The Petrology of Katungite

Abstract

The specimens representing the Katunga lavas are compact fine-grained porphyritic rocks of lustrous stone-grey to deep blue-grey colour, the phenocrysts being olivine (up to 3 mm. long) of various tints from nearly colourless to yellow-green, and melilite (up to 2 mm. square) in thin tabular, less conspicuous crystals which, megascopically, have a shining grey to black appearance. Some of the specimens from near the sources of the flows are internally altered and have become fawn or buff in colour, mottledby white zeolitic specks and reddish brown ferruginous pseudomorphs after olivine and melilite. Some weathered specimens have a brown ochreous crust, but within this the rock is normally fresh.