Rhyodacite lava from Pratt Point member of Little Sitkin dacite. Color: yellowish gray (5 P 7/1). Texture: hypocrystalline, vesicular, porphyritic with plagioclase phenocrysts as much as 5 mm and hornblende phenocrysts as much as 3 mm long. Plagioclase megaphenocrysts, 15-20 percent, are complexly zoned from cores of An55-60 to rims of An35-45; several feldspars poikilitic with small hornblende inclusions, 0.09 by 0.2 mm; minute clinopyroxene rods, 0.01 by 0.05 mm; and two-fluid inclusions in cavities with a maximum diameter of 0.04 mm. Hornblende, 36 percent, is pleochroic in shades of olive green, reddish brown, and black; several crystals have deeper pleochroism at borders than at center. Hypersthene, 1-2 percent, is the only pyroxene megaphenocryst; two crystals have a core of oxyhornblende. Quartz, 1 percent, occurs in large phenocrysts with resorbed outlines and conchoidal fractures, which tend to aid plucking of this mineral during grinding of the thin section. Anhedral magnetite forms 1 percent of the rock. The groundmass consists of clear pinkish glass (N=1.491) containing numerous small vesicles.
Geology of Little Sitkin Island, Alaska