Rhyodacite

Rhyodacite is an extrusive volcanic rock intermediate in composition between dacite and rhyolite. It is the extrusive equivalent of granodiorite. Phenocrysts of sodium-rich plagioclase, sanidine, quartz, and biotite or hornblende are typically set in an aphanitic to glassy light to intermediate-colored matrix.
Rhyodacite is a high silica rock containing 20% to 60% quartz with the remaining constituents being mostly feldspar. The feldspar is a mix of alkaline feldspar and plagioclase, with plagioclase forming 35% to 65% of the mix.[1] Rhyodacite often exists as explosive pyroclastic volcanic deposits.

A sample of rhyodacite from Slovakia

Rhyodacite lava flows occur, for example, in northwestern Ferry County (Washington),[1] and at An Sgùrr on the island of Eigg in Scotland.[2]

References
  1. "Geologic units containing rhyodacite". usgs.gov.
  2. Emeleus, C.H. and Bell, B.R. (2005) British Regional Geology: The Palaeogene Volcanic Districts of Scotland, 4th edition, Nottingham, British Geological Survey, page 77


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