Magnesium oxide, or magnesia, is a white solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of . It is formed by a covalent bond between one magnesium and one oxygen atom. Magnesium oxide is easily made by burning magnesium ribbon which oxidizes in a bright white light, resulting in a powder. It is hygroscopic in nature and care must be taken to protect it from moisture. Magnesium hydroxide forms in the presence of water (MgO + H2O → Mg(OH)2), but it can be reversed by heating it to separate moisture.
The crystal structure of MgO matches the crystal structure of NaCl.
- In medicine, magnesium oxide is used for relief for heartburn and sore stomach, as an antacid, magnesium supplement, and as a short-term laxative. It is also used to improve symptoms of indigestion. Side effects of magnesium oxide may include nausea and cramping.
- It is used by many libraries for preserving books by reacting with ambient moisture to dry the book storage areas.
- It is used as an insulator in industrial cables.
- It is used as a basic refractory material for lining crucibles.
- It is used as a principal ingredient in construction materials used for fireproofing.
- It is used as a reference white color in colorimetry, owing to its good diffusing and reflectivity properties. It may be smoked onto the surface of an opaque material to form an integrating sphere.
- It is used as an insulator in devices that exhibit the tunnel magnetoresistance effect.
- It is used extensively in electrical heating as a component of "CalRod"-styled heating elements. There are several mesh sizes available and most commonly used ones are 40 and 80 mesh per the American Foundry Society. The extensive use is due to its high dielectric strength and average thermal conductivity. MgO is usually crushed and compacted with minimal airgaps or voids. The electrical heating industry also experimented with aluminium oxide, but it is not used anymore.
- It is used in feeding animals and there are special grades available.
- An industrial use of magnesium oxide is in leather processing, as a basifying agent on chrome tanning process.
- Pressed MgO is used as an optical material. It is transparent from 300 nm to 7 µm. The refractive index is 1.72 at 1 µm and the Abbe number is 53.58. It is sometimes known by the Eastman Kodak trademarked name Irtran-5, although this designation is long since obsolete. Crystalline pure MgO is available commercially and has small use in infrared optics.
- It is packed around transuranic waste at the Waste Isolation Pilot Plant, to control the solubility of radionuclides.
Inhalation of magnesium oxide fumes can cause metal fume fever