00308176