| Text Description: |
Hornblende andesite. A compact, light pinkish-gray (5R 7/1) rock, with conspicuous crystals of white plagioclase and black hornblende, and a seriate texture. The median composition of the larger plagioclase crystals is about An[?]; the maximum size is about 2 mm. The hornblende forms brilliant narrow prisms, as much as 1.5 mm long, greenish-brown in thin section, and showing marginal resorption to clinopyroxene. The larger salite crystals have nZ 1.715 plus/minus 0.005, or about Ca50Mg35Fe15. The groundmass, with a mean grain size of about 0.002 mm, is apparently an intergrowth of quartz and orthoclase, with prisms up to 0.008 mm long of pyroxene and grains or iron ore about 0.003 mm across. Cristobalite lines vesicles and fills obscure cracks and replaces some of the groundmass. |