Oxidized low-silica dacite of Little Sitkin dacite. Color: pale red (5R 5/2). Texture: porphyritic with plagioclase phenocrysts as much as 4 mm long. Plagioclase, 25-30 percent, ranges in separate and individually zoned crystals from An29(?) to An55 with an average composition of about An50; many of the larger feldspars form a graphic "intergrowth" with dusty glass; some have a rim of clear feldspar in optical continuity with the fritted cores; two-fluid inclusions and small needles (0.002 mm by 0.04 mm) of apatite(?) or pyroxene(?) occur in feldspar. Orthopyroxene, 6 percent, occasionally cores clinopyroxene, 9 percent, crystals. Accessory minerals: magnetite, 3 percent, and tridymite, 1 percent. Fluidal groundmass highly charged with hematite dust.
Geology of Little Sitkin Island, Alaska