Dolomite

This article is about the mineral. For other uses, see Dolomite (disambiguation).
Dolomite

Dolomite (white) with magnesite (yellowish) from Spain
General
Category Carbonate minerals
Formula
(repeating unit)		 CaMg(CO3)2
Strunz classification 5.AB.10
Crystal system Trigonal
Crystal class Rhombohedral (3)
H-M symbol: (3)
Space group R3
Unit cell a = 4.8012(1), c = 16.002 [Å]; Z = 3
Identification
Color White, gray to pink
Crystal habit Tabular crystals, often with curved faces, also columnar, stalactitic, granular, massive.
Twinning Common as simple contact twins
Cleavage Perfect on {1011}, rhombohedral cleavage
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 3.5 to 4
Luster Vitreous to pearly
Streak White
Specific gravity 2.84–2.86
Optical properties Uniaxial (-)
Refractive index nω = 1.679–1.681 nε = 1.500
Birefringence δ = 0.179–0.181
Solubility Poorly soluble in dilute HCl
Other characteristics May fluoresce white to pink under UV; triboluminescent.
Ksp values vary between 1x10−19 to 1x10−17
References [1][2][3][4][5]

Dolomite (pronunciation: /ˈdɒləmaɪt/) is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally CaMg(CO3)2. The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dolomitic rock type is dolostone.

History

Most probably the mineral dolomite was first described by Carl Linnaeus in 1768.[6] In 1791, it was described as a rock by the French naturalist and geologist Déodat Gratet de Dolomieu (1750–1801), first in buildings of the old city of Rome, and later as samples collected in the mountains now known as the Dolomite Alps of northern Italy. Nicolas-Théodore de Saussure first named the mineral (after Dolomieu) in March 1792.[7]

Properties

The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, tan, gray, or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium and magnesium ions. It does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid as calcite does. Crystal twinning is common.

Solid solution exists between dolomite, the iron-dominant ankerite and the manganese-dominant kutnohorite.[8] 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. Lead, zinc, and cobalt also substitute in the structure for magnesium. The mineral dolomite is closely related to huntite Mg3Ca(CO3)4.

Because dolomite can be dissolved by slightly acidic water, areas of dolomite are important as aquifers and contribute to karst terrain formation.[9]

Formation

Modern dolomite formation has been found to occur under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. It is often thought that dolomite will develop only with the help of sulfate-reducing bacteria (e.g. Desulfovibrio brasiliensis).[10] However, low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter.[11]

Dolomite

Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999. Those laboratory experiments showed how the initial precipitation of a metastable "precursor" (such as magnesium calcite) will change gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and re-precipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "breaking Ostwald's step rule".[12]

There is some evidence for a biogenic occurrence of dolomite. One example is that of the formation of dolomite in the urinary bladder of a Dalmatian dog, possibly as the result of an illness or infection.[13]

Formation of dolomite from solution and its link with biogenic dolomite

In 2015, it was experimentally discovered that the direct crystallization of dolomite can occur from solution at temperatures between 60 and 220 °C. Dolomite forms through a three-stage process:

  1. A nanoparticulate magnesium-deficient, amorphous calcium carbonate (Mg-ACC) forms;
  2. After a temperature-dependent induction time, this Mg-ACC partially dehydrates and orders prior to its rapid (<5 minutes) crystallization to non-stoichiometric proto-dolomite (with a lower Mg/Ca ratio compared to dolomite). This occurs via the dissolution of Mg-ACC, followed by the secondary nucleation of proto-dolomite from solution.
  3. Finally, the proto-dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de novo and continuous formation of nanocrystalline proto-dolomite subunits that form spherical aggregates. In stage three of the reaction, the proto-dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale (hours to days), via an Ostwald-ripening mechanism. Such a three-stage crystallization can explain microbially induced proto-dolomites observed in modern hypersaline settings and may also be the route by which the Cryogenian cap dolomite deposits of the Neoproterozoic formed.[14]

Uses

Dolomite with chalcopyrite from the Tri-state district, Cherokee County, Kansas (size: 11.4×7.2×4.6 cm)

Dolomite is used as an ornamental stone, a concrete aggregate, and a source of magnesium oxide, as well as in the Pidgeon process for the production of magnesium. It is an important petroleum reservoir rock, and serves as the host rock for large strata-bound Mississippi Valley-Type (MVT) ore deposits of base metals such as lead, zinc, and copper. Where calcite limestone is uncommon or too costly, dolomite is sometimes used in its place as a flux for the smelting of iron and steel. Large quantities of processed dolomite are used in the production of float glass.

