Hypersthene andesite. Light gray (N7) hypersthene andesite. Structure porphyritic, vesicular, with hyalopilitic groundmass. Megaphenocrysts (2%) from 1.5 to 3 mm are of plagioclase only; the remainder of the constituents are nearly all less than 0.75 mm. The plagioclase phenocrysts have a median index of about 1.557, suggesting a median composition of about An55. Crystals are strongly zoned, many with zones of inclusions. Salite crystals, some as much as 1 mm long, are dark greenish-black in hand specimen and pale-greenish in thin section. The extinction angle of 41.5 degrees suggests a composition of Ca50Mg35Fe15 (equivalent to Di35He15). The hypersthene prisms have nZ 1.710, suggesting a composition of En66Fs34. Olivine is present in rare grains, up to 2 mm; they are black in hand specimen and have an iridescent luster on fracture surfaces. Under the microscope the olivine appears to be permeated by chains of tiny crystals of an opaque black mineral. As fragments of the olivine are strongly attracted by a hand magnet, this mineral is presumed to be magnetite. The mean index of the olivine is 1.982, suggesting a composition of Fo85Fa15. The principal accessories are magnetite, which forms irregular grains from 0.05 to 0.25 mm in size, and apatite, which forms stout pseudodichroic prisms. The groundmass consists of plagioclase laths, augite rods, a few prisms of apatite, small grains of magnetite or ilmenite, and a little interstitial orthoclase. Tridymite, in crystals as much as 0.5 mm across, is a common vesicle lining.
Magmatic differentiation in Tertiary and Quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska