Chemistry and origin of phlogopite megacrysts in kimberlite

Abstract

STUDIES of the megacrysts occurring in kimberlite have hitherto neglected the mica megacrysts. We have analysed mica megacrysts from one Lesotho and twelve South African kimberlites. The megacrysts, which vary in size from 4 mm to 3 cm, were taken from both ‘basaltic’ and micaceous kimberlites, and distinguished from the matrix phlogopites by (i) their greater size; (ii) their dark brown, rather than bronze or light brown, colour; (iii) their rounded shape and corroded margins; (iv) distortion of the {001} planes (in some, but not all, megacrysts); and (v) absence of inclusions of matrix phases such as perovskite and spinel which often occur in the matrix micas.