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