Sand
From Wikipedia, the free encyclopedia
Categories: Articles needing additional references from May 2007 | Granular materials | Soil | Sediments | Sedimentology
|
For other uses, see Sand (disambiguation).
Image:Sand dunes 2.jpg
Sand dunes in the desert
Image:Third beach sand.jpg
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 Φ sediment size scale, where size in Φ = -log base 2 of size in mm. On this scale, for sand the value of Φ varies from -1 to +4, with the divisions between sub-categories at whole numbers.
Constituents of sand
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. Study of sandStudy 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. Uses of sandImage:Sanden ICE3.jpg
At 300 km/h, an ICE 3 (DB class 403) releases sand from several bogies to the rails.
Image:Sand sorting tower.jpg
Sand sorting tower at a gravel extraction pit.
|
