Andesites are the volcanic equivalent
of a dioritic magma with aphanitic to porphyritic texture. Andesite is
an often dark-colored. fine-grained rock. which have 57 to 63 % SiO2 and
<6 % Na2O+K2O as weight (illustrated in the TAS diagram). In a general
sense, it is the intermediate type between basalt and granite. The mineral
assemblage is typically dominated by plagioclase as well pyroxene (clinopyroxene
and orthopyroxene) and/or amphibole (hornblende). Magnetite, zircon, apatite,
ilmenite, biotite, and garnet are common accessory minerals.
Texturally, most andesites are porphyritic rocks with prominent phenocrysts.
These phenocrysts are typically enclosed in the fine-grained groundmass.
Classification of andesites may be refined according to the most abundant
phenocryst. The quartz-alkalifeldspar-plagioclase-foide abundances in
andesite are illustrated in the Streckeisen QAPF diagram.