Dolomite 102519 222883488 2008-07-01T16:04:20Z 190.60.90.70 {{Refimprove|date=August 2007}}{{other uses}} {{Infobox mineral | name = Dolomite | category = [[Carbonate mineral]] | boxwidth = | boxbgcolor = | image = Dolomite Morocco.jpg | imagesize = | caption = | formula = [[calcium|Ca]][[magnesium|Mg]]([[carbon|C]][[oxygen|O]]<sub>3</sub>)<sub>2</sub> | molweight = | color = white, gray to pink | habit = tabular crystals, often with curved faces, also columnar, stalactitic, granular, massive. | system = [[trigonal|trigonal - rhombohedral, bar3]] | twinning = common as simple contact twins | cleavage = rhombohedral cleavage (3 planes) | fracture = brittle - conchoidal | mohs = 3.5 to 4 | luster = vitreous to pearly | refractive = nω = 1.679–1.681 nε = 1.500 | opticalprop = Uniaxial (-) | birefringence = δ = 0.179–0.181 | pleochroism = | streak = white | gravity = 2.84–2.86 | density = | melt = | fusibility = | diagnostic = | solubility = Poorly soluble in dilute [[Hydrochloric acid|HCl]] unless powdered. | diaphaneity = | other = May fluoresce white to pink under UV; triboluminescent. | references = <ref>Deer, W. A., R. A. Howie and J. Zussman (1966) ''An Introduction to the Rock Forming Minerals'', Longman, pp. 489–493. ISBN 0-582-44210-9.</ref><ref>http://rruff.geo.arizona.edu/doclib/hom/dolomite.pdf Handbook of Mineralogy</ref><ref>http://webmineral.com/data/Dolomite.shtml Webmineral</ref><ref>http://www.mindat.org/min-1304.html Mindat data</ref> }} [[Image:Dolomit z brochantytem, Maroko.JPG|thumb|right|250px|Dolomite from [[Morocco]].]] [[Image:Dolomit, Rumunia; pseudomorfoza po kalcycie.jpg|thumb|right|250px|Dolomite.]]<!--LEAVE THESE HERE TO AVOID CREATION OF BLANK LINES--> '''Dolomite''' ({{pronEng|ˈdɒləmaɪt}}) is the name of a [[sedimentary]] [[carbonate rock]] and a [[mineral]], both composed of [[calcium]] [[magnesium]] [[carbonate]] [[calcium|Ca]][[magnesium|Mg]]([[carbon|C]][[oxygen|O]]<sub>3</sub>)<sub>2</sub> found in [[crystal]]s. Dolomite rock (also [[dolostone]]) is composed predominantly of the mineral dolomite. [[Limestone]] that is partially replaced by dolomite is referred to as dolomitic limestone, or in old U.S. geologic literature as ''magnesian limestone''. Dolomite was first described in 1791 as the rock by the [[French people|French]] [[natural history|naturalist]] and [[geologist]], [[Déodat Gratet de Dolomieu]] (1750–1801) for exposures in the [[Dolomite Alps]] of northern [[Italy]]. ==Properties== The mineral dolomite crystallizes in the [[trigonal|trigonal-rhombohedral]] system. It forms white, gray to pink, commonly curved crystals, although it is usually massive. It has physical properties similar to those of the mineral [[calcite]], but does not rapidly dissolve or effervesce (fizz) in dilute [[hydrochloric acid]] unless it is scratched or in powdered form. The [[Mohs hardness]] is 3.5 to 4 and the [[specific gravity]] is 2.85. [[Refractive index]] values are nω = 1.679 - 1.681 and nε = 1.500. [[Crystal twinning]] is common. A solid solution series exists between dolomite and [[iron]] rich [[ankerite]]. Small amounts of iron in the structure give the crystals a yellow to brown tint. [[Manganese]] substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color noted in the image above. A series with the manganese rich [[kutnohorite]] may exist. [[Lead]] and [[zinc]] also substitute in the structure for magnesium. ==Formation== [[Image:dolomite09.jpg|thumb|left|250px|Pale pink crystals of dolomite with a slight pearly luster.]] Vast deposits are present in the geological record, but the mineral is relatively rare in modern environments. However, laboratory synthesis of stoichiometric dolomite has been carried out only at temperatures of greater than 100 degrees Celsius, conditions typical of burial in sedimentary basins—even though much dolomite in the rock record appears to have formed in low-temperature conditions. The high temperature is likely to speed up the movement of calcium and magnesium ions so that they can find their places in the ordered structure within a reasonable amount of time. This suggests that the lack of dolomite that is being formed today is likely due to kinematic factors. Modern dolomite does occur as a precipitating mineral in specialized environments on the surface of the earth today. In the 1950s and 60s, dolomite was found to be forming in highly saline lakes in the Coorong region of [[South Australia]]. Dolomite crystals also occur in deep-sea sediments, where organic matter content is high. This dolomite is termed "organogenic" dolomite. Recent research has found modern dolomite formation under [[Hypoxia (environmental)|anaerobic]] conditions in [[Supersaturation|supersaturated]] saline [[lagoon]]s along the [[Rio de Janeiro]] coast of [[Brazil]], namely, Lagoa Vermelha and Brejo do Espinho. One interesting reported case was the formation of dolomite in the kidneys of a [[Dalmatian (dog)|Dalmatian]] dog. This was believed to be due to chemical processes triggered by bacteria. Dolomite has been speculated to develop under these conditions with the help of [[sulfate-reducing bacteria]]. This joins other research in pointing out many new interesting links between large-scale [[geology]] and small-scale [[microbiology]] (see [[geomicrobiology]]). The actual role of bacteria in the low-temperature formation of dolomite remains to be demonstrated. The specific mechanism of dolomitization, involving sulfate-reducing bacteria, has not yet been demonstrated.