Magnesite, or magnesite, is a white hygroscopic solid mineral that occurs naturally as perlite and is the source of magnesium (see also oxides). Magnesium oxide is produced by directional firing of magnesite at 850 degrees Celsius with additional grinding in a special mill. Due to the high quality of the raw material and the absence of burnt particles, the product has a high chemical activity. It has the empirical formula MgO and consists of a lattice of Mg2+ ions and O2- ions, held together by ionic bonds. Magnesium hydroxide is formed in the presence of water (MgO + H2O → Mg(OH)2), but can be reversed by separating the water by heating.