porphyry, igneous rock

porphyry pôrˈfərē [key], igneous rock composed of large, conspicuous crystals (phenocrysts) and a groundmass in which the phenocrysts are embedded. Some authorities consider the expression “porphyritic rock” better usage than porphyry, since the term refers only to the texture of the rock—not its chemical, physical, or mineralogical composition or color. The texture is important in the determination of the circumstances under which the rock formed. The phenocrysts vary in size; the groundmass may be either glassy or made up of coarse or fine granules or crystals. The varieties of porphyry are many, the specimens being named by the character of the phenocrysts in the groundmass. They are found in main classes of igneous rocks, e.g., in granite, syenite, diorite, gabbro, and peridotite. Porphyritic felsites and porphyritic basalts are widely distributed. The porphyritic texture indicates two separate stages of solidification. In the first phase the phenocrysts form in the molten mass; in the second, the molten mass itself crystallizes into a solid. Porphyritic texture is especially common in extrusions, e.g., in lava.

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