Medium dark-gray (N4) hypersthene-olivine basalt flow. Plagioclase phenocrysts, ranging in size from 0.1 to 4 mm, are conspicuously zoned. The median composition, judging from the index, is about An[?]. The dark-green augite crystals, ranging in size from 0.1 to 3 mm, have nZ 1.718 corresponding to Ca50Mg32Fe18. Hypersthene prisms have nZ 1.703, suggesting a composition of En72Fs28. Olivine, in grains as much as 1 mm across, has nZ 1.70, corresponding to a composition of Fo85Fa15. Small grains of magnetite or ilmenite are the principal accessory. The groundmass consists of plagioclase laths about 0.01 mm long and augite rods and granules up to 0.005 mm wide, in a brown glass, with an index of 1.505.
Magmatic differentiation in Tertiary and Quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska