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rhyolite-porphyry

rhyolite-porphyry

SRI Petrography ID: 29

Names

[de] Rhyolith-Porphyr
[en] rhyolite-porphyry

Description

Porphyry is an igneous, volcanic rock with rhyolitic chemism, composed of large, conspicuous crystals (phenocrysts) and a fine grained to glassy groundmass in which the phenocrysts are embedded. Since the term “porphyry” refers only to the texture of the rock and not to its composition, the expression “porphyritic rock” is often used.
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) and dykes.

Color

variable (gray, rose)

Grain Size

Fine grained matrix, coarse grained phenocrysts

Texture

porphyric

Minerals

phenocrysts: mostly quartz and feldspar, mica

Occurences

rhyolite-porphyry is known all over the world in volcanic environments in continental crust, island arcs and dykes.

Comments

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.

Classification

Class

IGNEOUS ROCKS

Broader Terms

porphyric (inequigranular) crystalline rocks