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Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by alkali feldspar and sodic plagioclase along with one or more of the following mafic minerals: amphibole, biotite or pyroxene. Small amounts of nepheline may be present and apatite is a common accessory mineral.
Trachyandesitic magma can produce explosive Plinian eruptions, such as happened at Tambora in 1815.
Varieties
Sodium-rich trachyandesite is called benmoreite. Basaltic trachyandesite is transitional to basalt and comes in two varieties, mugearite (sodium-rich) and shoshonite (potassium-rich).
References
Trachyandesite Wikipedia(Text) CC BY-SA