Porphyry is a volcanic rock composed of large, conspicuous crystals
(phenocrysts) and a fine grained to glassy groundmass. Since the
term “porphyry” refers only to the texture of the rock and not to
its composition, the expression “porphyritic rock” is often
used.
There exist many varieties of porphyry, the specimens are named by
the character of the phenocrysts in the groundmass. They are found
in main classes of igneous rocks, especially in basalt, andesite,
granite, syenite, diorite, gabbro, and peridotite.
Porphyritic felsites and basalts are widely distributed. The
porphyritic texture indicates two separate stages of
solidification. The porphyritic texture is common in extrusions,
e.g., lava, dykes.
Quartz porphyry contains large quartz phenocrysts
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