Sandstone

What is a Sandstone ?

Sandstone is a type of rock made from sediment — a sedimentary rock. The sediment particles are clasts, or pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. It is composed mostly of sand particles, which are of a medium size; therefore, sandstone is a medium-grained clastic sedimentary rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). The sand grains that make up sandstone are aptly referred to as framework grains.

Minerals of Sandstone

Sandstone is formally defined strictly by particle size, but rocks made of carbonate minerals don't qualify as sandstone. Carbonate rocks are called limestone and given a whole separate classification, so sandstone really signifies a silicate-rich rock. (A medium-grained clastic carbonate rock, or "limestone sandstone," is called calcarenite.) This division makes sense because limestone is made in clean ocean water, whereas silicate rocks are made from sediment eroded off the continents.

Mature continental sediment consists of a handful of surface minerals, and sandstone, therefore, is usually almost all quartz. Other minerals—clays, hematite, ilmenite, feldspar, amphibole, and mica— and small rock fragments (lithics) as well as organic carbon (bitumen) add color and character to the clastic fraction or the matrix. A sandstone with at least 25 percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.

The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no matrix.

Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to nearly black, with gray, brown, red, pink and buff in between.

How Sandstone Forms

Sandstone forms where sand is laid down and buried. Usually, this happens offshore from river deltas, but desert dunes and beaches can leave sandstone beds in the geologic record too. The famous red rocks of the Grand Canyon, for instance, formed in a desert setting. Fossils can be found in sandstone, although the energetic environments where sand beds form don't always favor preservation.

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