Andesite of Williwaw Cove formation. Color: medium light gray (N 6). Texture: holocrystalline, porphyritic with both feldspar and pyroxene phenocrysts as much as 4 mm long. Plagioclase, 45-60 percent, ranges in composition from An45 to An60 and averages An51; complex oscillatory zoning and graphic fritting common within individual plagioclase grains. Subhedral clinopyroxene, 10 percent; euhedral orthopyroxene, 10 percent; and anhedral olivine, 1 percent, characterized by brown stains on some fractures. Anhedral magnetite, 5 percent, particularly abundant. Groundmass consists of mesh of feldspar and pyroxene microlites and magnetite dust. Groundmass plagioclase averages An43.
Geology of Little Sitkin Island, Alaska