Arkose () or arkosic sandstone[
]
Components
Quartz is commonly the dominant mineral component, and some
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 (fizz) slightly in dilute hydrochloric acid; sometimes the cement also contains
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.
are rare in arkose, due to the depositional processes that form it, although bedding is frequently visible.
Formation process
Arkose is generally formed from the weathering of feldspar-rich
igneous rock or
metamorphic rock, most commonly
granite, 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 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
unconformity in the immediate vicinity of granite terrains.
Gallery
File:Arkosesand.jpg|Arkosic sand in the Llano Uplift, Texas, with granite outcrops
File:GrusSand.JPG|Grus sand and the granitoid it's derived from
Uluru
The central
inselberg Uluru (Ayers Rock) is composed of late
Neoproterozoic/
Cambrian arkose, deposited in the
Amadeus Basin.
See also
