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A porphyritic, extrusive igneous rock containing phenocrysts of zoned plagioclase feldspar, augite (clinopyroxene), orthopyroxene, and some olivine set in a groundmass of andesine plagioclase and alkali feldspar, pyroxene, and iron oxides. Plagioclase accounts for 60–90% of the total feldspar content. This rock is transitionary to a true andesite, which is characterized by plagioclase forming more than 90% of the total feldspar content of the rock.

Subjects: Earth Sciences and Geography.

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