porphyry

Porphyry, Greek scholar

Porphyry (pôr`fĭrē), c.232–c.304, Greek scholar and Neoplatonic philosopher. He studied rhetoric under Cassius Longinus and philosophy under Plotinus. He later lectured in Rome on the philosophy of Plotinus and was the teacher of the Neoplatonist Iamblichus. He wrote lives of Pythagoras and of Plotinus and edited the Enneads of Plotinus. He wrote extensively against Christianity and on rhetorical and literary themes. His most influential work is the Isagoge, an introduction to the logic of Aristotle, which became a standard medieval text.

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porphyry, igneous rock

porphyry (pôr`fərē), 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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porphyry

1. any igneous rock with large crystals embedded in a finer groundmass of minerals

2. Obsolete a reddish-purple rock consisting of large crystals of feldspar in a finer groundmass of feldspar, hornblende, etc.

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Porphyry

original name Malchus. 232--305 ad, Greek Neo-Platonist philosopher, born in Syria; disciple and biographer of Plotinus