Magnesite

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Magnesite

General
Category	Mineral
Chemical formula	magnesium carbonate:MgCO₃
Identification
Color	White
Crystal habit	usually massive, rarely as rhombohedrons or hexagonal prisms
Crystal system	trigonal; bar 3 2/m
Cleavage	[1011] Perfect, [1011] Perfect, [1011] Perfect
Fracture	Brittle - Conchoidal
Mohs Scale hardness	3.5 - 5
Luster	Vitreous
Refractive index	Uniaxial (-) nω=1.508 - 1.510 nε=1.700
Streak	white
Specific gravity	3.0 - 3.2
Fusibility	infusible
Solubility	Effervesces in hot HCl

Magnesite is magnesium carbonate, MgCO₃. Iron (as Fe²⁺) substitutes for 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.

1 Occurrence
2 Formation
3 Uses
4 References and external links

[edit] 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.

[edit] 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.

[edit] 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 produc in refractory materials.