Magnesium carbonate

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 applications.

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Properties

The most common magnesium carbonate forms are the anhydrous salt called magnesite (MgCO₃) and the di, tri, and pentahydrates known as barringtonite (MgCO₃·2H₂O), nesquehonite (MgCO₃·3H₂O), and lansfordite (MgCO₃·5H₂O), respectively. Some basic forms such as artinite (MgCO₃·Mg(OH)₂·3H₂O), hydromagnestite (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 dissolve in acids. Magnesium carbonate crystallizes in the calcite structure wherein Mg²⁺ is surrounded by six oxygen atoms. The dihydrate has a triclinic structure, while the trihydrate has a monoclinic structure. The pentahydrate is a white crystalline solid with monoclinic crystals.

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 high temperatures, MgCO₃ decomposes to magnesium oxide and carbon dioxide, this process is called calcining:

MgCO₃ → MgO + CO₂

Uses

Magnesite and dolomite minerals are used to produce magnesium metal and basic refractory bricks. MgCO₃ is also used in flooring, fireproofing, fire extinguishing compositions, cosmetics, dusting powder, and toothpaste. Other applications are as filler material, smoke suppressant in plastics, a reinforcing agent in neoprene rubber, a drying agent, a laxative to loosen the bowels, and color retention in foods. In addition, high purity magnesium carbonate is used as antacid and as an additive in table salt to keep it free flowing.

In 1911 MgCO₃ was first added to salt to make it flow more freely.^[1]

Magnesium carbonate, most often referred to as 'chalk', is used as a drying agent for hands in rock climbing, gymnastics, and weight lifting.

References

1. ^ Morton Salt FAQ.

• Patnaik, Pradyot (2003). Handbook of Inorganic Chemicals. New York: McGraw Hill. 
  
• Trotman-Dickenson, A.F "(ed.)" (1973). Comprehensive Inorganic Chemistry. Oxford: Pergamon Press.