Magnesium Oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ions and O2-ions held together by ionic bonding. Magnesium hydroxide forms in the presence of water (MgO+H2O -- Mg(OH)2), but it can be reversed by heating it to separate moisture.