Andesites, Dacites, and Related Rocks
o Very finely crystalline extrusive rock of volcanic origin composed largely of plagioclase feldspar (oligoclase or andesine) with smaller amounts of dark-colored mineral (hornblende, biotite, or pyroxene), the extrusive equivalent of diorite.
o Very fine crystalline or glassy rock of volcanic origin, composed chiefly sodic plagioclase and free silica with subordinate dark-colored minerals.
- Any group of hornblende-biotite andesites and dacites containing cordierite.
- Augite-bearing andesite.
- A modified andesite, altered by hydrothermal processes, resembling a greenstone and consisting of calcite, epidote, serpentine, quartz, pyrite, and iron ore.
- A glassy andesite composed of bronzite, augite, magnetite, and a few large plagioclase and garnet crystals.
- A hypersthene andesite containing plagioclase crystals that have labradorite cores and sodic rims and a groundmass with microlites of sodic oligoclase.
- An andesite characterized by orthopyroxene as the mafic mineral, andesine as the plagioclase, and a glassy groundmass.
- A dacite type of extrusive rock composed of hypersthene and labradorite.
- A variety of dacite or rhyodacite containing very calcic plagioclase and pyroxene in a glassy groundmass.