Dolomite (mineral)

Dolomite (/ˈdɒl.əˌmt, ˈd.lə-/) 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.

Dolomite
Dolomite Luzenac.jpg
Dolomite (white) on talc
General
CategoryCarbonate minerals
Formula
(repeating unit)
CaMg(CO3)2
Strunz classification5.AB.10
Crystal systemTrigonal
Crystal classRhombohedral (3)
H-M symbol: (3)
Space groupR3
Unit cella = 4.8012(1), c = 16.002 [Å]; Z = 3
Identification
ColorWhite, grey to pink, reddish-white, brownish-white; colourless in transmitted light
Crystal habitTabular crystals, often with curved faces, also columnar, stalactitic, granular, massive.
TwinningCommon as simple contact twins
Cleavage3 directions of cleavage not at right angles
FractureConchoidal
TenacityBrittle
Mohs scale hardness3.5 to 4
LusterVitreous to pearly
StreakWhite
Specific gravity2.84–2.86
Optical propertiesUniaxial (-)
Refractive indexnω = 1.679–1.681 nε = 1.500
Birefringenceδ = 0.179–0.181
SolubilityPoorly soluble in dilute HCl
Other characteristicsMay fluoresce white to pink under UV; triboluminescent.
Ksp values vary between 1x10−19 to 1x10−17
References[1][2][3][4][5]
Dolomite and calcite look similar under a microscope, but thin sections can be etched and stained in order to identify the minerals. Photomicrograph of a thin section in cross and plane polarised light: the brighter mineral grains in the picture are dolomite, and the darker grains are calcite.

HistoryEdit

 
Cristallo in the Dolomites mountain range near Cortina d'Ampezzo, Italy. The Dolomite Mountains were named after the mineral.

As stated by Nicolas-Théodore de Saussure[6] the mineral dolomite was probably first described by Carl Linnaeus in 1768.[7] 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.

PropertiesEdit

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. Unless it is in fine powder form, it does not rapidly dissolve or effervesce (fizz) in cold dilute hydrochloric acid as calcite does.[8] Crystal twinning is common.

Solid solution exists between dolomite, the iron-dominant ankerite and the manganese-dominant kutnohorite.[9] 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 where dolomite is an abundant rock-forming mineral are important as aquifers and contribute to karst terrain formation.[10]

FormationEdit

Modern dolomite formation has been found to occur under anaerobic conditions in supersaturated saline lagoons such as those at the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. There are many other localities where modern dolomite forms, notably along sabkhas in the Persian Gulf,[11] but also in sedimentary basins bearing gas hydrates[12] and hypersaline lakes.[13] It is often thought that dolomite nucleates with the help of sulfate-reducing bacteria (e.g. Desulfovibrio brasiliensis),[14] but other microbial metabolisms have been also found to mediate in dolomite formation.[11] In general, low-temperature dolomite may occur in natural supersaturated environments rich in extracelullar polymeric substances (EPS) and microbial cell surfaces.[11] This is likely result from complexation of both magnesium and calcium by carboxylic acids comprising EPS.[15]

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 are yet to be performed. Usually, the initial inorganic precipitation of a metastable "precursor" (such as magnesium calcite) can easily be achieved. The precursor phase will theoretically change gradually into a more stable phase (such as partially ordered dolomite) 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".[16] High diagenetic temperatures, such as those of groundwater flowing along deeply rooted fault systems affecting some sedimentary successions or deeply buried limestone rocks allocate dolomitization.[17] But the mineral is also volumetrically important in some Neogene platforms never subjected to elevated temperatures. Under such conditions of diagenesis the long-term activity of the deep biosphere could play a key role in dolomitization, since diagenetic fluids of contrasting composition are mixed as a response to Milankovitch cycles.[18]

A recent biotic synthetic experiment claims to have precipitated ordered dolomite when anoxygenic photosynthesis proceeds in the presence of manganese(II).[19] A still perplexing example of an organogenic origin is that of the reported formation of dolomite in the urinary bladder of a Dalmatian dog, possibly as the result of an illness or infection.[20]

UsesEdit

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.

