Magnesium carbonate (MgCO3) is a white solid, readily found in nature as magnesite and which usually occurs in a hydrated form, clustered with water molecules. It has some industrial uses, such as in glass production, but some everyday uses as well.
Magnesium carbonate is used as an oral supplement for people with low magnesium in their blood, which occurs most often when someone uses diuretics or has lost fluids through diarrhea or vomiting, for example.
Highly pure magnesium carbonate is a common antacid, but in large doses also acts as a laxative. Antacids often contain aluminum hydroxide for balance, since it has a constipating effect.
Most of the hand chalk that athletes like gymnasts, rock climbers and weightlifters use to dry their hands is magnesium carbonate. This chalk absorbs water readily and is not the same as blackboard chalk, which is calcium carbonate.
Its insulating properties, as well as the fact that it is a non-toxic, fairly light and non-flammable substance, make magnesium carbonate ideal for heavy-duty insulation. This includes shipbuilding, boiler manufacturing, and for heavy appliances like ovens and dishwashers.
Because of its water-absorbing properties, some manufacturers add food-grade magnesium carbonate to salt and flour as an anti-clumping agent.