Magnesium oxide (MgO), or magnesia, is awhite hygroscopic solid mineral that occurs naturally as periclase and is asource of magnesium (see also oxide). It has an empirical formula of MgO andconsists 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.

    Calcining at different temperaturesproduces magnesium oxide of different reactivity. High temperatures 1500 – 2000°C diminish the available surface area and produces dead-burned (often calleddead burnt) magnesia, an unreactive form used as a refractory. Calciningtemperatures 1000 – 1500 °C produce hard-burned magnesia, which has limitedreactivity and calcining at lower temperature, (700–1000 °C) produceslight-burned magnesia, a reactive form, also known as caustic calcinedmagnesia. Although some decomposition of the carbonate to oxide occurs attemperatures below 700 °C, the resulting materials appears to reabsorb carbondioxide from the air.