Editing Felsic

{{short description|Igneous rock rich in silica and feldspar}}
{{Use American English|date=January 2021}}
In [[geology]], '''felsic''' is a [[grammatical modifier|modifier]] describing [[igneous rock]]s that are relatively rich in elements that form [[feldspar]] and [[quartz]].<ref name= EG>Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. {{ISBN|978-0393196566}}</ref> It is contrasted with [[mafic]] rocks, which are richer in [[magnesium]] and [[iron]]. Felsic refers to [[silicate minerals]], [[magma]], and [[rock (geology)|rocks]] which are enriched in the lighter elements such as [[silicon]], [[oxygen]], [[aluminium]], [[sodium]], and [[potassium]]. Molten felsic magma  and lava is more [[viscosity|viscous]] than molten mafic magma and lava. Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas.

Felsic rocks are usually light in color and have [[specific gravity|specific gravities]] less than 3. The most common felsic rock is [[granite]]. Common felsic minerals include quartz, [[muscovite]], [[orthoclase]], and the sodium-rich [[plagioclase]] feldspars ([[albite]]-rich).

==Terminology==

=== Acid rock ===
In modern usage, the term ''acid rock'', although sometimes used as a synonym, normally now refers specifically to a high-silica-content (greater than 63% SiO<sub>2</sub> by weight) [[volcanic rock]], such as [[rhyolite]].  Older, broader usage is now considered archaic.{{Citation needed|date=March 2019}}  That usage, with the contrasting term "basic rock" (MgO, FeO, [[mafic]]), was based on an ancient concept, dating from the 19th century, that "[[silicon dioxide|silicic acid]]" (H<sub>4</sub>SiO<sub>4</sub> or Si(OH)<sub>4</sub>) was the chief form of silicon occurring in siliceous rocks. Although this intuition makes sense from an [[acid-base]] perspective in [[aquatic chemistry]] considering water-rock interactions and silica dissolution, siliceous rocks are not formed by this protonated monomeric species, but by a tridimensional network of SiO<sub>4</sub><sup>4–</sup> tetrahedra connected to each other. Once released in water and hydrolyzed, these silica entities can indeed form [[silicic acid]] in aqueous solution. 

=== Etymology ===
The term "felsic" is a derivation of the words "'''fel'''dspar" and "'''si'''lica".<ref>{{cite web |title=felsic, adj |url=https://doi.org/10.1093/OED/1115329128 |website=Oxford English Dictionary |date=2023 |doi=10.1093/OED/1115329128 |access-date=14 June 2024}}</ref> The similarity of the resulting term ''felsic'' to the German ''felsig'', "rocky" (from ''Fels'', "rock"), is accidental.  ''Feldspar'' is from the German ''Feldspat'',  a [[Compound (linguistics)|compound]] of the German ''Feld'', meaning field, plus ''spat[h]'', meaning mineral.<ref>{{OEtymD|feldspar|accessdate=2008-02-08}}</ref>

==Classification of felsic rocks==
[[File:LvMS-Lvg.jpg|thumb|A felsic [[volcanic lithic fragment]], as seen in a [[petrographic microscope]].  Scale box is in millimeters.]]
In order for a rock (rather than a [[mineral]]) to be classified as felsic, it generally needs to contain more than 75% felsic minerals (namely quartz, [[orthoclase]] and [[plagioclase]]). Rocks with greater than 90% felsic minerals can also be called [[leucocratic]],<ref>{{Cite web|url=http://thecse.com/sites/default/files/SWK-20161118-MMICP_Technical_Report_update_November2016.pdf |archive-url=https://web.archive.org/web/20171010105513/http://thecse.com/sites/default/files/SWK-20161118-MMICP_Technical_Report_update_November2016.pdf |archive-date=2017-10-10 |url-status=live|title=Technical report Mackenzie mountains iron-copper property, Northwest Territories, Canada|last=Aurora Geosciences Ltd}}</ref> from the Greek words for white and dominance.

[[Felsite]] is a [[petrology|petrologic]] field term used to refer to very fine-grained or [[aphanitic]], light-colored [[volcanic]] rocks which might be later reclassified after a more detailed microscopic or chemical analysis.

In some cases, felsic volcanic rocks may contain [[phenocryst]]s of [[mafic]] minerals, usually [[hornblende]], [[pyroxene]] or a feldspar mineral, and may need to be named after their phenocryst mineral, such as 'hornblende-bearing felsite'.

The chemical name of a felsic rock is given according to the [[TAS classification]] of Le Maitre (1975). However, this only applies to volcanic rocks. If the rock is analyzed and found to be felsic but is [[Metamorphic rock|metamorphic]] and has no definite volcanic [[protolith]], it may be sufficient to simply call it a 'felsic schist'. There are examples known of highly [[Shear (geology)|sheared]] granites which can be mistaken for rhyolites.

For [[phaneritic]] felsic rocks, the [[QAPF diagram]] should be used, and a name given according to the [[granite]] nomenclature. Often the species of mafic minerals is included in the name, for instance, hornblende-bearing granite, [[pyroxene]] [[tonalite]] or [[augite]] megacrystic [[monzonite]], because the term "granite" already assumes content with feldspar and quartz.

The rock texture thus determines the basic name of a felsic rock.
[[File:Granite Yosemite P1160483.jpg|thumb|Close-up of [[granite]] from [[Yosemite National Park]].]]
[[File:RhyoliteUSGOV.jpg|thumb|A specimen of [[rhyolite]].]]

{| class="wikitable"
|-
| '''Rock texture'''||'''Name of felsic rock'''
|-
| [[Pegmatitic]]||[[Granite]] [[pegmatite]]
|-
| Coarse-grained ([[phaneritic]])||[[Granite]]
|-
| Coarse-grained and [[porphyritic]]||Porphyritic [[granite]]
|-
| Fine-grained ([[aphanitic]])||[[Rhyolite]]
|-
| Fine-grained and porphyritic||Porphyritic [[rhyolite]]
|-
| [[Pyroclastic rock|Pyroclastic]]||Rhyolitic [[tuff]] or [[breccia]]
|-
| [[Vesicular texture|Vesicular]]||[[Pumice]]
|-
| [[Amygdule|Amygdaloidal]]||None
|-
| [[Vitreous lustre|Vitreous (Glassy)]]||[[Obsidian]] or porcellanite
|}

==See also==
*[[QAPF diagram]]
*[[List of minerals]]
*[[List of rock types]]
*[[Bowen's reaction series]]
*[[Archean felsic volcanic rocks]]

==Notes==
{{Reflist}}

==References==
* Le Maitre, L. E., ed. 2002. ''Igneous Rocks: A Classification and Glossary of Terms'' 2nd edition, Cambridge

{{igneous rocks}}

[[Category:Igneous petrology]]
[[Category:Mineralogy concepts]]
[[Category:Petrology]]