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rhyodacite (toscanite) A fine-grained, extrusive, igneous rock characterized by an adamellite mineral assemblage and composition. Most rhyodacites are porphyritic, with quartz and plagioclase as common phenocryst types. The term ‘toscanite’ was used originally by H. S. Washington in 1897 to describe rocks of rhyodacite composition from Tuscany, Italy; this older term is now little used. Rhyodacites are erupted above subducted plates and belong to the calcalkaline magma series.