Close-up of sand from a beach in Vancouver, showing a surface area of (approximately) between 1-2 square centimetres.
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.
As the term is used by geologists, sand particles range in diameter from 0.0625 (or 1/16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. The next smaller size class in geology is silt: particles smaller than 0.0625 mm down to 0.004 mm in diameter. The next larger size class above sand is gravel, with particles ranging from 2 mm up to 64 mm (see particle size for standards in use). Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). Sand is commonly divided into five sub-categories based on size: very fine sand (1/16 - 1/8 mm diameter), fine sand (1/8 mm - 1/4 mm), medium sand (1/4 mm - 1/2 mm), coarse sand (1/2 mm - 1 mm), and very coarse sand (1 mm - 2 mm). These sizes are based on the F sediment size scale, where size in F = -log base 2 of size in mm. On this scale, for sand the value of F varies from -1 to +4, with the divisions between sub-categories at whole numbers.
[edit] Constituents of sand
An electron micrograph showing grains of sand
Close up of black volcanic sand from Perissa, in Santorini, Greece
The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), usually in the form of quartz, which, because of its chemical inertness and considerable hardness, is resistant to weathering.
The composition of sand is highly variable, depending on the local rock sources and conditions. The bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material.[1] The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from the weathering and erosion of a (usually nearby) granite. Some sands contain magnetite, chlorite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are typically green in color, as are sands derived from basalt (lava) with a high olivine content. Many sands, especially those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow colour. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones.
Sand is transported by wind and water and deposited in the form of beaches, dunes, sand spits, sand bars and related features.
[edit] Study of sand
Study of individual grains can reveal much historical information as to the origin and kind of transport of the grain. Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. 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.
People who collect sand as a hobby are known as arenophiles or psammofiles.
[edit] Uses of sand
At 300 km/h, an ICE 3 (DB class 403) releases sand from several bogies to the rails.
Sand sorting tower at a gravel extraction pit.
- Sand is often a principal component of concrete.
- Molding sand, also known as foundry sand, is moistened or oiled and then shaped into molds for sand casting. This type of sand must be able to withstand high temperatures and pressure, allow gases to escape, have a uniform, small grain size and be non-reactive with metals.
- It is the principal component in glass production.
- Graded sand is used as an abrasive in sandblasting and is also used in media filters for filtering water.
- Brick manufacturing plants use sand as an additive with a mixture of clay and other materials for manufacturing bricks.
- Sand is sometimes mixed with paint to create a textured finish for walls and ceilings or a non-slip floor surface.
- Sandy soils are ideal for certain crops such as watermelons, peaches, and peanuts and are often preferred for intensive dairy farming because of their excellent drainage characteristics.
- Sand is used in landscaping, it is added to make small hills and slopes (for example, constructing golf courses).
- Beach nourishment - transportation to popular beaches where seasonal tides or artificial changes to the shoreline cause the original sand to flow out to sea.[2]
- Sandbags are used for protection against floods and gun fire. They can be easily transported when empty, then filled with local sand.
- Sand castle building is a popular activity. There are competitive sand castle building competitions (See sand art and play).
- Sand animation is a type of performance art and a technique for creating animated films.
- Aquaria are often lined with sand instead of gravel. This is a low cost alternative which some believe is better than gravel.
- Railroads use sand to improve the traction of wheels on the rails.
[edit] Hazards
While sand is generally harmless, one must take care with some activities involving sand such as sandblasting. Bags of silica sand used for sandblasting now carry labels warning the user to wear respiratory protection and avoid breathing the fine silica dust. There have been a number of lawsuits in recent years where workers have developed silicosis, a lung disease caused by inhalation of fine silica particles over long periods of time. Material safety data sheets (MSDS) for silica sand state that "excessive inhalation of crystalline silica is a serious health concern".[1]
In areas of high pore water pressure sand can partially liquefy to form quicksand. Quicksand, once dried, produces a considerable barrier to escape for creatures caught within, who often die from exposure as a result.
[edit] See also
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[edit] References
[edit] External links
(Source: Wikipedia) |