Editing Rhyolite (section)

== Description ==
[[File: Rhyolite qapf.jpg|thumb|QAPF diagram with rhyolite field highlighted]]
[[File:TAS-Diagramm-rhyolite.png|thumb|TAS diagram with rhyolite field highlighted]]
Rhyolite is an [[Extrusive rock|extrusive]] igneous rock, formed from magma rich in [[Silicate mineral|silica]] that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of [[mafic]] minerals, and it is typically very fine-grained ([[aphanitic]]) or [[glassy phase|glassy]].<ref name="blatt-tracy-1996-55-74">{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |pages=55, 74 |edition=2nd}}</ref>

An extrusive igneous rock is classified as rhyolite when [[quartz]] constitutes 20% to 60% by volume of its total content of quartz, [[alkali feldspar]], and [[plagioclase]] ([[QAPF diagram|QAPF]]) and alkali feldspar makes up 35% to 90% of its total feldspar content. [[Feldspathoid]]s are not present. This makes rhyolite the extrusive equivalent of granite. However, while the [[IUGS]] recommends classifying volcanic rocks on the basis of their mineral composition whenever possible, volcanic rocks are often glassy or so fine-grained that mineral identification is impractical. The rock must then be classified chemically based on its content of silica and [[alkali metal oxide]]s ([[Potassium oxide|K<sub>2</sub>O]] plus [[Sodium oxide|Na<sub>2</sub>O]]). Rhyolite is high in silica and total alkali metal oxides, placing it in the R field of the [[TAS classification|TAS diagram]].<ref>{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}</ref><ref>{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref><ref>{{Cite web|url=http://geology.csupomona.edu/alert/igneous/igclass.htm|title=Classification of igneous rocks|archive-url=https://web.archive.org/web/20110930102012/http://geology.csupomona.edu/alert/igneous/igclass.htm|archive-date=30 September 2011|url-status=dead}}</ref><ref name="philpotts-ague-2009">{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd}}</ref>{{rp|140–146}}

The alkali feldspar in rhyolites is [[sanidine]] or, less commonly, [[orthoclase]]. It is rarely [[anorthoclase]]. These feldspar minerals sometimes are present as phenocrysts. The plagioclase is usually [[sodium]]-rich ([[oligoclase]] or [[andesine]]). [[Cristobalite]] and [[trydimite]] are sometimes present along with the quartz. [[Biotite]], [[augite]], [[fayalite]], and [[hornblende]] are common accessory minerals.<ref name="blatt-tracy-1996-55-74"/>