Magnesium oxide

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Crystal structure
Structure of magnesium oxide
__ Mg 2+      __ O 2−
Crystal system


Space group

Fm 3 m (No. 225)Template: room group / 225

Lattice parameters

a = 421 pm

Coordination numbers

Mg [6], O [6]

Surname Magnesium oxide
other names
  • magnesia
  • Magnesia usta
  • burnt magnesia
  • Sintered magnesia
  • Fused magnesia
  • Bitter earth
  • Epsom salt earth
  • Talc earth
Ratio formula MgO
Brief description

colorless octahedral or cubic crystals

External identifiers / databases
CAS number 1309-48-4
EC number 215-171-9
ECHA InfoCard 100,013,793
PubChem 14792
ChemSpider 14108
DrugBank DB01377
Wikidata Q214769
Drug information
ATC code
Molar mass 40.32 g mol −1
Physical state



3.58 g cm −3 (25 ° C)

Melting point

2800 ° C

boiling point

3600 ° C


practically insoluble in water

Refractive index


safety instructions
Please note the exemption from the labeling requirement for drugs, medical devices, cosmetics, food and animal feed
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases

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

Magnesium oxide is produced by burning magnesium in the air. This reaction is strongly exothermic with −592.8 kJ mol −1 from the solid phase and −608.4 kJ mol −1 from the liquid phase .

It can also be obtained by the reaction of ethanol vapor and magnesium at an elevated temperature. In addition to magnesium oxide, ethane is formed .

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Since magnesium oxide was originally made by calcination, it is still called Magnesia usta , or burnt magnesia , today.


Powdered magnesium oxide

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 .

Magnesium oxide is also used to remove silica from water and as an adsorbent and vulcanization retarder in organic chemistry.

Sintered magnesia sticks with a diameter of 1–2 mm are used for the borax bead sample , the end of which can be easily broken off.

Pressed fine powder of MgO is also used as blackboard chalk in schools.


The " magnesia " used in climbing and gymnastics is not magnesium oxide, but essentially magnesium carbonate with magnesium hydroxide .

Individual evidence

  1. ^ RWG Wyckoff Crystal Structures , Wiley, New York 1963
  2. Entry on MAGNESIUM OXIDE in the CosIng database of the EU Commission, accessed on March 7, 2020.
  3. Entry on magnesium oxide. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.
  4. a b c d e Entry on magnesium oxide in the GESTIS substance database of the IFA , accessed on December 21, 2019 (JavaScript required)
  5. 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.
  6. 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.
  7. 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 .
  8. a b 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
  9. 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
  10. U.S. Patent 4,731,389