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Diatomaceous earth (/ˌdaɪ.ətəˈmeɪʃəs/DY-ə-tə-MAY-shəs), also known asdiatomite (/daɪˈætəmaɪt/dy-AT-ə-myte),celite, orkieselguhr, is a naturally occurring, soft,siliceoussedimentary rock that can be crumbled into a fine white to off-white powder. It has aparticle size ranging from more than 3 mm to less than 1 μm, but typically 10 to 200 μm.[1] Depending on thegranularity, this powder can have anabrasive feel, similar topumice powder, and has a lowdensity as a result of its highporosity. The typical chemical composition of oven-dried diatomaceous earth is 80–90%silica, with 2–4%alumina (attributed mostly toclay minerals), and 0.5–2%iron oxide.[2]
Diatomaceous earth consists of thefossilized remains ofdiatoms, a type of hard-shelledmicroalgae, that have accumulated over millions of years.[3] It is used as afiltration aid, mild abrasive in products including metal polishes andtoothpaste, mechanicalinsecticide,absorbent for liquids, matting agent for coatings, reinforcing filler in plastics and rubber, anti-block in plastic films, porous support for chemical catalysts,cat litter, activator incoagulation studies, a stabilizing component ofdynamite, athermal insulator, and a soil for potted plants and trees as in the art ofbonsai.[4][5] It is also used ingas chromatography packed columns made with glass or metal as stationary phase.
Diatomaceous earth consists of the fossilized remains of diatoms that accumulated over millions of years. It is usually composed of 80% to 90% silica, 2% to 4% alumina minerals, and 0.5% to 2% iron oxide, although the precise composition of every deposit is different.[3] Deposits may contain different amounts of silica depending on thesedimentation conditions, the presence of other sediments (clay, sand, volcanic ashes), and the age of the deposit (diagenesis,silica (SiO2) dissolution/precipitation, diatoms tests ageing). The species of diatom may also differ among deposits. The species of diatom is dependent upon the age andpaleoecology of the deposit. In turn, the shape of a diatom is determined by its species.
Many deposits throughoutBritish Columbia, such as Red Lake Earth, are from theMiocene epoch and contain a species of diatom known asMelosira granulata. These diatoms have a small globular shape. A deposit containing diatoms from this epoch can provide certain benefits over others. For example, diatoms from theEocene epoch are not as effective in their ability to absorb fluids because as older diatoms recrystallize, their small pores become filled with silica.[6]
| Part of a series related to |
| Biomineralization |
|---|
 |
Teeth, scales, tusks etc |
Diatomite forms by the accumulation of theamorphous silica (opal,SiO2·nH2O) remains of dead diatoms (microscopic single-celledalgae) inlake sediment ormarine sediments. Thefossil remains consist of a pair of symmetrical shells orfrustules.[2] Marine diatomites are found in association with a wide variety of other rock types but lacustrine diatomites are almost always associated with volcanic rock. Diatomaceouschert consists of diatomite that has beencemented with silica.[7]
Diatoms are able to extract silica from water that is less than 1% saturated in amorphous silica (saturation index (SI): -2). Their frustules remain undissolved because they are surrounded by an organic matrix. Clay minerals may also precipitate on the frustules and protect them from dissolution in sea water. When the diatom dies, the frustule is stripped of its organic layer and exposed to sea water. As a result, only 1% to 10% of frustules survive long enough to be buried under sediments and some of this is dissolved within the sediments. Only an estimated 0.05% to 0.15% of the original amount of silica produced by diatoms is preserved in the sedimentary record.[8]
In 1836 or 1837, German peasant Peter Kasten discovered diatomaceous earth (German:Kieselgur) when sinking a well on the northern slopes of theHaußelberg hill, onLüneburg Heath inNorth Germany.[9][10]
The extraction site on Lüneburg Heath was 1863–1994 Neuohe, while the storage sites were:
| Storage site | from | to |
|---|---|---|
| Wiechel | 1871 | 1978 |
| Hützel | 1876 | 1969 |
| Hösseringen | 1880 c. 1880 | 1894 |
| Hammerstorf | 1880 c. 1880 | 1920 |
| Oberohe | 1884 | 1970 |
| Schmarbeck | 1896 | 1925 c. 1925 |
| Steinbeck | 1897 | 1928 |
| Breloh | 1907 | 1975 |
| Schwindebeck | 1913 | 1973 |
| Hetendorf | 1970 | 1994 |
Thedeposits are up to 28 meters (92 ft) thick and are all of freshwater diatomaceous earth.[citation needed]
UntilWorld War I, almost the entire worldwide production of diatomaceous earth was from this region.[citation needed]
In Poland diatomaceous earth deposits are found in Jawornik, and are composed mostly of diatomaceous skeletons (frustules).[11]
In Germany, diatomaceous earth was also extracted atAltenschlirf[12] on theVogelsberg (Upper Hesse) and atKlieken[13] (Saxony-Anhalt).
