Sedimentary Rock – Sandstone Composition & Structure

Sandstones are mechanical in their origin, being formed by the consolidation into rock masses of beds of sand and gravel. Usually the constituted grains are rounded and water-worn, but at times they be more or less angular in shape. With the variation in the may be more or less angular in shape. With the variation in the size of the mineral particles the rocks themselves vary in their grain. Coarse-grained sandstones formed from gravels are known as conglomerates., The cement which serves to bind the sand grains together may be deposited silica, a carbonate, usually calcite, an iron oxide, hematite or limonite, or fine-grained argillaceous or claylike material. The color of the rock will depend in large measure upon the character of the cement. The rocks which have silica or calcite as their binding material are light in color, usually pale yellow, buff, white to gray, while those that contain an iron oxide are red to reddish brown. It is to be noted that when a sandstone breaks it is usually the cement that is fractured, while the individual grains grains remain unbroken, so that surfaces of the rocks have a granular appearance and feeling. The chief mineral of sandstones is quartz, but at times a rock may contain notable amounts of feldspar and is then termed an arkose. Graywacke is a sandstone, usually of a gray color, which in addition to quartz and feldspar contains particles of other rocks and minerals.

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