Definition of rhyodacite
The extrusive equivalent of granodiorite. The principal minerals, sodic plagioclase, sanidine, quartz, and biotite or hornblende, commonly occur as phenocrysts in a finely crystalline groundmass of alkali feldspar and quartz. Accessory minerals are apatite and magnetite, and occasionally augite.
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Locality Updated: Tapada dos Mercados Mine, Guarda (Sé), Guarda, Guarda District, PortugalFrom Rui Nunes, 11th Mar 2014 12:26:35