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Magnesium carbonate, MgCO3, is a white solid that occurs in nature as a mineral. Several hydrated and basic forms of magnesium carbonate also exist as minerals. In addition, MgCO3 has a variety of uses.
Contents    [hide]
  • 1 Forms
  • 2 Reactions
  • 3 Uses
  • 4 Food additive
  • 5 Toxicology
  • 6 Compendial status
  • 7 Notes and references
  • 8 See also
  • 9 External links
[edit]Forms  
The most common magnesium carbonate forms are the anhydrous salt called magnesite (MgCO3) and the di, tri, and pentahydrates known asbarringtonite (MgCO3·2H2O), nesquehonite (MgCO3·3H2O), and lansfordite (MgCO3·5H2O), respectively. Some basic forms such as artinite(MgCO3·Mg(OH)2·3H2O), hydromagnesite (4MgCO3·Mg(OH)2·4H2O), and dypingite (4MgCO3· Mg(OH)2·5H2O) also occur as minerals. Magnesite consists of white trigonal crystals. The anhydrous salt is practically insoluble in water, acetone, and ammonia. All forms of magnesium carbonate react in acids. Magnesium carbonate crystallizes in the calcite structure where in Mg2+ is surrounded by six oxygen atoms. The dihydrate one has a triclinic structure, while the trihydrate has a monoclinic structure.
References to 'light' and 'heavy' magnesium carbonates actually refer to the magnesium hydroxy carbonates hydromagnesite and dypingite(respectively).[2]
[edit]Reactions  
Although magnesium carbonate is ordinarily obtained by mining the mineral magnesite, the trihydrate salt, MgCO3·3H2O, can be prepared by mixing solutions of magnesium and carbonate ions under an atmosphere of carbon dioxide. Magnesium carbonate can also be synthesized by exposing a magnesium hydroxide slurry to carbon dioxide under pressure (3.5 to 5 atm) below 50 °C, which gives soluble magnesium bicarbonate:
Mg(OH)2 + 2 CO2 → Mg(HCO3)2
Following the filtration of the solution, the filtrate is dried under vacuum to produce magnesium carbonate as a hydrated salt:
Mg2+ + 2 HCO3- → MgCO3 + CO2 + H2O
When dissolved with acid, magnesium carbonate decomposes with release of carbon dioxide:
MgCO3 + 2 HCl → MgCl2 + CO2 + H2O
MgCO3 + H2SO4 → MgSO4 + CO2 + H2O
At a temperature range between (250 °C - 800 °C), MgCO3 decomposes to magnesium oxide and carbon dioxide with reaction enthalpy 118 kJ / mole, this process is called calcining:
MgCO3 -{250-800 °C}→ MgO + CO2
Above 500 °C the process reaches a decomposition rate of 100% MgO, the publicated decomposition temperature in cases of material safety is 662 °C.
[edit]Uses