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porphyric (inequigranular) crystalline rocks

porphyric (inequigranular) crystalline rocks

SRI Petrography ID: 7

Names

[de] Porphyrische (ungleichkörnige) Kristallingesteine
[en] porphyric (inequigranular) crystalline rocks

Description

Porphyry is an igneous rock composed of large, conspicuous crystals (phenocrysts) embedded in a fine grained to glassy groundmass (matrix). Since the term “porphyry” refers only to the texture of the rock and not to its composition, the expression “porphyritic rock” is often used.
Porphyrics are specified by the phenocrysts in the groundmass (e.g. quartz porphyry). 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. However this texture is common in many volcanics.

Color

variable (brown, gray, rose)

Grain Size

Fine grained matrix, coarse grained phenocrysts

Texture

porphyric

Minerals

phenocrysts: plagioclase, pyroxene and/or hornblende, biotite (+ magnetite, zircon, apatite, ilmenite, garnet).

Occurences

examples: Andes (South America), North Am. (USA: Arizona, Cascades), Italy (Eolian Islands, Tuscany, Umbria), Kamchatka, Japan

Classification

Class

IGNEOUS ROCKS

Narrower Terms

andesit-porphyry

basalt-porphyry

feldspare-porphyry

porphyry

quartz-porphyry

rhyolite-porphyry