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Andesites, Dacites, and Related Rocks

Definition

·       Andesite

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.

·        Dacite

o   Very fine crystalline or glassy rock of volcanic origin, composed chiefly sodic plagioclase and free silica with subordinate dark-colored minerals.

Varieties



    • 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.

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