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Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt.

Composition

The composition of sand varies, depending on the local rock sources and conditions. Calcium carbonate is the second most common type of sand. Somewhat more rarely, sand may be composed of calcium sulfate, such as gypsum and selenite. Because of its chemical inertness and considerable hardness, quartz is the most common mineral resistant to weathering. Sand from Coral Pink Sand Dunes State Park in Utah consists of quartz grains with a hematite coating that gives the orange color. All forms of silicon dioxide are white or colorless, although impure samples can be colored or opaque.

Grain Size and Texture

In the United States, sand is commonly divided into five sub-categories based on size. Very coarse sand is the 1 mm to 2 mm class. Sand can also refer to a textural class of soil, that is, a soil containing more than 85 percent sand-sized particles by mass.

Origin

Sand dunes are a consequence of dry conditions or wind deposition. Over time, wind blows away fine particles, such as clay and dead organic matter, leaving only sand and larger rocks. Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded.

Uses

Sand is often a principal component of concrete. Desert sand, although plentiful, is not suitable for concrete. Fifty billion tons of beach sand and fossil sand are used each year for construction. Sand rich in silica is the principal component in common glasses. Quartz sand is a raw material for the production of silicon. Graded sand serves as an abrasive in cleaning, preparing, and polishing. Mixing sand with paint produces a textured finish for walls and ceilings or non-slip floor surfaces. Engine drivers and rail transit operators use sand to improve the traction of wheels on the rails. Media filters use sand for filtering water.

Resources

Sand is a non-renewable resource over human timescales, and sand suitable for making concrete is in high demand. Many small rivers have been depleted, causing environmental concern and economic losses to adjacent land.

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