Andesite

A dark-colored, fine-grained, mostly extrusive rock that is approximately the fine-grained equivalent of diorite. When porphyritic, andesites contain phenocrysts composed primarily of zoned sodic plagioclase (esp. andesine) and one or more mafic minerals (e.g., biotite, hornblende, pyroxene, usually <20%), with a groundmass composed generally of the same minerals as the phenocrysts, although the plagioclase may be more sodic, and quartz is generally present in small amounts.

Andesite grades into latite with increasing alkali feldspar content, or into dacite with more alkali feldspar and quartz. Basalt is similar but with more mafic minerals (especially olivine) and minimal or no quartz or alkali feldspar; basaltic andesite is intermediate.

Andesitic rocks (andesitoids), including andesite and basaltic andesite, plot on the QAPF diagram with 0-20% quartz, <10% feldspathoids and with >65% of feldspars being plagioclase; this puts them in fields 9, 9*, 9’, 10, 10* and 10’ (basaltic and andesitic rocks). They differ from basalts in being more silica rich (>52%), with less mafic minerals. Andesite itself is defined as being in field 9. Because it is difficult to determine the mineralogy when fine grained, andesite is also defined by chemistry on the TAS diagram: 57-63% SiO₂, (Na₂O+K₂O)<~6%. Depending on which definition is used, andesite may plot variably on these diagrams.