An electron microscopic study of amphibole lamellae in augite

Ion-thinned samples of augite from four plutonic igneous rocks have been examined in the electron microscope at 100 kV. Lamellae less than 0.08 m thick were observed parallel to (010) in all four samples. Electron diffraction shows that the lamellae consist of clino-amphibole, space group I2/m, with crystallographic axes parallel to those of the augite. Analytical electron microscopy of the lamellae in one specimen shows that they are a hornblende. The amphibole lamellae have nucleated at the interface between the augite host and exsolution lamellae of orthopyroxene parallel to (100) of the augite. The interface between the amphibole lamellae and the augite is coherent.It is thought that the amphibole lamellae have formed by exsolution from the augite, implying the existence of finite solid solution between members of the pyroxene and amphibole groups of minerals.