Magnesite
| Magnesite | |
|---|---|
Magnesite from Bahia, Brazil (9.7 x 7.1 x 6 cm)
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| General | |
| Category | Carbonate mineral |
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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 | |
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.Manganese, cobalt and nickel may also occur in small amounts.
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 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.
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:
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