Magnesium oxide

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.

Magnesium oxide was historically known as magnesia alba (literally, the white mineral from magnesia – other sources give magnesia alba as MgCO₃), to differentiate it from magnesia negra, a black mineral containing what is now known as manganese.

While "magnesium oxide" normally refers to MgO, magnesium peroxide MgO₂ is also known as a compound. According to evolutionary crystal structure prediction, MgO₂ is thermodynamically stable at pressures above 116 GPa (gigapascals), and a semiconducting suboxide Mg₃O₂ is thermodynamically stable above 500 GPa. Because of its stability, MgO is used as a model system for investigating vibrational properties of crystals.