Arkose

A silicate-sandstone containing at least 25% feldspar, it is derived from arkosic sand.

A feldspar-rich sandstone, typically coarse-grained and pink or reddish, that is composed of angular to subangular grains that may be either poorly or moderately well sorted; usually derived from the rapid disintegration of granite or granitic rocks, and often closely resembles granite; e.g., the Triassic arkoses of the Eastern United States. Quartz is usually the dominant mineral, with feldspar (chiefly microcline) constituting at least 25%. Cement (silica or calcite) is commonly rare, and matrix material (usually less than 15%) includes clay minerals (esp. kaolinite), mica, and iron oxide; fine-grained rock fragments are often present. 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 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. Arkose may also occur at the base of a sedimentary series as a thin blanketlike residuum derived from and resting on granitic rock. Etymol: French, probably from Greek archaios, ancient, primitive. Also spelled arcose.