sandstone

sandstone, lithified accumulation of sand-sized grains (0.063 to 2 mm [0.0025 to 0.08 inch] in diameter). It is the second most common sedimentary rock after shale, constituting about 10 to 20 percent of the sedimentary rocks in the Earth’s crust. Because of their abundance, diverse textures, and mineralogy, sandstones are important indicators of erosional and depositional processes.

Texturally, sandstones consist of two components: (1) a framework composed of sand-sized grains and (2) interstitial volume between grains, which may be empty, especially in modern sandstones, or, in the case of most ancient sandstones, filled with either a chemical cement of silica or calcium carbonate or a fine-grained matrix.

The principal mineral constituents of the framework are quartz, feldspar, and rock fragments. The relative proportions of these have genetic implications, first for the source area and second for the rate of deposition.

Texture and mineralogical properties are used for sandstone classification, though considerable debate exists as to which properties to emphasize. The two major classes of sandstone are arenite and wacke. The boundary between the two is based on the amount of matrix present in the sample. Arenites contain less matrix than wacke. Though the exact boundary is debated, often 5 percent matrix is the accepted value, whereas some experts place this boundary at 15 percent. Another common classification uses the name arkose for rocks rich in feldspar. These names are further modified by the components present in the rock. Lithic arenite, feldspathic wacke, and quartz arenite are common examples.