Wikepedia has this to say...Magnesium carbonate, also known as the mineral magnesite when found in nature, is a chemical compound with a variety of uses. It is a white, odorless, low density compound, with chemical formula: MgCO3. The chemical is used to make magnesium oxide. The compound has a molecular mass of 84.31 and a specific gravity of 3.009.
Magnesium carbonate is added to table salt to prevent caking and used in gymnastics, weightlifting, and climbing to provide a good grip.
Basic magnesium carbonate is also called Artinite.
In spite of being carcinogenic, it is used in some gutkas in India as a whitening agent.