Magnesite

Magnesite

Magnesite from Bahia, Brazil (9.7 x 7.1 x 6 cm)
General
Category Carbonate mineral
Formula
(repeating unit)		 MgCO3
Strunz classification 5.AB.05
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group R3c
Identification
Color Colorless, white, pale yellow, pale brown, faintly pink, lilac-rose
Crystal habit Usually massive, rarely as rhombohedrons or hexagonal prisms
Cleavage [1011] perfect
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 3.5 - 4.5
Luster Vitreous
Streak white
Diaphaneity Transparent to translucent
Specific gravity 3.0 - 3.2
Optical properties Uniaxial (-)
Refractive index nω=1.508 - 1.510 nε=1.700
Birefringence 0.191
Fusibility infusible
Solubility Effervesces in hot HCl
Other characteristics May exhibit pale green to pale blue fluorescence and phosphorescence under UV; triboluminescent
References [1][2][3][4]

Magnesite is a mineral with the chemical formula MgCO3 (magnesium carbonate). Mixed crystals of iron(II) carbonate and magnesite (mixed crystals known as ankerite) possess a layered structure: monolayers of carbonate groups alternate with magnesium monolayers as well as iron(II) carbonate monolayers.[5] Manganese, cobalt and nickel may also occur in small amounts.

Occurrence

Magnesite occurs as veins in and an alteration product of ultramafic rocks, serpentinite and other magnesium rich rock types in both contact and regional metamorphic terrains. These magnesites often are cryptocrystalline and contain silica in the form of opal or chert.

Magnesite is also present within the regolith above ultramafic rocks as a secondary carbonate within soil and subsoil, where it is deposited as a consequence of dissolution of magnesium-bearing minerals by carbon dioxide within groundwaters.

Formation

Magnesite can be formed via talc carbonate metasomatism of peridotite and other ultramafic rocks. Magnesite is formed via carbonation of olivine in the presence of water and carbon dioxide at elevated temperatures and high pressures typical of the greenschist facies.

Magnesite can also be formed via the carbonation of magnesium serpentine (lizardite) via the following reaction:

2 Mg3Si2O5(OH)4 + 3 CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O.

However, when performing this reaction in the laboratory, the trihydrated form of magnesium carbonate (nesquehonite) will form at room temperature.[6] This very observation led to the postulation of a "dehydration barrier" being involved in the low-temperature formation of anhydrous magnesium carbonate.[7] Laboratory experiments with formamide, a liquid resembling water, have shown how no such dehydration barrier can be involved. The fundamental difficulty to nucleate anhydrous magnesium carbonate remains when using this non-aqueous solution. Not cation dehydration, but rather the spatial configuration of carbonate anions creates the barrier in the low-temperature nucleation of magnesite.[8]

Magnesite has been found in modern sediments, caves and soils. Its low-temperature (around 40 °C [104 °F]) formation is known to require alternations between precipitation and dissolution intervals.[9][10]

Magnesite in a natural form (from Lubeník in Slovakia)

Magnesite was detected in meteorite ALH84001 and on planet Mars itself. Magnesite was identified on Mars using infra-red spectroscopy from satellite orbit.[11] Controversy still exists over the temperature of formation of this magnesite. Low-temperature formation has been suggested for the magnesite from the Mars derived ALH84001 meteorite.[12][13] The low-temperature formation of magnesite might well be of significance toward large-scale carbon sequestration.[14]

Magnesium-rich olivine (forsterite) favors production of magnesite from peridotite. Iron-rich olivine (fayalite) favors production of magnetite-magnesite-silica compositions.

Magnesite can also be formed by way of metasomatism in skarn deposits, in dolomitic limestones, associated with wollastonite, periclase, and talc.

