Magnesite

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Magnesite
Magnesite.jpg
General
CategoryCarbonate mineral
Formula
(repeating unit)
MgCO3
Strunz classification05.AB.05
Identification
ColorColorless, white, pale yellow, pale brown, faintly pink, lilac-rose
Crystal habitUsually massive, rarely as rhombohedrons or hexagonal prisms
Crystal systemTrigonal - Hexagonal Scalenohedral H-M Symbol 32/m Space Group: R3c
Cleavage[1011] perfect
FractureConchoidal
TenacityBrittle
Mohs scale hardness3.5 - 4.5
LusterVitreous
Streakwhite
DiaphaneityTransparent to translucent
Specific gravity3.0 - 3.2
Optical propertiesUniaxial (-)
Refractive indexnω=1.508 - 1.510 nε=1.700
Birefringence0.191
Fusibilityinfusible
SolubilityEffervesces in hot HCl
Other characteristicsMay exhibit pale green to pale blue fluorescence and phosphorescence under UV; triboluminescent
References[1][2][3][4]
 
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Magnesite
Magnesite.jpg
General
CategoryCarbonate mineral
Formula
(repeating unit)
MgCO3
Strunz classification05.AB.05
Identification
ColorColorless, white, pale yellow, pale brown, faintly pink, lilac-rose
Crystal habitUsually massive, rarely as rhombohedrons or hexagonal prisms
Crystal systemTrigonal - Hexagonal Scalenohedral H-M Symbol 32/m Space Group: R3c
Cleavage[1011] perfect
FractureConchoidal
TenacityBrittle
Mohs scale hardness3.5 - 4.5
LusterVitreous
Streakwhite
DiaphaneityTransparent to translucent
Specific gravity3.0 - 3.2
Optical propertiesUniaxial (-)
Refractive indexnω=1.508 - 1.510 nε=1.700
Birefringence0.191
Fusibilityinfusible
SolubilityEffervesces in hot HCl
Other characteristicsMay 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). It occupies one end of a solid solution series with siderite (FeCO3), as the iron ion Fe2+ substitutes for the magnesium ion Mg2+. Calcium, manganese, cobalt and nickel may also occur in small amounts.

Occurrence[edit]

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[edit]

Magnesite can be formed via talc carbonate metasomatism of peridotite and other ultrabasic 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:
serpentine + carbon dioxide → talc + magnesite + water

2 Mg3 Si2O5(OH)4 + 3 CO2 → Mg3Si4O10(OH)2 + 3 MgCO3 + H2O.

However when performing this reaction in the laboratory, the trihydrated form of magnesium carbonate (nesquehonite) will form at room temperature. This very observation led to the postulation of a "dehydration barrier" being involved in the low-temperature formation of anhydrous magnesium carbonate.[5]

Magnesite has been found in modern sediments, caves and soils. Its low-temperature (around 40 °C) formation is known to require alternations between precipitation and dissolution intervals.[6][7]

Magnesite was detected in meteorite ALH84001 and on planet Mars itself. Magnesite was identified on Mars using infra-red spectroscopy from satellite orbit.[8] 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.[9][10] The low-temperature formation of magnesite might well be of significance toward large-scale carbon sequestration.[11]

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[edit]

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.

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.

At times magnesite is dyed to make beads used as ornaments.

References[edit]

  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. ^ Leitmeier, H.(1916): Einige Bemerkungen über die Entstehung von Magnesit und Sideritlagerstätten, Mitteilungen der Geologischen Gesellschaft in Wien, vol.9, pp. 159–166. Lippmann, F. (1973): Sedimentary carbonate minerals. Springer Verlag, Berlin, 228 p.
  6. ^ Deelman, J.C. (1999): "Low-temperature nucleation of magnesite and dolomite", Neues Jahrbuch für Mineralogie, Monatshefte, pp. 289–302.
  7. ^ Alves dos Anjos et al. (2011): Synthesis of magnesite at low temperature. Carbonates and Evaporites, vol.26, pp.213-215.
  8. ^ Ehlmann, B. L. et al. (2008): Orbital identification of carbonate-bearing rocks on Mars. Science, vol.322, no.5909, pp.1828-1832.
  9. ^ 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.
  10. ^ 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.
  11. ^ Oelkers, E. H.; Gislason, S. R. and Matter, J. (2008): Mineral carbonation of CO2. Elements, vol.4, pp.33-337.