There is a layer of diatomaceous earth more than 6 meters (20 ft) thick in the nature reserve ofSoos in the Czech Republic.[14]
Deposits on theIsle of Skye, off the west coast of Scotland, were mined until 1960.[15]
InColorado and inClark County, Nevada, United States, there are deposits that are up to several hundred meters thick in places. Marine deposits have been worked in theSisquoc Formation inSanta Barbara County, California nearLompoc and along theSouthern Californiacoast. This is the world's largest deposit of diatomite.[16] Additional marine deposits have been worked inMaryland,Virginia,Algeria and theMoClay of Denmark. Freshwater lake deposits occur in Nevada,Oregon,Washington andCalifornia. Lake deposits also occur ininterglacial lakes in the eastern United States, in Canada and in Europe in Germany, France, Denmark and the Czech Republic. The worldwide association of diatomite deposits andvolcanic deposits suggests that the availability of silica fromvolcanic ash may be necessary for thick diatomite deposits.[17]
Diatomaceous earth is sometimes found ondesert surfaces. Research has shown that the erosion of diatomaceous earth in such areas (such as theBodélé Depression in theSahara) is one of the most important sources of climate-affecting dust in the atmosphere.[18]
The siliceousfrustules ofdiatoms accumulate in fresh and brackish wetlands and lakes. Some peats and mucks contain a sufficient abundance of frustules such that they can be mined. Most of Florida's diatomaceous earths have been found in the muck of wetlands or lakes. The American Diatomite Corporation, from 1935 to 1946, refined a maximum of 145 tons per year from their processing plant nearClermont, Florida. Muck from several locations inLake County, Florida was dried and burned (calcined) to produce the diatomaceous earth.[19]
It was formerly extracted from LakeMývatn in Iceland.
The commercial deposits of diatomite are restricted toTertiary orQuaternary periods. Older deposits from as early as theCretaceous Period are known, but are of low quality.[17]
Diatomite deposits rich in fossils have been located in New Zealand, but mining of theFoulden Maar deposits on an industrial scale, for conversion to animal feed, has drawn strong opposition.[20]
Diatomaceous earth is available commercially in several formats:
In 1866,Alfred Nobel discovered thatnitroglycerin could be made more stable if absorbed in diatomite (kieselguhr in German).[21] This allowed safer transport and handling than pure nitroglycerin in liquid form. Nobel patented this mixture asdynamite in 1867; the mixture is also called guhr dynamite in reference to the kieselguhr.[22][23]
TheCelle engineer, Wilhelm Berkefeld, recognized the ability of the diatomaceous earth to filter and developed tubular filters (known as filter candles) fired from diatomaceous earth.[24] During thecholeraepidemic inHamburg in 1892, theseBerkefeld filters were used successfully.
One form of diatomaceous earth is used as afilter medium, especially for swimming pools. It has a high porosity because it is composed of microscopically small, hollow particles. Diatomaceous earth (sometimes referred to by trademarked brand names such as Celite) is used in chemistry as a filtration aid, to increase flow rate, and filter very fine particles that would otherwise pass through or clogfilter paper. It is also used to filter water, particularly in the drinking water treatment process and infish tanks, and other liquids, such as beer and wine. It can also filter syrups, sugar, and honey without removing or altering their color, taste, or nutritional properties.[25]
The oldest use of diatomite is as a very mild abrasive and has been used intoothpaste, metalpolishes, and some facial scrubs.
Diatomite is of value as aninsecticide because of its abrasive and physico-sorptive properties.[26] The fine powderadsorbslipids from the waxy outer layer of theexoskeletons of many species of insects; this layer acts as a barrier that resists the loss of water vapour from the insect's body. Damaging the layer increases the evaporation of water from their bodies, so that they dehydrate, often fatally.
This also works againstgastropods and is commonly employed in gardening to defeatslugs.[27] However, since slugs inhabit humid environments, efficacy is low. Diatomaceous earth is sometimes mixed with an attractant or other additives to increase its effectiveness.
The shape of the diatoms contained in a deposit has not been proven to affect their functionality when it comes to the adsorption of lipids; however, certain applications, such as that for slugs and snails, work best when a particularly shaped diatom is used, suggesting that lipid adsorption is not the only factor involved. For example, in the case of slugs and snails, large, spiny diatoms work best to lacerate the epithelium of the mollusk. Diatom shells will work to some degree on the vast majority of animals that undergoecdysis in sheddingcuticle, such asarthropods ornematodes. It also may have other effects onlophotrochozoans, such asmollusks orannelids.
Medical-grade diatomite has been studied for its efficacy as adeworming agent in cattle; in both studies cited the groups being treated with diatomaceous earth did not fare any better than control groups.[28][29] It is commonly used in lieu ofboric acid and can be used to help control and possibly eliminatebed bugs,[30]house dust mite,cockroach,ant, andflea infestations.[31]
Diatomaceous earth is widely applied for insect control in grain storage.[32] It is used to control cannibalistic behaviors inconfused flour beetles, which infest flour storages.
