|Molar mass||40.3044 g/mol|
|Melting point||2,852 °C (5,166 °F; 3,125 K)|
|Boiling point||3,600 °C (6,510 °F; 3,870 K)|
|0.00062 g/100 mL (0 °C)
0.0086 g/100 mL (30 °C)
|Solubility||Soluble in acid, ammonia
insoluble in alcohol
|Band gap||7.8 eV|
|Thermal conductivity||45–60 W·m−1·K−1|
Refractive index (nD)
|6.2 ± 0.6 D|
|Halite (cubic), cF8|
|Fm3m, No. 225|
a = 4.212Å
|Octahedral (Mg2+); octahedral (O2−)|
|37.8 J/mol K|
Std enthalpy of
Gibbs free energy (ΔfG˚)
|Main hazards||Metal fume fever, Irritant|
|Safety data sheet||ICSC 0504|
|R-phrases||R36, R37, R38|
|US health exposure limits (NIOSH):|
|TWA 15 mg/m3 (fume)|
IDLH (Immediate danger
|750 mg/m3 (fume)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is: / ?)(|
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 normally "magnesium oxide" means compound MgO, magnesium peroxide MgO2 is also known as a metastable compound. According to evolutionary crystal structure prediction, MgO2 is thermodynamically stable at pressures above 116 GPa (gigapascals), and a totally new semiconducting suboxide Mg3O2 is thermodynamically stable above 500 GPa.
Magnesium oxide is produced by the calcination of magnesium carbonate or magnesium hydroxide or by the treatment of magnesium chloride with lime followed by heat.[clarification needed] Calcining at different temperatures produces magnesium oxide of different reactivity. High temperatures 1500 - 2000 °C diminish the available surface area and produces dead-burned (often called dead burnt) magnesia, an unreactive form used as a refractory. Calcining temperatures 1000 - 1500 °C produce hard-burned magnesia which has limited reactivity and calcining at lower temperature, (700-1000°C) produces light-burned magnesia, a reactive form, also known as caustic calcined magnesia. Although some decomposition of the carbonate to oxide occurs at temperatures below 700°C, this appears rapidly reversible due to absorption of carbon dioxide from the air. 
A refractory material is one that is physically and chemically stable at high temperatures. "By far the largest consumer of magnesia worldwide is the refractory industry, which consumed about 56 % of the magnesia in the United States in 2004, the remaining 44 % being used in agricultural, chemical, construction, environmental, and other industrial applications."
In medicine, magnesium oxide is used for relief of heartburn and sour 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. In quantities sufficient to obtain a laxative effect, side effects of long-term use include enteroliths resulting in bowel obstruction.
- MgO is used as a basic refractory material for crucibles and as a principal fireproofing ingredient in construction materials. As a construction material, magnesium oxide wallboards have several attractive characteristics: fire resistance, moisture resistance, mold and mildew resistance, and strength.
- 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 extensively as an electrical insulator in tubular construction 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 also used as an insulator in heat-resistant electrical cable.
- MgO doping has been shown to effectively inhibit grain growth in ceramics and improve their fracture toughness by transforming the mechanism of crack growth at nanoscale.
- Pressed MgO is used as an optical material. It is transparent from 0.3 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 obsolete. Crystalline pure MgO is available commercially and has a small use in infrared optics.
- MgO is packed around transuranic waste at the Waste Isolation Pilot Plant, to control the solubility of radionuclides.
- MgO has an important place as a commercial plant fertilizer  and as animal feed. 
- An aerosolized solution of MgO is used in library science and collections management for the deacidification of at-risk paper items. In this process, the alkalinity of MgO (and similar compounds) neutralizes the relatively high acidity characteristic of low-quality paper, thus slowing the rate of deterioration.
- MgO is also used as a protective coating in plasma displays.
- Magnesium oxide is used as an oxide barrier in spin-tunneling devices. Owing to the crystalline structure of its thin films, which can be deposited by magnetron sputtering, for example, it shows characteristics superior to those of the commonly used amorphous Al2O3. In particular, spin polarization of about 85 % has been achieved with MgO versus 40–60 % with alluminium oxide. The value of tunnel magnetoresistance is also significantly higher for MgO (600 % at room temperature and 1,100 % at 4.2 K) than Al2O3 (ca. 70 % at room temperature). MgO is thermally stable up to about 700 K, vs. 600 K for Al2O3.
Magnesium oxide is easily made by burning magnesium ribbon, which produces a very bright white light, and a powdery ash. The bright flame is very hard to extinguish and it emits a harmful intensity of UV light. Inhalation of magnesium oxide fumes can cause metal fume fever. When burned in open air, the magnesium gets hot enough to produce noticeable amounts of yellow magnesium nitride. Burning in a covered crucible, letting in just enough air to support combustion, will reduce the burning temperature, minimizing the production of the nitride.
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