magnesite

A member of the calcite-type carbonates having the formula MgCO₃. It forms dolomite [CaMg(CO₃)₂] with calcite (CaCO₃) in the system CaCO₃MgCO₃. Pure magnesite is not common in nature because there exists a complete series of solid solutions between MgCO₃ and FeCO₃, which is constantly present in magnesite in its natural occurrence. See also Carbonate minerals; Magnesium.

Magnesite is usually white, but it may be light to dark brown if iron-bearing. The hardness of magnesite is 3½ to 4½ on the Mohs scale, and the specific gravity is 3.00. See also Hardness scales.

Magnesite deposits are of two general types: massive and crystalline. Massive magnesite is an alteration product of serpentine which has been subjected to the action of carbonate waters. Crystalline magnesite is usually found in association with dolomite. It is generally thought to be a secondary replacement of magnesite in preexisting dolomite by magnesium-rich fluids.

Magnesite is an important industrial mineral. Various types of magnesite or magnesia (MgO) are produced by different thermal treatments. The caustic-calcined magnesite or magnesia is used in the chemical industry for the production of magnesium compounds, while dead-burned or sintered magnesite or magnesia is used in refractory materials. Fused magnesia is used as an insulating material in the electrical industry because of its high electrical resistance and high thermal conductivity.

A natural magnesium carbonate.

Environment

Associated with altered and weathered serpentine and in sedimentary beds from which calcium has been completely leached.

Usually in dull white, sometimes spherical, microcrystalline masses developed in weathering. Small prismatic needles on serpentine; also in large transparent Iceland spar-type crystals and cleavages. Also coarsely granular, like a marble.

White, colorless, light tints. Luster glassy to dull; hardness 3Ɖ-5, specific gravity 3.0-3.2, fracture conchoidal to smooth; cleavage rhombohedral. Brittle; transparent to translucent.

Magnesium carbonate (47.6% MgO, 52.4% CO ₂ ), often with some iron and calcium.

The tongue adheres to matte, porcelaneous, massive material. Dissolves with bubbles in hot hydrochloric acid.

The white, dull, fine-grained porcelaneous masses can be identified by their behavior in acid. Both the marble-grained and the more recently discovered transparent rhombohedrons can be confused with calcite or dolomite but are heavier and remain little affected in cold hydrochloric acid.

Usually results from a hot-water (hydrothermal) alteration of serpentine, which creates solid white veins in the parent rock. Surface alteration can produce the dull white spheres. Small free-growing crystals were described from serpentine fissures on Staten I., New York. Huge quantities of the dull white material have been mined as sources of magnesia and magnesium in Washington and California. Good crystals (mostly hexagonal, rhombohedron-terminated brownish prisms) have been found associated with strontianite and dolomite at Oberndorf, in Styria, Austria, in a magnesite quarry. Gabbs, Nevada, is the most commercial deposit in the U.S.

The best specimen source is a marblelike variety with very coarse grain, which is being exploited in Brumado, Bahia, Brazil. This deposit has produced hundreds of large Iceland spar-like crystals and cleavages, grown in cavities in the bed accompanied by quartz, topaz, uvite and dravite tourmaline, uranium minerals, and others. This fascinating, perhaps unique, stratified deposit probably represents a final stage after a magnesia-enrichment process changes limestones into dolomites followed, in this case by metamorphic recrystallization into magnesite. A magnesite quarry near Pamplona, Spain, has similar Iceland spar-like rhombohedrons, but all are dolomite.

Magnesite is not to be confused with Magnetite or Magnemite.

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
Streak	white
Specific gravity	3.0 - 3.2
Refractive index	Uniaxial (-) nω=1.508 - 1.510 nε=1.700
Fusibility	infusible
Solubility	Effervesces 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.

Contents 1 Occurrence 2 Formation 3 Uses 4 References and external links

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

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

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

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.

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.

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

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