<ref>http://www.the-conference.com/JConfAbs/5/1038.pdf Role of Sulfate Reducing Bacteria During Microbial Dolomite Precipitation as Deduced from Culture Experiments</ref> Dolomite appears to form in many different types of environment and can have varying structural, textural and chemical characteristics. Some researchers have stated "there are dolomites and dolomites", meaning that there may not be one single mechanism by which dolomite can form. Much modern dolomite differs significantly from the bulk of the dolomite found in the rock record, leading researchers to speculate that environments where dolomite formed in the geologic past differ significantly from those where it forms today. Reproducible laboratory syntheses of dolomite (and [[magnesite]]) leads first to the initial precipitation of a metastable "precursor" (such as magnesium calcite), to be changed gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and reprecipitation. The general principle governing the course of this irreversible [[geochemistry|geochemical]] reaction has been coined [[Ostwald's step rule]]. For a very long time scientists had difficulties synthesizing dolomite. However, in a 1999 study, through a processes of dissolution alternating with the intervals of precipitation measurable levels of dolomite were synthesized at low temperatures and pressures.<ref>Deelman, J.C. (1999): [http://www.jcdeelman.demon.nl/dolomite/files/NeuesJahrbuchMineral.pdf "Low-temperature nucleation of magnesite and dolomite"], ''Neues Jahrbuch für Mineralogie'', Monatshefte, Jg.1999, pp.289–302.</ref> ==Uses== Dolomite is used as an [[ornamental stone]], a concrete aggregate and as a source of [[magnesium oxide]]. It is an important [[petroleum]] [[oil reservoir|reservoir]] rock, and serves as the host rock for large strata-bound [[Mississippi Valley-Type]] (MVT) [[ore]] deposits of [[base metal]]s (that is, readily oxidized metals) such as [[lead]], [[zinc]], and [[copper]]. Where [[calcite]] [[limestone]] is uncommon or too costly, dolomite is sometime used in its place as a [[flux (metallurgy)|flux]] (impurity remover) for the [[smelting]] of iron and steel. Large quantities of processed dolomite are used in the production of float glass (flat glass) In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes to lower their acidity ("sweeten" them). Home and container gardening are common examples of this use. ===As nutritional supplement=== In nutrition, dolomite is sold sometimes as a [[dietary supplement]] on the assumption that it should make a good simultaneous source of the two important elemental nutrients [[calcium]] and [[magnesium]]. However, since dolomites from [[Mississippi Valley-Type]] ore regions such as the Old [[Lead Belt]] and New Lead Belt in southeastern [[Missouri]] [[United States]] often include significant levels of [[lead]] and other toxic elements,{{Fact|date=January 2008}} users should always verify that such dolomite supplements are from non-ore regions before ingesting them. Further, laboratory experiments conducted at the [[University of Alberta]] demonstrate that dolomite is practically insoluble in stomach acid and is eliminated from the body before significant magnesium or calcium can be absorbed.{{Fact|date=January 2008}} A far safer strategy is to avoid using dolomite as a supplement altogether, and instead taking equivalent amounts of [[milk of magnesia]] and [[calcium]] supplements. The chemical processes used to create such individual supplements effectively eliminate the risk of ingesting the toxic metals often associated with raw dolomite. ==See also== * [[List of minerals]] * [[Evaporite]] ==References== {{reflist}} [[Category:Sedimentary rocks]] [[Category:Calcium minerals]] [[Category:Magnesium minerals]] [[Category:Carbonate minerals]] [[Category:Limestone]] [[Category:Dolomite group]] [[bg:Доломит]] [[cs:Dolomit]] [[de:Dolomit (Mineral)]] [[et:Dolomiit]] [[es:Dolomita]] [[eo:Dolomito]] [[eu:Dolomita]] [[fr:Dolomite]] [[hr:Dolomit]] [[id:Dolomit]] [[it:Dolomite]] [[he:דולומיט (מינרל)]] [[lt:Dolomitas]] [[hu:Dolomit]] [[ja:ドロマイト]] [[no:Dolomitt]] [[pt:Dolomita]] [[ro:Dolomit (mineral)]] [[ru:Доломит]] [[sk:Dolomit (minerál)]] [[sr:Доломит]] [[sh:Dolomit]] [[fi:Dolomiitti]] [[sv:Dolomit]] [[uk:Доломіт]] [[zh:白云石]]