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.

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.

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.