|__ Mg 2+ __ O 2−|
Fm 3 m (No. 225)
a = 421 pm
Mg , O 
colorless octahedral or cubic crystals
|External identifiers / databases|
|Molar mass||40.32 g mol −1|
3.58 g cm −3 (25 ° C)
2800 ° C
3600 ° C
practically insoluble in water
Switzerland: 3 mg m −3 (measured as respirable dust )
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C|
Magnesium oxide ( magnesia ), also obsolete bitter earth , is the oxide of magnesium . The salt has the formula MgO and consisting of Mg 2+ - and O 2- - ions . It crystallizes in the sodium chloride structure .
Magnesium oxide occurs naturally as the volcanic mineral periclase . They are white to gray, due to inclusions also dark green, glass-shining regular crystals of hardness 5.5 to 6 (according to Mohs).
Presentation and extraction
Furthermore, MgO is by calcination of magnesium hydroxide Mg (OH) 2 , magnesium sulfate or magnesium carbonate MgCO 3 obtained as a by-product in the sea water treatment in which, by, precipitation with calcium hydroxide is obtained magnesium hydroxide.
Since magnesium oxide was originally made by calcination, it is still called Magnesia usta , or burnt magnesia , today.
Magnesium oxide forms colorless crystals in the sodium chloride structure. It has a high melting point of around 2800 ° C. If it is obtained from the thermal conversion of other magnesium compounds, the chemical behavior depends very much on the production temperature and the production process. When calcining naturally occurring magnesium carbonate ( magnesite ) at around 800 ° C, so-called "caustic burned magnesite" is created. The carbon dioxide escapes during the firing process , but the temperature is too low to allow sintering . The resulting magnesium oxide grains therefore still have the outer shape of the magnesium carbonate, are porous and very reactive due to the large inner surface. With water, magnesium hydroxide is formed in a short time through hydration . The reaction of some magnesium salts dissolved in water with caustic burned magnesite leads to the crystallization of a rock-hard mass. If the magnesium salt is magnesium chloride MgCl 2 , it is called Sorel cement .
At 1700 ° C to 2000 ° C, sintered magnesia forms , above 2800 ° C (for example in an electric arc furnace ) fused magnesia forms . Both types hardly react with water. Sintered magnesia is also known as "dead-burned magnesia", a highly compacted material with large primary crystals and low reactivity, which is often used to manufacture refractory products . In contrast to quicklime , magnesia has not yet been produced industrially in the medium temperature range above 1000 ° C.
It is added to foods as an acidity regulator or release agent. It is generally approved in the EU as a food additive with the designation E 530 without maximum quantity restriction ( quantum satis ) for foodstuffs.
For technical applications, magnesium oxide is offered in slightly contaminated form as caustic burned magnesite (KM) and is used to extract non-ferrous metals , steel refiners and glass . Mixed with magnesium chloride or magnesium sulphate solutions , caustic burned magnesite is used as so-called Sorel cement, for example for the production of industrial floors. Such magnesia cement bonds well with organic porous substances such as wood, cork and leather. In the construction sector, magnesium oxide is also used as a binder for mineral foams .
Caustic magnesite is also used as a magnesium carrier in the fertilizer and animal feed industry.
In medicine, magnesium oxide is used for substitution therapy.
Due to its high melting point of 2800 ° C, sintered magnesia is used to manufacture magnesia-carbon bricks , which are basic refractory materials . End products include refractory linings for laboratory equipment, vessels such as converters, electric arc furnaces , ladles in steel production , insulating sheaths for thermocouples and insulating tubes between the filament and cathode of electron tubes .
Pressed fine powder of MgO is also used as blackboard chalk in schools.
- RWG Wyckoff Crystal Structures , Wiley, New York 1963
- Entry on MAGNESIUM OXIDE in the CosIng database of the EU Commission, accessed on March 7, 2020.
- Entry on magnesium oxide. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.
- David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Index of Refraction of Inorganic Crystals, pp. 10-247.
- Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for 1309-48-4 or magnesium oxide ), accessed on November 2, 2015.
- Yuan Chunmiao, Yu Lifu, Li Chang, Li Gang, Zhong Shengjun: Thermal analysis of magnesium reactions with nitrogen / oxygen gas mixtures. In: Journal of Hazardous Materials. Vol. 260, 2013, pp. 707-714, doi : 10.1016 / j.jhazmat.2013.06.047 .
- Dipl.-Ing. Bonar Marbun: Kinetics of the hydration of CaO and MgO , p. 3, dissertation, February 2006, Faculty of Natural and Material Sciences, Clausthal University of Technology
- Dipl.-Ing. Bonar Marbun: Kinetics of the hydration of CaO and MgO , p. 9, dissertation, February 2006, Faculty of Natural and Material Sciences, Clausthal University of Technology
- U.S. Patent 4,731,389