Magnesium Oxide, MgO

The mineral periclase, MgO, crystallises in transparent regular octahedra which have a density of 3.7-3.9 and a hardness of 5.5-6. When the amorphous variety of the oxide is volatilised in an electric furnace it deposits in the crystalline form. Amorphous magnesia remains as a residue when magnesium salts of oxyacids are heated in air or when magnesium burns in air or oxygen.

Amorphous magnesium oxide has been most commonly prepared by gentle prolonged heating of the carbonate or hydroxide, and the natural carbonate has long been used for this purpose. The denser the original carbonate and the higher the temperature of production, the denser the resulting oxide.

Since waste magnesium chloride liquors remain after the preparation of potassium chloride from carnallite, magnesium chloride has often been used as a starting-point for preparing magnesium oxide. Magnesium chloride has been (a) heated with steam, (b) converted into hydroxide by lime, (c) converted into oxychloride by heating with magnesite and decomposed by water into hydroxide and chloride, (d) precipitated as hydroxide by calcium oxychloride solution, (e) treated with calcined dolomite.

Magnesium sulphate has also been treated with calcined dolomite.

Ordinary magnesium oxide is a white, tasteless, odourless powder. Its density is 3.1982 after being heated to 350° C, and 3.654 after fusion in an electric furnace. Between these temperatures its density rises as it is heated more strongly. Ordinary commercial magnesia is a fine, very bulky powder, of density 3.07-3.2. The volume occupied by a given weight may vary from 1 to 3.5, according to the carbonate from which it is prepared, and it is commonly classed as "heavy" and "light." 12 "Heavy" carbonates require a higher temperature than "light" to decompose them, and the resulting oxide is denser.

Moist magnesium oxide is alkaline to litmus. The higher the temperature to which it has been raised the more slowly it dissolves in water, and the rate of solution also varies with the source: oxide prepared from magnesite at a white heat appears to be practically insoluble.

Polymerisation probably accounts for the increased density of magnesium oxide with increased temperature, and for its correspondingly increased resistance to the action of water.

The influence of preliminary treatment on the solution of magnesium oxide probably explains the variation from 500 mgm. to 5 mgm. per litre obtained by different observers for its solubility. Magnesia absorbs water and carbon dioxide in contact with air, and is soluble in acids. The oxide obtained by gently heating the native carbonate "sets " to a firm mass with a little water, and the oxide from the nitrate prepared by treating artificial carbonates with nitric acid behaves similarly. The product of the ignition of artificial carbonates does not " set."

It melts at about 2250° C. and then vaporises freely; its boiling-point is near its melting-point. Carbon seems to reduce it at temperatures as low as 1700° C. In the electric furnace the reaction

CO+Mg = C+MgO

must be prevented, either by removing the carbon monoxide or alloying the magnesium with copper, to obtain the metal. Otherwise deposits of magnesia are obtained.

Crystalline magnesium oxide sublimes when the amorphous oxide is heated either in an electric furnace or strongly with hydrochloric acid. It also crystallises out of a fusion of potassium and magnesium hydroxides, and Debray obtained it by heating magnesium sulphate strongly with an alkali sulphate. Artificially prepared crystalline magnesium oxide is usually octahedral and corresponds to natural periclase, though Houdard, by heating the amorphous variety with magnesium sulphide, obtained cubical crystals with a density of 3.5692 – 3.5705.

Crystalline magnesium oxide dissolves slowly in warm hydrochloric or nitric acid, rapidly in sulphuric acid, and is scarcely attacked by water.

Thomsen's value of 143.4 Cal. for the heat of formation of magnesium oxide is probably too low. Its molecular heat at 2780° C. is about 11.6 Cal.

Magnesium oxide emits a bright white light at a red heat, and has been used instead of lime in the oxyhydrogen lantern. Since it is very refractory it is used for making crucibles, firebricks, furnace linings, etc. It conducts heat rather well, but it can easily be moulded and fired without any addition of other substances. In medicine it is used as a purgative, and as an antidote to poisoning by arsenious acid, copper sulphate, and acids. Magnesia is also an important constituent of cements and hydraulic mortars. It has a brilliant red fluorescence when mixed with a little chromic oxide, and phosphoresces under certain conditions.