MFQ MINING AND TRADING COMPANY Magnesite is an important industrial mineral composed of magnesium carbonate (MgCO 3. Pure magnesite is theoretically 47.8% magnesia (MgO) and 52.2% carbon dioxide (CO2). It is the source of two-thirds of the worldâ??s magnesia (MgO); 25% is extracted from seawater. uses: Magnesite 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. Small amounts of magnesite are also used as a gem and lapidary material. Magnesite is also used to make fertilizers, magnesium chemicals, and is refined into magnesium metal