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

Calcined dolomite is also used as a catalyst for destruction of tar in the gasification of biomass at high temperature.[21] 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.[22]

In addition to being an industrial mineral, dolomite is highly valued by collectors and museums when it forms large, transparent crystals. The specimens that appear in the magnesite quarry exploited in Eugui, Esteribar, Navarra (Spain) are considered among the best in the world.[23]

See alsoEdit

ReferencesEdit

  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 Archived 2008-04-09 at the Wayback Machine. Handbook of Mineralogy. (PDF) . Retrieved on 2011-10-10.
  3. ^ Dolomite Archived 2005-08-27 at the Wayback Machine. Webmineral. Retrieved on 2011-10-10.
  4. ^ Dolomite Archived 2015-11-18 at the Wayback Machine. Mindat.org. Retrieved on 2011-10-10.
  5. ^ Krauskopf, Konrad Bates; Bird, Dennis K. (1995). Introduction to geochemistry (3rd ed.). New York: McGraw-Hill. ISBN 9780070358201. Archived from the original on 2017-02-26.
  6. ^ Saussure le fils, M. de (1792): Analyse de la dolomie. Journal de Physique, vol.40, pp.161-173.
  7. ^ Linnaeus, C. (1768): Systema naturae per regnum tria naturae, secundum classes, ordines, genera, species cum characteribus & differentiis. Tomus III. Laurentii Salvii, Holmiae, 236 p. On p.41 of this very book, Linnaeus stated (in Latin): "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"".
  8. ^ "Dolomite Mineral - Uses and Properties". geology.com.
  9. ^ Klein, Cornelis and Cornelius S. Hurlbut Jr., Manual of Mineralogy, Wiley, 20th ed., p. 339-340 ISBN 0-471-80580-7
  10. ^ Kaufmann, James. Sinkholes Archived 2013-06-04 at the Wayback Machine. USGS Fact Sheet. Retrieved on 2013-9-10.
  11. ^ a b c Petrash, Daniel A.; Bialik, Or M.; Bontognali, Tomaso R.R.; Vasconcelos, Crisógono; Roberts, Jennifer A.; McKenzie, Judith A.; Konhauser, Kurt O. (2017-08-01). "Microbially catalyzed dolomite formation: From near-surface to burial". Earth-Science Reviews. 171: 558–582. doi:10.1016/j.earscirev.2017.06.015. ISSN 0012-8252.
  12. ^ Snyder, Glen T.; Matsumoto, Ryo; Suzuki, Yohey; Kouduka, Mariko; Kakizaki, Yoshihiro; Zhang, Naizhong; Tomaru, Hitoshi; Sano, Yuji; Takahata, Naoto; Tanaka, Kentaro; Bowden, Stephen A. (2020-02-05). "Evidence in the Japan Sea of microdolomite mineralization within gas hydrate microbiomes". Scientific Reports. 10 (1): 1876. doi:10.1038/s41598-020-58723-y. ISSN 2045-2322. PMC 7002378. PMID 32024862.
  13. ^ Last, William M. (1990-05-01). "Lacustrine dolomite—an overview of modern, Holocene, and Pleistocene occurrences". Earth-Science Reviews. 27 (3): 221–263. doi:10.1016/0012-8252(90)90004-F. ISSN 0012-8252.
  14. ^ 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. S2CID 4371495.
  15. ^ 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 3767548. PMID 23964124.
  16. ^ Deelman, J.C. (1999): "Low-temperature nucleation of magnesite and dolomite" Archived 2008-04-09 at the Wayback Machine, Neues Jahrbuch für Mineralogie, Monatshefte, pp. 289–302.
  17. ^ Warren, J. (2000-11-01). "Dolomite: occurrence, evolution and economically important associations". Earth-Science Reviews. 52 (1–3): 1–81. doi:10.1016/S0012-8252(00)00022-2. ISSN 0012-8252.
  18. ^ Petrash, Daniel A.; Bialik, Or M.; Staudigel, Philip T.; Konhauser, Kurt O.; Budd, David A. (2021). "Biogeochemical reappraisal of the freshwater–seawater mixing-zone diagenetic model". Sedimentology. 68 (5): 1797–1830. doi:10.1111/sed.12849. ISSN 1365-3091. S2CID 234012426.
  19. ^ Daye, Mirna; Higgins, John; Bosak, Tanja (2019-06-01). "Formation of ordered dolomite in anaerobic photosynthetic biofilms". Geology. 47 (6): 509–512. doi:10.1130/G45821.1. hdl:1721.1/126802. ISSN 0091-7613.
  20. ^ 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.
  21. ^ A Review of the Literature on Catalytic Biomass Tar Destruction Archived 2015-02-04 at the Wayback Machine National Renewable Energy Laboratory.
  22. ^ Short Sharp Science: Particle quest: Hunting for Italian WIMPs underground Archived 2017-05-17 at the Wayback Machine. Newscientist.com (2011-09-05). Retrieved on 2011-10-10.
  23. ^ Calvo M.; Sevillano, E. (1991). "The Eugui quarries, Navarra, Spain". The Mineralogical Record. 22: 137–142.

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