High-silica dacite of the Double Point dacite. Color: brownish gray (5 YR 5/l). Texture: hypocrystalline, porphyritic with feldspar phenocrysts as much as 3 mm long. Zoned and twinned plagioclase, 25-30 percent, varies in composition from An35 to An45 and averages An40. Accessory minerals are euhedral orthopyroxene, 2 percent; snbhedral clinopyroxene, 2 percent; magnetite, 1 percent; and tridymite, 1 percent. The fluidal glassy to cryptocrystalline groundmass carries pyroxene, magnetite and plagioclase microlites. The plagioclase microlites have an average composition of An43.
Geology of Little Sitkin Island, Alaska