Medium light-gray (N5) olivine-hypersthene basalt, with seriate pilotaxitic texture and dense structure. Plagioclase crystals, as much as 3 mm in size, have a median composition of about An75 for the larger crystals and show strong zoning. The salite grains, which reach a maximum size of 1 mm, have nZ 1.73, nX 1.692, and 2V 60 degrees, suggesting a composition of Ca50Mg[?]-[?]Fe15-30. The crystals are visibly zoned, with the higher index material on the outside; the highest and lowest indices obviously do not represent the same material. The highest index measured for hypersthene was 1.699, suggesting a composition of En74Fs26. The olivine, which attains a maximum size of 0.5 mm, has nZ 1.699, corresponding to Fa15. Magnetite in irregular grains up to 0.1 mm, and apatite, in pseudodichroic prisms, are the principal accessories. Orthoclase (?) was apparently the last mineral to crystallize.
Magmatic differentiation in Tertiary and Quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska