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andesit-porphyry

andesit-porphyry

SRI Petrography ID: 30

Names

[de] Andesit-Porphyr
[en] andesit-porphyry

Description

Andesite-porphyry is a porphyry with andesitic chemism.
The term “porphyry” is used for igneous rock composed of large, conspicuous crystals (phenocrysts) and a fine grained to glassy groundmass (matrix) 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.
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, sometimes even in plutonic rocks as 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.

Color

variable (gray, rose)

Grain Size

Fine grained to glassy matrix, coarse grained phenocrysts

Texture

porphyric

Minerals

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

Occurences

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

Classification

Class

IGNEOUS ROCKS

Broader Terms

porphyric (inequigranular) crystalline rocks

Narrower Terms

Egypt-andesite-porphyry

France-andesite-porphyry

Germany-andesite-porphyry

Greece-andesite-porphyry

Italy-andesite-porphyry

USA-andesite-porphyry