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Quartz Latites (Rhyodacites) and Rhyolites

 

Definitions

    • Is the effusive equivalent of quartz monzonite.

    • A group of extrusive porphyritic igneous rocks containing quartz., plagioclase, and biotite phenocrysts in a fine-grained to glassy groundmass composed of alkali feldspar and silica minerals.

    • A light-colored, aphanitic volcanic rock composed largely alkali feldspar and free silica with minor amounts of mafic minerals; the extrusive equivalent of granite.

Varieties

    • Rhyolite in which the matrix is holocrystalline.

    • Is acid glass, filled with crystallites and microlites.

    • A type of volcanic glass distinguished by a waxy, dull, resinous, pitchy luster.

    • It contains more water (up to 10%) than does obsidian (usually less than 4%).

    • A green to black extrusive rock characterized by acmite- augite or diopside, anorthoclase, and cossyrite phenocrysts in an acmite or feldspar matrix that is either pumiceous, partly glassy, fine-grained holocrystalline trachytic, or microlitic.

    • A white, sodic rhyolite containing alkalic amphibole or pyroxene.

    • Synonym for quartz latite (rhyodacite)

    • Is a leucocratic albite rhyolite.

    • Is a term for alkalic rhyolite that recent usage tends to restrict to sodic rocks often found in spilitic associations. Terms such as albite rhyolite or albitized dacite are as appropriate and more generally comprehensible than these euphonious but hard-to-remember rock names.

    • Is a dike rock containing phenocrysts of microcline or microcline-perthite, aegerine, and a little hornblende in a matrix of abundant quartz, aegerine, and microcline or perthite.

    • An alkalic microgranite containing a considerable amount of riebeckite.

    • Is a rock with phenocrysts of quartz and sanidine with patches of riebeckite in a fine-grained matrix that is often micropegmatitic.

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