| Text Description: |
Specimen 6 is from the Finger Bay volcanics. It is porphyritic, brownish-gray albitized pillow lava from the southwest shore of Boot Bay, Adak Island. About 15 percent of the rock is tabular cloudy albite crystals (4 mm) altering to clay minerals and containing small patches of epidote; 5 percent of the rock is fresh, euhedral to subhedral augite crystals (1.1 mm) . Some of these phenocrysts are clustered together in subophitic patches. About 10 percent of the rock is subspherical, chertlike greenish masses (1-4 mm), either vesicle fillings or foreign pellets, containing chlorite, stilpnomelane, iron-rich mineraloids, and central masses of quartz. Some of the quartz is intergrown with orthoclase. About 70 percent of the rock is a cloudy fine-grained groundmass containing devitrified glass (chlorite, nontronite, epidote), altered albite microlites and skeletal crystals (0.3 mm), and elongate clinopyroxene microlites (0.3 mm), commonly in splayed aggregates. The groundmass is crowded with needlelike skeleta1 crystals of opaque oxide. The original composition of this rock is unknown; the altered rock might be called albitized trachyandesite. Albite, quartz, and orthoclase may all be secondary. |
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