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porphyry

porphyry

SRI Petrography ID: 26

Names

[de] Porphyr
[en] porphyry

Description

Porphyry is an igneous (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.

Common porphyry types:
- andesite porphyry
- basalt porphyry
- rhyolite porphyry
- feldspar porphyry
- quartz porphyry

Color

Often dark coloured, variable phenocrysts (often feldspar (white, grey or pink), quartz (white, grey), olivine (green, black), hornblende etc.)

Grain Size

Fine grained to glassy matrix, coarse grained phenocrysts

Texture

porphyric

Minerals

phenocrysts: feldspar (plagioclase, alkali-feldspar), quartz, olivine, pyroxene, amphibole,…

Occurences

variable, worldwide in volcanic environments; historical use: e.g. Egypt

Classification

Class

IGNEOUS ROCKS

Broader Terms

porphyric (inequigranular) crystalline rocks

Narrower Terms

Egypt-porphyry

Germany-porphyry

Greece-porphyry

Italy-porphyr

Norway-porphyry

Poland-porphyry

Sweden-prophyry