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andesite

andesite

SRI Petrography ID: 23

Names

[de] Andesit
[en] andesite

Description

Andesites are medium to dark coloured vesicular volcanic rocks, mostly fine grained with typically porphyric texture (larger crystals in a fine groundmass). The phenocrysts (the larger crystals) comprise plagioclase, pyroxene, biotite and iron-titanium oxides. The groundmass is fine-grained, frequently glassy.
Some andesites contain phenocrysts of olivine, of amphibole and biotite.

Color

medium to dark grey, brownish to red

Grain Size

Mostly fine grained

Texture

porphyric to aphanitic

Minerals

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

Occurences

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

Comments

Andesite can be considered as extrusive equivalent of plutonic diorite.

Classification

Class

IGNEOUS ROCKS

Broader Terms

fine-grained crystalline rocks