jtwsr
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Geology and occurrence
Diatomite forms by the accumulation of the amorphous silica (opal, SiO2·nH2O) remains of dead diatoms (microscopic single-celled algae) in lacustrine or marine sediments. The fossil remains consist of a pair of symmetrical shells or frustules.[1]
The commercial deposits of diatomite are restricted to Tertiary or Quaternary Periods. Older deposits from as early as the Cretaceous Period are known but are of low quality.[2] Marine deposits have been worked in the Lompoc area of Santa Barbara County, California and along the southern California coast. Additional marine deposits have been worked in Maryland, Virginia, Algeria and the Molar-earths of Denmark. Fresh water lake deposits in Nevada, Oregon, Washington and California. Lake deposits also occur in interglacial lakes in the eastern US and Canada and in Europe in Germany, France, Denmark and the Czech Republic. The worldwide association of diatomite deposits and volcanic deposits suggests that the availability of silica from volcanic ash may be needed for thick diatomite deposits.[2]
[edit] Applications
[edit] Industrial
In 1866, Alfred Nobel discovered that nitroglycerin could be made much more stable if absorbed in diatomite. This allows much safer transport and handling than nitroglycerin in its raw form. He patented this mixture as dynamite in 1867, and the mixture is also referred to as guhr dynamite.
[edit] Filtration
Individual diatom cell walls often maintain their shape even in commercially processed filter media, such as this one for swimming poolsThe most common use (68%)[citation needed] of diatomaceous earth is as a filter medium, especially for swimming pools. It has a high porosity, because it is composed of microscopically-small, coffin-like, hollow particles. It is used in chemistry under the brand name Celatom or Celite as a filtration aid, to filter very fine particles that would otherwise pass through or clog filter paper. It is also used to filter water, particularly in the drinking water treatment process and in fish tanks, and other liquids, such as beer and wine. It can also filter syrups and sugar. Other industries such as paper, paints, ceramics, soap and detergents use it as a fulling material.
[edit] Abrasive
Live marine diatoms from Antarctica (magnified)The oldest use of diatomite is as a very mild abrasive and, for this purpose, it has been used both in toothpaste and in metal polishes, as well as in some facial scrubs.
[edit] Pest control
Diatomite is also used as an insecticide, due to its physico-sorptive properties. The fine powder absorbs lipids from the waxy outer layer of insects' exoskeletons, causing them to dehydrate. Arthropods die as a result of the water pressure deficiency, based on Fick's law of diffusion. This also works against gastropods and is commonly employed in gardening to defeat slugs. However, since slugs inhabit humid environments, efficacy is very low. It is sometimes mixed with an attractant or other additives to increase its effectiveness. Medical-grade diatomite is sometimes used to de-worm both animals and humans. It is most commonly used in lieu of boric acid, and can be used to help control and eventually eliminate a cockroach infestation. This material has wide application for insect control in grain storage.[3]
[edit] Absorbent
Its absorbent qualities make it useful for spill clean-up and the U.S. Centers for Disease Control recommends it to clean up toxic liquid spills. These qualities also lend themselves to use in facial masks to absorb excess oils.
It has been employed as a primary ingredient in a type of cat litter. The type of silica used in cat litter comes from freshwater sources and does not pose a significant health risk to pets or humans.
[edit] Thermal
Its thermal properties enable it to be used as the barrier material in some fire resistant safes.
[edit] DNA purification
Diatomite (Celite) can be used for the removal of DNA in the presence of a highly concentrated chaotropic agent such as sodium iodide, guanidinium hydrochloride and guanidinium thiocyanate. As with other silicates, the diatomites will remove double stranded DNA but not RNA or proteins. The DNA can be extracted from the diatomites using low ionic strength buffers, including water, at neutral to slightly alkaline pH. Crude diatomites of a uniform size must first be washed in a heated acid such as 5M HCl[4]. Calcination can further improve consistency of the material, while mild caustic treatment may improve adsorption with lower levels of chaotrophs.
[edit] Use in agriculture
Natural freshwater diatomaceous earth is used in agriculture for grain storage as an anti-caking agent, as well as insecticide.[5] It is approved by the US Department of Agriculture as a feed supplement.
It is also used as a neutral anthelmentic (de-wormer). Some farmers add it to their livestock and poultry feed to improve the health of animals.[6] "Food Grade Diatomaceous Earth" is widely available in agricultural feed supply stores. It is acceptable as organic feed additive for livestock.
[edit] Hydroponics
Freshwater diatomite can be used as a growing medium in hydroponic gardens.
It is also used as a growing medium in potted plants, particularly as bonsai soil. Bonsai enthusiasts use it as a soil additive, or pot a bonsai tree in 100% Diatomaceous earth. Like perlite, vermiculite, and expanded clay, it retains water and nutrients while draining fast and freely, allowing high oxygen circulation within the growing medium.
[edit] Marker in livestock nutrition experiments
Natural diatomaceous earth (dried, not calcined) is regularly used in livestock nutrition research as a source of acid insoluble ash (AIA), which is used as an indigestible marker. By measuring the content of AIA relative to nutrients in test diets and feces or digesta sampled from the terminal ileum (last third of the small intestine) the percentage of that nutrient digested can be calculated using the following equation:
% Nutrient Digestibility = (1 - (% Nutrientfeces/digesta / % Nutrientfeed) X (% AIAfeed / % AIAfeces/digesta)) X 100
Natural diatomaceous earth (freshwater) is preferred by many researchers over chromic oxide, which has been widely used for the same purpose, but which is also a known carcinogen and therefore a potential hazard to research personnel.
[edit] Specific varieties
Tripolite is the variety found in Tripoli, Libya.
Bann clay is the variety found in the Lower Bann valley in Northern Ireland.
Moler (Mo-clay) is the variety found in northwestern Denmark, especially on the islands of Fur and Mors.
Perma-Guard is the type most often used in US agriculture for grain storage, and as feed supplement.
[edit] Climatologic importance
The Earth's climate is affected by dust in the atmosphere, so locating major sources of atmospheric dust is important for climatology. Recent research indicates that surface deposits of diatomaceous earth play an important role. For instance, the largest single atmospheric dust source is the Bodélé depression in Chad, where storms push diatomite gravel over dunes, generating dust by abrasion.[7]
[edit] Safety considerations
The absorbent qualities of diatomite can result in a significant drying of the hands if handled without gloves. The flux-calcined form contains a highly crystalline form of silica, resulting in sharp edges. The sharpness of this version of the material makes it dangerous to breathe and a dust mask is recommended when working with it.
The type of hazard posed by inhalation depends on the form of the silica. Crystalline silica poses a serious inhalation hazard because it can cause silicosis. Amorphous silica can cause dusty lungs, but does not carry the same degree of risk as crystalline silica. Natural or dried diatomite generally contains very low percentages of crystalline silica. Diatomite produced for pool filters is treated with high heat (calcining) and a fluxing agent (soda ash), causing the formerly amorphous silicon dioxide to assume its crystalline form.
In the United States, the crystalline silica content in the dusts is regulated by the Occupational Safety and Health Administration (OSHA), and there are guidelines for the maximum amounts allowable in the product and in the air near the breathing zone of workers.[8]
[edit] See also
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Jeff Wilson
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01-21-2010 12:45 AM
