Under
IUGS guidelines, rhyodacites are not formally defined in either the
QAPF classification, used to classify
igneous rocks by their mineral content, or the
TAS classification, used to classify volcanic rocks chemically. However, the IUGS allows the use of the term to describe rocks close to the boundary between the rhyolite and dacite fields in each classification scheme. Rhyodacite then describes a fine-grained igneous rock containing between 20% and 60%
quartz and in which
plagioclase makes up about two-thirds of the total
feldspar content.[1][2] Such a rock will contain between 69% and 72% silica by weight.[3]
The U.S. Geological Survey defines rhyodacite as volcanic rock containing 20% to 60% quartz and with plagioclase making up 35% to 65% of the total feldspar content. This makes rhyodacite the extrusive equivalent of monzogranite.[4]
In addition to its quartz and feldspar content, rhyodacite commonly contains
phenocrysts of
andesine,
biotite,
hornblende, and
pyroxene. Quartz and
sanidine phenocrysts are less common than in rhyolite, and rhyolite has only scant
mafic phenocrysts.[5]
^Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143.
ISBN9780521880060.
^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