| Text Description: |
Medium-light gray (N 6), porphyritic dacite (S; high-silica andesite G). Major phases are clinopyroxene, orthopyroxene (hypersthene), plagioclase (An 58 and possibly An 40), and glass. Phenocrysts of plagioclase and pyroxene as much as 7 mm in length in a very fine grained to aphanitic groundmass of plagioclase, glass, and opaque minerals. In thin section, areas of the groundmass are more transparent than others; these more transparent areas contain fewer opaque minerals and are located in areas of augite concentration. Sample contains two types of plagioclase phenocrysts; some are poikilitic and have overgrown edges, and others are unaltered, free of inclusions, and may have resorbed edges. Most plagioclase phenocrysts of both types have compositions between An 60 to 70, although some may have compositions nearer to An 40. |