# Arkose

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{{short description|Type of sandstone containing at least 25% feldspar}}
{{Infobox rock
|name=Arkose
|alternative_name=
|type=Sedimentary
|image=Arkose with K-feldspar (pinkish-orangish) and quartz (gray) grains.jpg
|image_size=250
|alt=
|caption=Arkose with K-feldspar (pinkish-orangish) and quartz (gray) grains
|coordinates=
|composition=>25% [feldspar](/source/feldspar)
|composition_secondary=
}}

'''Arkose''' ({{IPAc-en|ˈ|ɑːr|k|oʊ|s|,_|-|k|oʊ|z}}) or '''arkosic sandstone'''<ref>{{cite book |editor-last=Jackson|editor-first=Julia A. |title=Glossary of Geology |date=1997 |publisher=[American Geological Institute](/source/American_Geosciences_Institute) |location=[Alexandria, Virginia](/source/Alexandria%2C_Virginia) |isbn=0922152349 |edition=4th |chapter=arkosic sandstone}}</ref> is a [detrital](/source/detrital) [sedimentary rock](/source/sedimentary_rock), specifically a type of [sandstone](/source/sandstone) containing at least 25% [feldspar](/source/feldspar).<ref name=Folk1974>
{{cite book |last=Folk|first=R. L. |year=1974 |title=Petrology of Sedimentary Rocks |publisher=Hemphill |isbn=0-914696-14-9 |url=http://www.lib.utexas.edu/geo/folkready/folkprefrev.html |access-date=2007-05-05 |archive-url=https://web.archive.org/web/20060214063526/http://www.lib.utexas.edu/geo/folkready/folkprefrev.html |archive-date=2006-02-14 |url-status=dead}}
</ref><ref name=Stow2005>{{cite book |last=Stow|first=D. A. V. |year=2005 |title=Sedimentary Rocks in the Field |publisher=[Manson](/source/CRC_Press) |isbn=1-874545-69-3}}</ref> '''Arkosic sand''' is [sand](/source/sand) that is similarly rich in feldspar, and thus the potential precursor of arkose. 
==Components==
[Quartz](/source/Quartz) is commonly the dominant mineral component, and some [mica](/source/mica) is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of [calcite](/source/calcite) cement, which causes it to effervesce (fizz) slightly in dilute [hydrochloric acid](/source/hydrochloric_acid); sometimes the cement also contains [iron oxide](/source/iron_oxide). 
==Colouration and presence of fossils==
Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tend toward the coarser end of the scale. [Fossil](/source/Fossil)s are rare in arkose, due to the depositional processes that form it, although [bedding](/source/bed_(geology)) is frequently visible.
==Formation process==
Arkose is generally formed from the weathering of feldspar-rich [igneous](/source/igneous_rock) or [metamorphic](/source/metamorphic_rock), most commonly [granitic, rocks](/source/granite), which are primarily composed of quartz and feldspar (called '[grus](/source/Grus_(geology))' as a sand). These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significant [chemical weathering](/source/chemical_weathering) and decomposition; therefore arkose is designated a ''texturally immature'' sedimentary rock. Arkose is often associated with [conglomerate](/source/conglomerate_(geology)) deposits sourced from granitic terrain and is often found above [unconformities](/source/unconformity) in the immediate vicinity of granite terrains.
==Gallery==
<gallery widths=200>
File:Arkosesand.jpg|Arkosic sand in the [Llano Uplift](/source/Llano_Uplift), Texas, with [granite](/source/granite) outcrops
File:GrusSand.JPG|Grus sand and the granitoid it's derived from
</gallery>
==Uluru==
The central [Australia](/source/Australia)n [inselberg](/source/inselberg) [Uluru](/source/Uluru) (Ayers Rock) is composed of late [Neoproterozoic](/source/Neoproterozoic)/[Cambrian](/source/Cambrian) arkose, deposited in the [Amadeus Basin](/source/Amadeus_Basin).<ref name=Sweet1992>{{cite book |last1=Sweet|first1=I. P. |last2=Crick|first2=I. H. |year=1991 |title=Uluṟu & Kata Tjuṯa: A Geological History |publisher=[Australian Geological Survey Organization](/source/Geoscience_Australia) |isbn=0-644-25681-8 |url=}}</ref>

==See also==
*{{annotated link|Lithic sandstone}}

==References==
{{reflist}}
{{Rock type}}
Category:Sedimentary rocks
Category:Sandstone
Category:Feldspar
Category:Uluru

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Adapted from the Wikipedia article [Arkose](https://en.wikipedia.org/wiki/Arkose) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Arkose?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.