Magnesite

A member of the calcite-type car-bonates 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

The calcite-type structure of magnesite has a = 0.4632 nm, c = 1.5032 nm, z = 6, and space group R3c. The interatomic distances are CO, 0.12851 nm, and MgO, 0.21018 nm, which produce a MgO octahedron with less distortion than that of CaO6 in calcite. Magnesite decomposes to MgO and CO2 in the temperature range of 625–643°C as determined by differential thermal analysis. Crystallization of magnesite in aqueous solution is inhibited by species of magnesium in solution and strong positive catalysis by ionic strength and CO2 pressure. Therefore, highly saline conditions provide a favorable environment for magnesite crystallization.

Magnesite is uniaxial and optically negative with extreme birefringence (0.191). Refractive indices are ∊ = 1.509 (extraordinary index) and ω = 1.700 (ordinary index). Magnesite is usually white, but it may be light to dark brown if iron-bearing. The hardness of magnesite is on the Mohs scale, and the specific gravity is 3.00. See also: Birefringence; Crystal optics; 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. Massive magnesite deposits are found in Greece, Turkey, India, China, North Korea, and Australia, while those of crystalline magnesite occur in the United States, Austria, Spain, Russia, and the Czech Republic.

Magnesite is an important industrial mineral. Various types of magnesite or magnesia (MgO) are produced by different thermal treatment. 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.

Luke L. Y. Chang

Bibliography

  •  L. Chang, R. A. Howie, and J. Zussman, Rock-Forming Minerals, vol. 5B: Nonsilicates, 2d ed.,
  • alifazeli=egeology.blogfa.com