Andesite

Andesites consist of intermediate composition plagioclase (typically, An30-50), and less than 35% of ferromagnesian minerals such as clinopyroxene, hornblende, or olivine. Andesites are most abundant in subduction-related arc volcanoes.

The photographs above (crossed polarizers on left, plane polarized light on right) show zoned and twinned plagioclase phenocrysts in a matrix of glass and plagioclase microclites. Plagioclase microlites are subparallel, producing trachytic texture.

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