Geology Rocks And Minerals
Group - volcanic. Colour - variable, but light coloured. Texture - usually porphyritic, but can be aphanitic e.g. obsidian. Mineral content - groundmass generally of quartz and plagioclase, with lesser amounts of orthoclase, biotite, amphibole augite, pyroxene hornblende, and glass phenocrysts of plagioclase and quartz, often with amphibole and or biotite, sometimes orthoclase.