Magnesite
General
Category	Carbonate mineral
Chemical formula	MgCO3
Strunz classification	05.AB.05
Identification
Color	Colorless, white, pale yellow, pale brown, faintly pink, lilac-rose
Crystal habit	Usually massive, rarely as rhombohedrons or hexagonal prisms
Crystal system	Trigonal - Hexagonal Scalenohedral H-M Symbol 32/m Space Group: R3c
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
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 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.

Occurrence[link]

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[link]

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

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

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.

Magnesite is also found in a number of Precambrian carbonate hosted sediments, and is thought to have formed as an evaporite.

Uses[link]

Dyed and polished magnesite beads

Magnesite can be used as a slag former in steelmaking furnaces, in conjunction with lime, 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.

Magnesite can also be used as a binder in flooring material.

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

It is dyed to make beads, as is howlite.

References[link]

• Smithsonian Rock and Gem ISBN 0-7566-0962-3