| Sedimentary rock | |
_and_quartz_(gray)_grains.jpg) Arkose with K-feldspar (pinkish-orangish) and quartz (gray) grains | |
| Composition | |
|---|---|
| >25%feldspar |
Arkose (/ˈɑːrkoʊs,-koʊz/) orarkosic sandstone[1] is adetritalsedimentary rock, specifically a type ofsandstone containing at least 25%feldspar.[2][3]Arkosic sand issand that is similarly rich in feldspar, and thus the potential precursor of arkose.
Quartz is commonly the dominant mineral component, and somemica is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts ofcalcite cement, which causes it to effervesce (fizz) slightly in dilutehydrochloric acid; sometimes the cement also containsiron oxide.
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.Fossils are rare in arkose, due to the depositional processes that form it, althoughbedding is frequently visible.
Arkose is generally formed from the weathering of feldspar-richigneous ormetamorphic, most commonlygranitic, rocks, which are primarily composed of quartz and feldspar (called 'grus' as a sand). These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significantchemical weathering and decomposition; therefore arkose is designated atexturally immature sedimentary rock. Arkose is often associated withconglomerate deposits sourced from granitic terrain and is often found aboveunconformities in the immediate vicinity of granite terrains.
The centralAustralianinselbergUluru (Ayers Rock) is composed of lateNeoproterozoic/Cambrian arkose, deposited in theAmadeus Basin.[4]