In order to be effective as an insecticide, diatomaceous earth must beuncalcinated (i.e., it must not be heat-treated prior to application)[33][better source needed] and have a mean particle size below about 12 μm (i.e., food grade—see below).[citation needed]
Although considered to be relatively low-risk, pesticides containing diatomaceous earth are not exempt from regulation in the United States under theFederal Insecticide, Fungicide, and Rodenticide Act and must be registered with theEnvironmental Protection Agency.[34]
Its thermal properties enable it to be used as the barrier material in some fire-resistant safes.[citation needed] It is also used in evacuated powder insulation for use with cryogenics.[35] Diatomaceous earth powder is inserted into the vacuum space to aid in the effectiveness of vacuum insulation. It was used in the classicalAGA cookers as a thermal heat barrier.[citation needed]
Diatomaceous earth also finds some use as asupport forcatalysts, generally serving to maximize a catalyst'ssurface area andactivity. For example,nickel can be supported on the material—the combination is called Ni–Kieselguhr—to improve its activity as ahydrogenation catalyst.[36]
Natural freshwater diatomaceous earth is used in agriculture for grain storage as ananticaking agent, as well as an insecticide.[37] It is approved by the U.S. Food and Drug Administration as afeed additive[38] to preventcaking.[39]
Some believe it may be used as a naturalanthelmintic (dewormer), although studies have not shown it to be effective.[28][29] Some farmers add it to their livestock andpoultry feed to prevent the caking of feed.[40] "Food-Grade Diatomaceous Earth" is widely available in agricultural feed supply stores.
Freshwater diatomite can be used as a growing medium inhydroponic gardens.
It is also used as a growing medium in potted plants, particularly asbonsai soil. Bonsai enthusiasts use it as a soil additive, or pot a bonsai tree in 100% diatomaceous earth. In vegetable gardening it is sometimes used as asoil conditioner, because likeperlite,vermiculite, andexpanded clay, it retains water and nutrients, while draining fast and freely, allowing high oxygen circulation within the growing medium.
Natural dried, not calcinated diatomaceous earth 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:
where:
Natural freshwater diatomaceous earth is preferred by many researchers over chromic oxide, which has been widely used for the same purpose, the latter being a known carcinogen and, therefore, a potential hazard to research personnel.
Spent diatomaceous earth from thebrewing process can be added to ceramic mass for the production of red bricks with higher open porosity.[41]
Diatomaceous earth is considered a prominent inorganic non-metallic material that can be used for the production of various ceramics, including production of porous ceramics under low temperature hydrothermal technology.[42]
Diatomaceous earth (DE) is used in some home products where dryness or the ability to wick away moisture is needed. In particular there are bath mats made of DE which absorb water from the bather and allow it to spread the material and rapidly evaporate. There are also spoons made of DE for scooping sugar and other hydroscopic kitchen ingredients.
Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by usinghydrolytic enzymes to break down the organic algal material.[43][44]
The Earth'sclimate is affected bydust in theatmosphere, so locating major sources of atmospheric dust is important forclimatology. Research indicates that surface deposits of diatomaceous earth play an important role. Research shows that significant dust comes from theBodélé Depression inChad, where storms push diatomite gravel overdunes, generating dust byabrasion.[45]
Inhalation ofcrystalline silica harms the lungs, causingsilicosis.Amorphous silica is considered to have low toxicity, but prolonged inhalation causes lung changes.[46] Diatomaceous earth is mainlyamorphous silica but contains some crystalline silica, especially in the saltwater forms.[47] In a 1978 study of workers, those exposed to natural diatomaceous earth for over five years had no significant lung changes while 40% of those exposed to the calcined form had developedpneumoconiosis.[48] Today's standard diatomaceous earth formulations are safer to use, as they are predominantly made of amorphous silica and contain little or no crystalline silica.[49]
The crystalline silica content of diatomaceous earth is regulated in the United States by theOccupational Safety and Health Administration (OSHA). There are guidelines from theNational Institute for Occupational Safety and Health that set maximum amounts allowable in the product (1%) and in the air near the breathing zone of workers, with arecommended exposure limit at 6 mg/m3 over an 8-hour workday.[49] OSHA has set apermissible exposure limit for diatomaceous earth as 20 mppcf (80 mg/m3/%SiO2). At levels of 3,000 mg/m3, diatomaceous earth is immediately dangerous to life and health.[50]
In the 1930s, workers with long-term occupational exposure in thecristobalite diatomaceous earth industry who were exposed to high levels of airborne crystalline silica over decades were found to have an increased risk ofsilicosis.[51]
Diatomite produced for pool filters is treated with high heat (calcination) and a fluxing agent (soda ash), causing the formerly harmless amorphous silicon dioxide to assume its crystalline form.[49]