In horticulture, dolomite and dolomitic limestone are added to soils and soilless potting mixes as a pH buffer and as a magnesium source. Home and container gardening are common examples of this use.

Dolomite is also used as the substrate in marine (saltwater) aquariums to help buffer changes in pH of the water.

Calcined dolomite is also used as a catalyst for destruction of tar in the gasification of biomass at high temperature.[15]

Particle physics researchers like to build particle detectors under layers of dolomite to enable the detectors to detect the highest possible number of exotic particles. Because dolomite contains relatively minor quantities of radioactive materials, it can insulate against interference from cosmic rays without adding to background radiation levels.[16]

Dolomite is a popular choice for motorcycle speedway tracks throughout Australia and New Zealand.

Dolomite is used in the ceramic industry and in studio pottery as a glaze ingredient, contributing magnesium and calcium as glass melt fluxes.

See also

Wikimedia Commons has media related to Dolomite.

References

  1. ↑ 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.
  2. ↑ Dolomite. Handbook of Mineralogy. (PDF) . Retrieved on 2011-10-10.
  3. ↑ Dolomite. Webmineral. Retrieved on 2011-10-10.
  4. ↑ Dolomite. Mindat.org. Retrieved on 2011-10-10.
  5. ↑ Krauskopf, Konrad Bates; Bird, Dennis K. (1995). Introduction to geochemistry (3rd ed.). Newyork: McGraw-Hill. ISBN 9780070358201.
  6. ↑ On p.41 of part 3 of his book "Systema naturae per regna tria naturae etc." (1768), Linnaeus stated: "Marmor tardum - Marmor paticulis subimpalpabilibus album diaphanum. Hoc simile quartzo durum, distinctum quod cum aqua forti non, nisi post aliquot minuta & fero, effervescens." In translation: "Slow marble - Marble, white and transparent with barely discernable particles. This is as hard as quartz, but it is different in that it does not, unless after a few minutes, effervesce with "aqua forti"".
  7. ↑ Saussure le fils, M. de (1792): Analyse de la dolomie. Journal de Physique, vol.40, pp.161-173.
  8. ↑ Klein, Cornelis and Cornelius S. Hurlbut Jr., Manual of Mineralogy, Wiley, 20th ed., p. 339-340 ISBN 0-471-80580-7
  9. ↑ Kaufmann, James. Sinkholes. USGS Fact Sheet. Retrieved on 2013-9-10.
  10. ↑ Vasconcelos C.; McKenzie J. A.; Bernasconi S.; Grujic D.; Tien A. J. (1995). "Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures". Nature. 337 (6546): 220–222. Bibcode:1995Natur.377..220V. doi:10.1038/377220a0.
  11. ↑ Roberts, J. A.; Kenward, P. A.; Fowle, D. A.; Goldstein, R. H.; Gonzalez, L. A. & Moore, D. S. (1980). "Surface chemistry allows for abiotic precipitation of dolomite at low temperature". Proceedings of the National Academy of Sciences of the United States of America. 110 (36): 14540–5. Bibcode:2013PNAS..11014540R. doi:10.1073/pnas.1305403110. PMC 3767548Freely accessible. PMID 23964124.
  12. ↑ Deelman, J.C. (1999): "Low-temperature nucleation of magnesite and dolomite", Neues Jahrbuch für Mineralogie, Monatshefte, pp. 289–302.
  13. ↑ Mansfield, Charles F. (1980). "A urolith of biogenic dolomite – another clue in the dolomite mystery". Geochimica et Cosmochimica Acta. 44 (6): 829–839. Bibcode:1980GeCoA..44..829M. doi:10.1016/0016-7037(80)90264-1.
  14. ↑ Rodriguez-Blanco, J.D., Shaw, S. and Benning, L.G. (2015) A route for the direct crystallization of dolomite. American Mineralogist, 100, 1172-1181. doi: 10.2138/am-2015-4963
  15. ↑ A Review of the Literature on Catalytic Biomass Tar Destruction National Renewable Energy Laboratory.
  16. ↑ Short Sharp Science: Particle quest: Hunting for Italian WIMPs underground. Newscientist.com (2011-09-05). Retrieved on 2011-10-10.
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