Magnesite is a mineral with the chemical formula MgCO 3 .Iron, manganese, cobalt and nickel may occur as admixtures, but only in small amounts.Magnesite has a chemical composition of MgCO 3 , and when it is heated it will dissociate into MgO and CO 2 .MgO has an extremely high melting temperature, and that makes it a good refractory material in many steelmaking, metallurgical, and ceramic processes.MgO is one of the most commonly used materials for making the bricks used to line kilns, industrial ovens, and blast furnaces.MgO is also used to