What makes arkose pink

Sedimentary Rocks deluxe rock kit. Get your sample of Arkose today at www. You must be logged in to post a comment. Mini Me Geology Blog About. Arkose — Beautiful Sedimentary Rock. You might also like:. Also spelled arcose. Arkose is typically grey to reddish in color. 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, although bedding is frequently visible.

Arkose is commonly a current-deposited sandstone of continental origin, occurring as a thick, wedge-shaped mass of limited geographic extent as in a fault trough or a rapidly subsiding basin ; it may be strongly cross-bedded and associated with the coarse granite-bearing conglomerate, and it may denote an environment of high relief and vigorous erosion of strongly uplifted granitic rocks in which the feldspar was not subjected to prolonged weathering or transport before burial.

The defining property large feldspar content needs a good explanation because feldspar although more common than quartz in the rocks is much less resistant to weathering and is therefore usually destroyed converted to clay even before the quartz grains are liberated from the feldspar-rich rocks.

Arkose may also occur at the base of a sedimentary series as a thin blanketlike residuum derived from and resting on a granitic rock. Hence, it was long believed that arkosic sandstones form in climatic regions which do not support weathering because they are either too dry or too cold.

It definitely is the case in some instances because chemical weathering which is the most important form of weathering is heavily dependent on the availability of water and to a lesser extent warm temperature which accelerates chemical reactions. However, it is obvious that many arkose formations had no such conditions during their formation. Nowadays it is believed that arkose forms where there is a crystalline source material rich in feldspar mostly granite or gneiss and a high relief with consequent rapid erosion which gives no time to break up feldspar.

This is the case with granitic batholiths which are surrounded by down-faulted blocks grabens which collect the eroded clastic detritus rich in feldspar. That has lead to a field description according to which any sandstone that contains appreciable amounts of feldspar in named an arkose. Feldspar is usually pink in color and is therefore easily seen. Arkosic sandstones generally contain small amounts of matrix small dust grains filling the pore space between larger sand grains.

According to this definition, arkose might contain as little as Arkose may be associated with reddish sandstones redbeds , but they should not be confused. Most redbeds are not rich in feldspar. It is a mineral hematite that covers sand grains as a fine pigment and gives a reddish color to redbeds. Some volcanic sands pyroclastic sediments may resemble it because they frequently contain reddish fragments volcanic glass , weathered scoria , fragments of tuff.

Torridonian sandstone below is a precambrian arkose from Scotland. Metamorphosed arkose or feldspar-rich quartzite. Aust-Agder, Norway. Width of sample 19 cm.

Heavily metamorphosed and recrystallized arkose. Green mineral is epidote. Salhus, the Bergen Arcs, Norway. Width of view 20 cm. Quartz and K-feldspar usually microcline are the most important minerals in arkose. Plagioclase is clearly less important than K-feldspar and only in rare cases is plagioclase the dominant feldspar in this rock type. Why is this the case? Probably because arkosic rocks mostly tend to occur in tectonic settings continental rifts which favor the emplacement of K-rich granites instead of Na- and Ca-rich granodiorites and tonalites.

Plagioclase feldspar is also clearly less resistant to weathering than K-feldspar, so its chances to survive as a sand grain are not that good.

Swauk arkose in Washington State USA is an example of arkose sandstone where plagioclase is the dominant feldspar.