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 O2â?? ions held together by ionic bonding. Magnesium hydroxide forms in the presence of water (MgO + H2O â?? Mg(OH)2), but it can be reversed by heating it to separate moisture.