Some Naturally Occurring Members of the Clinoenstatite - Clinoferrosilite Mineral Series

USING the electron probe micro-analyser, we have-examined the composition of basal exsolution lamellæ in two calcium-rich clino-pyroxenes, one an augite in a, norite from Bon Accord Quarry, near Pretoria, Transvaal, and the other a ferro-augite in a two-pyroxene granulite from Broken Hill, New South Wales. These lamellæ have been identified optically and by X-ray diffraction as a second clino-pyroxene (space group P 2 1 / c ) lying in an oriented relationship to the host ( a and b crystallographic axes are common, c axes inclined to one another at an angle of a few degrees 1 ). Such lamellæ have previously been called pigeonite, implying a calcium content of 4 per cent or more by weight calcium oxide (ref. 2).