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Magnesite



Magnesite is not to be confused with Magnetite or Magnemite.
Magnesite

General
CategoryMineral
Chemical formulamagnesium carbonate:MgCO3
Identification
ColorWhite
Crystal habitusually massive, rarely as rhombohedrons or hexagonal prisms
Crystal systemtrigonal; bar 3 2/m
Cleavage[1011] Perfect, [1011] Perfect, [1011] Perfect
FractureBrittle - Conchoidal
Mohs Scale hardness3.5 - 5
LusterVitreous
Refractive indexUniaxial (-) nω=1.508 - 1.510 nε=1.700
Streakwhite
Specific gravity3.0 - 3.2
Fusibilityinfusible
SolubilityEffervesces in hot HCl

Magnesite is magnesium carbonate, MgCO3. Iron (as Fe2+) substitutes for magnesium (Mg) with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts. Dolomite, (Mg,Ca)CO3, is almost indistinguishable from magnesite.

Additional recommended knowledge

Contents

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 terranes. These magnesites often are cryptocrystalline and contain silica as 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 ultrabasic rocks. Magnesite is formed via carbonation of olivine in the presence of water and carbon dioxide, and is favored at moderate temperatures and pressures typical of greenschist facies;

Magnesite can also be formed via the carbonation of magnesian serpentine (lizardite) via the following reaction:
Serpentine + carbon dioxide → Talc + magnesite + Water

2Mg_3Si_2O_5(OH)_4 + 3CO_2 \rarr Mg_3Si_4O_10(OH)_2 + 3MgCO_3 + H_2O

Forsterite magnesia-rich olivine compositions favor production of magnesite from peridotite. Fayalitic (iron-rich) olivine favors production of magnetite-magnesite-silica compositions.

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

Uses

Magnesite can be used as a slag former in steelmaking furnaces, in conjunction with lime, in order to protect the magnesium oxide lining. It can also be used as a catalyst and filler in the production of synthetic rubber and in the preparation of magnesium chemicals and fertilizers.

Similar to the production of lime, magnesite can be burned in the presence of charcoal to produce MgO, otherwise known as periclase. Such periclase is an important product in refractory materials.

References and external links

  • Dana's Manual of Mineralogy ISBN 0-471-03288-3
  • Smithsonian Rock and Gem ISBN 0-7566-0962-3
  • Mineral Galleries
  • Webmineral.com
  • Mindat.org
  • Minerals.net
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Magnesite". A list of authors is available in Wikipedia.
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