Magnesium oxide (MgO), also known as magnesia, is a white, hygroscopic solid mineral that occurs naturally as periclase and serves as a raw material for magnesium production. It consists of a crystal lattice formed by Mg²⁺ and O²⁻ ions held together by ionic bonds; in the presence of water, it forms magnesium hydroxide (MgO + H₂O → Mg(OH)₂), though heating can drive off the water to convert magnesium hydroxide back into magnesium oxide.
Historically, magnesium oxide was known as "magnesia alba" (literally "white magnesia"-referring to the white mineral containing magnesium oxide, as well as other sources like magnesium carbonate) to distinguish it from "magnesia nigra" (black magnesia), a manganese-containing black mineral.
Although the term "magnesium oxide" typically refers to MgO, magnesium peroxide is another compound in this category. Evolutionary crystal structure prediction indicates that magnesium peroxide is thermodynamically stable at pressures above 116 GPa (gigapascals), while the semiconducting suboxide Mg₃O₂ is thermodynamically stable above 500 GPa. Due to its stability, magnesium oxide is used as a model system for studying crystal vibrational properties.