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basalt-porphyry

basalt-porphyry

SRI Petrography ID: 31

Names

[de] Basalt-Porphyr
[en] basalt-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.
Basalt-porphyry is a porphyry with basaltic chemism.

Color

Dark coloured, mostly black or brown (if weathered)

Grain Size

Fine grained matrix, coarse grained phenocrysts

Texture

porphyric

Minerals

plagioclase, pyroxene, biotite, olivine, apatite, magnetite;
matrix homogeneous, phenocrysts (olivine, pyroxene)

Occurences

Iceland, Mount Etna (Italy), Hawaii

Classification

Class

IGNEOUS ROCKS

Broader Terms

porphyric (inequigranular) crystalline rocks