Quartz gabbro from elevation 1,000 feet on the west-facing hill slope 2 3/4 miles southwest of Tarasof Point. The texture is hypidiomorphic-granular. Felsic minerals compose 75-80 percent of the rock: 4-6 percent quartz, <0.1 percent orthoclase, and 70-75 percent plagioclase slightly altered to sericite (<0.1 percent). The plagioclase composition, by extinction angle, is An54 [mean of 36 crystals, standard deviation is 7 percent An, average amount of zoning per individual plagioclase crystal is 9 percent An per crystal]. Mafic minerals compose 20-25 percent: 0.5-1.5 percent clinopyroxene (remnants in hornblende and euhedra in feldspar), 8-10 percent hornblende (an indeterminate amount derived from clinopyroxene), 6-8 percent biotite slightly altered to penninite (<0.1 percent) and cliozoisite (<0.1 percent), 2-5 percent magnetite, <0.1 percent hematite, 0.5-1.5 percent apatite, 0.5-1 percent tourmaline, and 0.1-1 percent penninite (interstitial).
Geology of Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska