magnesite (măgˈnəsĪt) [key], mineral, magnesium carbonate, MgCO3, white, yellow, or gray in color. It originates through the alteration of olivine or of serpentine by waters carrying carbon dioxide; through the replacement of calcium by magnesium in calcareous rocks, sometimes limestone but more often dolomite; and through precipitation from waters rich in magnesium that have undergone reaction with sodium carbonate. Caustic magnesite is not thoroughly calcined, 3% to 4% of carbon dioxide being left; mixed with magnesium chloride it makes oxychloride cement, which is extensively used for floorings and as a stucco. Dead-burned magnesite is calcined in kilns until it contains less than 1% of carbon dioxide; it is made into an excellent firebrick. Magnesite is also used in the manufacture of Epsom salts, face powder, boiler wrappings, and disinfectants.