Editing Andesite (section)

== Description ==
[[File:Basalt qapf.jpg|thumb|[[QAPF diagram]] with basalt/andesite field highlighted in yellow. Andesite is distinguished from basalt by SiO<sub>2</sub> > 52%.]]
[[File:TAS-Diagramm-andesite.png|thumb|upright=1.35| Andesite is field O2 in the [[TAS classification]].]]
Andesite is an [[aphanitic]] (fine-grained) to [[porphyritic]] (coarse-grained) igneous rock that is intermediate in its content of [[silica]] and low in [[alkali metals]]. It has less than 20% quartz and 10% [[feldspathoid]] by volume, with at least 65% of the [[feldspar]] in the rock consisting of [[plagioclase]]. This places andesite in the [[basalt]]/andesite field of the [[QAPF diagram]]. Andesite is further distinguished from basalt by its silica content of over 52%.<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-2009-139-143">{{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 |pages=139–143}}</ref> However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and andesite is then defined chemically as volcanic rock with a content of 57% to 63% silica and not more than about 6% alkali metal oxides. This places the andesite in the O2 field of the [[TAS classification]]. [[Basaltic andesite]], with a content of 52% to 57% silica, is represented by the O1 field of the TAS classification but is not a distinct rock type in the QAPF classification.<ref name="Philpotts-ague-2009-139-143"/>

Andesite is usually light to dark grey in colour, due to its content of [[hornblende]] or [[pyroxene]] minerals.<ref name="macdonald-etal-1983-127"/> but can exhibit a wide range of shading. Darker andesite can be challenging to distinguish from basalt, but a common [[wiktionary: rule of thumb|rule of thumb]], used away from the laboratory, is that andesite has a [[color index (geology)|color index]] less than 35.<ref name="Philpotts-ague-2009-139">Philpotts and Ague 2009, p. 139</ref>

The plagioclase in andesite varies widely in sodium content, from [[anorthite]] to [[oligoclase]], but is typically [[andesine]], in which [[anorthite]] makes up about 40 [[mol%]] of the plagioclase. The pyroxene minerals that may be present include [[augite]], [[pigeonite]], or [[orthopyroxene]]. [[Magnetite]], [[zircon]], [[apatite]], [[ilmenite]], [[biotite]], and [[garnet]] are common accessory minerals.<ref>{{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=0-7167-2438-3 |page=57 |edition=2nd}}</ref> [[Alkali feldspar]] may be present in minor amounts.

Andesite is usually [[porphyritic]], containing larger crystals ([[phenocryst]]s) of plagioclase formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained [[Matrix (geology)|matrix]].  Phenocrysts of pyroxene or hornblende are also common.<ref name="blatt-tracy-1996-57">Blatt and Tracy 1996, p.57</ref> These minerals have [[Bowen's reaction series|the highest melting temperatures]] of the typical [[mineral]]s that can crystallize from the melt<ref name="bowen">{{Cite journal | last1 = Tilley | first1 = C. E. | author-link1 = Cecil Edgar Tilley| doi = 10.1098/rsbm.1957.0002 | title = [[Norman L. Bowen|Norman Levi Bowen]] 1887-1956 | journal = Biographical Memoirs of Fellows of the Royal Society | volume = 3 | pages = 6–26 | year = 1957 | jstor = 769349| s2cid = 73262622 }}</ref> and are therefore the first to form solid crystals. Classification of andesites may be refined according to the most abundant [[phenocryst]]. For example, if hornblende is the principal phenocryst mineral, the andesite will be described as a ''hornblende andesite''.