Petrography and mineralogy of the peralkaline silicic rocks

Abstract

The mineralogy and textures of quartz-normative peralkaline trachytes and the comendites and pantellerites are described in detail, using published examples and new information on rocks from the Naivasha area, central Kenya. Mineral assemblages are tabulated for each rock type, and attention is drawn to the mineralogical differences between glassy and crystalline varieties. The petrography of peralkaline ignimbrites is briefly discussed.

The chemical compositions and optical properties of the major mineral phases in peralkaline silicic rocks are tabulated, with a brief discussion on their stability relationships.