Basic Carbonates of Magnesium

When MgCO₃ is heated, basic carbonates result from a progressive loss of CO₂.

Magnesia alba is commercially prepared as a white powder by precipitating magnesium sulphate or chloride with sodium carbonate. Magnesia alba levis results from precipitation in the cold, and magnesia alba ponderosa from precipitation in the heat. Both the light, bulky form and the heavier variety are used as laxatives in medicine, in dentistry, and for toilet purposes. They ignite respectively to the light and heavy oxides of magnesium. The light carbonate, " when examined under the microscope, is found to consist of amorphous particles with numerous slender prisms intermixed."

Various other processes have been patented or proposed for preparing magnesia alba. Pattinson carefully ignited dolomite, dissolved out the magnesium carbonate by carbon dioxide under 5.6 atmospheres pressure, and, after separating calcium carbonate, injected steam. Magnesia has been agitated with alkaline bicarbonate, and magnesium hydroxide, precipitated by milk of lime from the " bittern " obtained by extracting salt from sea-water, has been dissolved in carbonic acid, heated rapidly to 70° C., and then slowly to the boiling-point.

Magnesia alba is only slightly soluble in water (the solution has an alkaline reaction), but is soluble in acids and in solutions of ammonium salts.

Its composition is usually said to approximate to that of hydromagnesite, Mg(OH)₂.3MgCO₃.3H₂O, but analyses of its composition were never very consistent, and it seems quite certain that its composition is very variable.

The product of the decomposition of MgCO₃.3H₂O by water has been said to approximate to the composition usually ascribed to magnesia alba. The precipitation of basic carbonates by sodium carbonate from magnesium salts has, following Rose, been usually ascribed to hydrolytic decomposition of the normal carbonate, but Davis suggests that the numerous basic carbonates of magnesium, as they have been described, are varying mixtures of different substances. Some of them may be solid solutions of MgO and MgCO₃.

The naturally occurring hydromagnesite, 3MgCO₃.Mg(OH)₂.3H₂O, crystallises in the monoclinic system with a hardness of 3.5 and a density of 2.15. Hydrogioberite, (Mg.OH)₂CO₃.2H₂O, occurs in light grey compact masses of density 2.16.