Uses

Dyed and polished magnesite beads

Similar to the production of lime, magnesite can be burned in the presence of charcoal to produce MgO, which, in the form of a mineral, is known as periclase. Large quantities of magnesite are burnt to make magnesium oxide: an important refractory material used as a lining in blast furnaces, kilns and incinerators. Calcination temperatures determine the reactivity of resulting oxide products and the classifications of light burnt and dead burnt refer to the surface area and resulting reactivity of the product, typically as determined by an industry metric of the iodine number. 'Light burnt' product generally refers to calcination commencing at 450 °C and proceeding to an upper limit of 900 °C - which results in good surface area and reactivity. Above 900 °C, the material loses its reactive crystalline structure and reverts to the chemically inert 'dead-burnt' product- which is preferred for use in refractory materials such as furnace linings.

Magnesite can also be used as a binder in flooring material. Furthermore, it is being used as a catalyst and filler in the production of synthetic rubber and in the preparation of magnesium chemicals and fertilizers.

In fire assay, magnesite cupels can be used for cupellation as the magnesite cupel will resist the high temperatures involved.

Magnesite can be cut, drilled, and polished to form beads that are used in jewelry-making. Magnesite beads can be dyed into a broad spectrum of bold colors, including a light blue color that mimics the appearance of turquoise.

Occupational safety and health

People can be exposed to magnesite in the workplace by inhaling it, skin contact, and eye contact.

United States

The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for magnesite exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday.[15]

References

  1. ↑ http://rruff.geo.arizona.edu/doclib/hom/magnesite.pdf Handbook of Mineralogy
  2. ↑ http://www.mindat.org/min-2482.html Mindat.org
  3. ↑ http://webmineral.com/data/Magnesite.shtml Webmineral data
  4. ↑ Klein, Cornelis and Cornelius S. Hurlbut, Jr., Manual of Mineralogy, Wiley, 20th ed., p. 332 ISBN 0-471-80580-7
  5. ↑ Beran, A. and Zemann, J. (1977): Refinement and comparison of the crystal structures of dolomite and of an Fe-rich ankerite. Tschermaks Mineralogische und Petrographische Mitteilungen, vol.24, pp.279-286.
  6. ↑ Leitmeier, H.(1916): Einige Bemerkungen über die Entstehung von Magnesit und Sideritlagerstätten, Mitteilungen der Geologischen Gesellschaft in Wien, vol.9, pp. 159–166.
  7. ↑ Lippmann, F. (1973): Sedimentary carbonate minerals. Springer Verlag, Berlin, 228 p.
  8. ↑ Xu, J; Yan, C.; Zhang, F.; Konishi, H., Xu, H. & Teng, H. H. (2013): Testing the cation-hydration effect on the crystallization of Ca - Mg- CO3 systems. Proc. Natl. Acad. Sci. US, vol.110 (44), pp.17750-17755.
  9. ↑ Deelman, J.C. (1999): "Low-temperature nucleation of magnesite and dolomite", Neues Jahrbuch für Mineralogie, Monatshefte, pp. 289–302.
  10. ↑ Alves dos Anjos et al. (2011): Synthesis of magnesite at low temperature. Carbonates and Evaporites, vol.26, pp.213-215.
  11. ↑ Ehlmann, B. L. et al. (2008): Orbital identification of carbonate-bearing rocks on Mars. Science, vol.322, no.5909, pp.1828-1832.
  12. ↑ McSween Jr, H. Y and Harvey, R. P.(1998): An evaporation model for formation of carbonates in the ALH84001 Martian meteorite. International Geology Review, vol.49, pp.774-783.
  13. ↑ Warren, P. H. (1998): Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite. Journal of Geophysical Research, vol.103, no.E7, 16759-16773.
  14. ↑ Oelkers, E. H.; Gislason, S. R. and Matter, J. (2008): Mineral carbonation of CO2. Elements, vol.4, pp.333-337.
  15. ↑ "CDC - NIOSH Pocket Guide to Chemical Hazards - Magnesite". www.cdc.gov. Retrieved 2015-11-19.
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