Definitions

arkose

arkose

[ahr-kohs]

Coarse sandstone that has formed by the disintegration of granite without appreciable decomposition. It thus consists primarily of quartz and feldspar grains. In the absence of stratification, arkose may bear superficial resemblance to granite, and it sometimes has been described as reconstituted granite, or granite wash. Like the granite from which it was formed, arkose is pink or gray.

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Arkose is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar., Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. The other mineral components may vary, but quartz is commonly dominant, and minor mica is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of calcite cement, which causes it to effervesce slightly in dilute hydrochloric acid; sometimes the cement also contains iron oxide. Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tends toward the coarser end of the scale. Fossils are rare in arkose, due to the depositional processes that form it, although bedding is frequently visible.

Arkose is generally formed from the weathering of feldspar-rich igneous or metamorphic rocks, most commonly granitic rocks, which are primarily composed of quartz and feldspar. These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significant chemical weathering and decomposition; therefore arkose is designated a texturally immature sedimentary rock. Arkose is often associated with conglomerate deposits sourced from granitic terrain and is often found above unconformities over such granitic terrain.

The famous central Australian monolith Uluru (Ayers Rock) is composed of late Neoproterozoic or Cambrian arkose, deposited in the Amadeus Basin.

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