Low-silica dacite of Little Sitkin dacite. Color: grayish black (N 2). Texture: hypocrystalline, porphyritic with plagioclase as much as 5 mm in length. Most plagioclase megaphenocryst and microphenocrysts, 25-30 percent, group around An56 but range from An25 to An78; complex oscillatory zoning common; many feldspars contain vermicular areas of dusty brown glass. Orthopyroxene, 8-10 percent, occurs as individual euhedrons and as cores in many clinopyroxene, 6 percent, crystals. Accessory minerals: magnetite, 2 percent, and oxyhornblende, 1 percent (xenocrysts?) Fluidal glassy groundmass contains angular crystal fragments, 3 percent, and euhedral rods of pyroxene (?).
Geology of Little Sitkin Island, Alaska