A suite of volcanic rocks from south-west Uganda containing kalsilite (a polymorph of KAlSiO4)

Abstract

During the last few years large collections of rock specimens from the Toro-Ankole volcanic fields, east and south-east of Ruwenzori, have been sent to Durham by the Geological Survey of Uganda for petrological investigation. The specimens recently studied include representatives of a suite of highly potassic ultrabasic lavas. Some of these resemble melanocratic olivine-nephelinites or olivine-rich ankaratrites, but the nepheline-like felspathoid which they contain, instead of being nepheline, is a polymorph of KAlSiO₄. For chemical reasons this mineral was at first thought to be kaliophilite. Other types, closely related, contain leucite, biotite, or melilite, in addition to the supposed kaliophilite.