Magnesium Carbonate, MgCO₃

If a solution containing magnesium sulphate and sodium bicarbonate is supersaturated with carbon dioxide and heated so that the gas can only escape slowly, microscopic rhombohedra of MgCO₃ separate. Similar colourless transparent crystals occur naturally as magnesite, which also occurs amorphous in compact masses. The naturally occurring crystals, which are often yellow, grey, or brown, are isomorphous with calcite, have a density of 3.0-3.1 and a hardness of 3.5-4.5. If a solution of magnesium carbonate in carbonic acid is evaporated to dryness at atmospheric pressure, the small residual crystals are isomorphous with orthorhombic aragonite - magnesium carbonate is thus isodimorphous with calcium carbonate. Rhombohedral magnesite is also prepared by heating precipitated magnesium carbonate with water and solid carbon dioxide at 185° C. at a pressure of 20-30 atmospheres.

Many mountain chains consist largely of dolomite, a double carbonate of magnesium and calcium, which is widely distributed, similar in form to magnesite, with a density of 2.85 and a hardness of 3.5-4. Dissolved magnesium carbonate occurs in natural waters containing carbon dioxide - including sea-water. It is prepared amorphous by precipitating magnesium sulphate with sodium carbonate and calcining, by decomposing calcium carbonate with magnesium chloride, or by heating sodium carbonate and magnesium sulphate from 160°-175° C. in sealed tubes.

A litre of water at 19.5° C. dissolves 0.970 grm. MgCO₃ when pure, 26.5 grm. when charged with carbon dioxide at 1 atmosphere, and 50.6 grm. when charged with carbon dioxide at 6 atmospheres. The solubility diminishes as the temperature rises - becoming nil at 100° C. under a pressure of 765 mm. According to Treadwell and Reuter, magnesium hydrogen carbonate is only stable when free carbonic acid is present, and at 15° C. and 760 mm. a solution in which the partial pressure of carbon dioxide is 0 contains 0.7156 grm. MgCO₃ per litre. In solutions of magnesium carbonate in carbonated water some magnesium carbonate remains dissolved even after prolonged boiling, and deposition appears to proceed slowly.

The solubility of magnesium carbonate in carbonated water is increased by various salts - including cold alkaline borates.

Magnesium carbonate has an alkaline reaction, is rapidly decomposed by acids, and dissociates into MgO and CO₂ when heated. At 825° C.

the pressure of the dissociation products is 1 atmosphere, and CO₂ is apparently retained on heating to redness. Magnesite is more resistant than the artificial carbonate to acids and heat.

KCO₃.MgCO₃.4H₄O, when carefully heated up to 150° or 200° C., leaves a transparent residue of crystals with the original form. Water dissolves out potassium carbonate and leaves crystalline anhydrous magnesium carbonate. Magnesium carbonate is obtained anhydrous and dry by heating the double carbonate of ammonium and magnesium. This carbonate is very unlike the natural mineral and Senarmont's artificial magnesite. It retains the crystalline form of the original carbonate, abstracts water from the air as vigorously as lime, absorbs ammonia gas, and sets like plaster in contact with a little water. A litre of water dissolves about 2 grm. of this carbonate, though it only dissolves about 1 grm. in the hydrated condition. When suspended in water the anhydrous carbonate is gradually transformed into the crystalline trihydrate.

MgCO₃.3H₂O is produced by the action of sodium or potassium bicarbonate on magnesium sulphate solutions at ordinary temperatures; by deposition at 60° C. from a solution of magnesium carbonate or hydroxide in carbonic acid; by the addition of alcohol to the solution of magnesium bicarbonate, or by exposing the latter to air; by leaving the precipitate in contact with the liquid when sodium carbonate precipitates a magnesium salt; by precipitating magnesium sulphate with ammonium carbonate; and by warming the pentahydrate with water.

It is fiocculent when first precipitated by alcohol, but soon becomes crystalline, and its habit of crystallising in rhombic needles may identify it with nesquehonite, which has the same composition. The finely divided salt effloresces in air, and its density at 18° C. is 1.808. It becomes the monohydrate at 100° C., which also separates from solutions of magnesium bicarbonate at a fairly high temperature. The trihydrate loses its third molecule of water, with partial decomposition, at much higher temperatures.

Its constitution may be OH.MgCO₃H.2H₂O, and its decomposition by boiling water may be represented by

OH.MgCO₃H.2H₂O = 2H₂O+OH.MgCO₃H
and
OH.MgCO₃H+H₂O = Mg(OH)₂+H₂O+CO₂.

Crystallised MgCO₃.3H₂O has apparently no definite solubility in water, and over a wide range of temperature its reaction with water approximates to

5MgCO₃+2H₂O = Mg(OH)₂.3MgCO₃+Mg(HCO₃)₂.

Natural nesquehonite has a hardness of 2.5 and a density of 1.84.

At lower temperatures than about 15° C. the pentahydrate separates instead of the trihydrate. The crystals deposited from solutions of magnesia in carbonic acid are monoclinic, usually hexagonal prisms, and are practically identical with landsfordite, though Dana assigned to this the composition 4MgO.3CO₂.22H₂O. They are said to be soluble in 267 parts of cold water, and in solution they apparently, especially on warming, transform into the trihydrate.

According to Leitmeier, when hydrated magnesium carbonates deposit from the water of the mineral springs at Rohitsch, the pentahydrate does not form above 6° C., and it loses water in the air at 20° C.

Natural landsfordite has a hardness of 2.5 and a density of 1.5-1.7.

Various other hydrates, including a dihydrate, a tetrahydrate, and others, have been reported, but are doubtful.