Low-silica dacite of Little Sitkin dacite. Color: dark gray (N 3). Texture: hypocrystalline, porphyritic with prominent feldspar phenocrysts as much as 6 mm long. Plagioclase, 20-25 percent, ranges from An36 to An62 in individual zoned phenocrysts and between different crystals; average composition is about An50; graphic fritting texture and two-fluid inclusions present in centers or selected zones of some feldspar crystals. Clinopyroxene, 6 percent, appears as reaction rims on some orthopyroxene, 5 percent, and olivine, 1 percent. Accessory: magnetite, 1-2 percent. Groundmass is clear brown glass choked with plagioclase (average composition, An43) and acicular orthopyroxene(?) microlites. Index of refraction of an artificial glass of the powdered rock is greater than 1.540.
Geology of Little Sitkin Island, Alaska