Double Carbonates of Magnesium

Knorre obtained octahedral crystals of Na₂CO₃MgCO₃, belonging to the tetragonal system, by digesting precipitated magnesium carbonate with excess of sodium bicarbonate solution. The crystals are isometric and doubly refractive. De Schulten obtained flat rhombohedra, with a density of 2.729 at 15° C., by heating mixed solutions of magnesium nitrate and sodium carbonate in a closed flask at 100° C. Na₂CO₃ is said to precipitate Na₂CO₃.MgCO₃.15H₂O from magnesium sulphate solution at temperatures below 10° C. Knorre could not confirm Deville's quartz-like crystals obtained by digesting magnesia alba with sodium bicarbonate at 60°-70° C.

K₂CO₃.MgCO₃.4H₂O is produced in rhombic prisms by digesting MgCO₃.3H₂O with potassium bicarbonate solution, or by the action of excess of concentrated potassium carbonate solution on a solution of magnesium chloride. Cold water slowly decomposes it into MgCO₃.3H₂O, and hot water into magnesia alba.

KCO₃.MgCO₃.4H₂O separates from the reaction

3(MgCO₃.3H₂O)+2KCl+CO₂ = 2(KCO₃.MgCO₃.4H₂O)+MgCl₂

in triclinic crystals, which have a density of 1.984 and do not alter in air. Potassium carbonate is obtained from the double salt by extracting with water after ignition or under pressure at 140° C. It was first prepared by the reaction

MgCl₂+3KCO₃+3H₂O = KCO₃.MgCO₃.4H₂O+2KCl+CO₂.

The double salt crystallises out slowly in snow-white crystals. MgCO₃.3H₂O precipitates sometimes in this reaction and at other times succeeds the double salt. The proportionate concentrations of KCO₃ and MgCl₂ determine precipitation. The reversible reaction

KCO₃.MgCO₃.4H₂O ⇔ MgCO₃.3H₂O+KCO₃+H₂O

is related to these observations, and indicates that the double salt can be prepared by acting with potassium bicarbonate on MgCO₃.3H₂O suspended in water.

Efflorescent crystals of RbHCO₃.MgCO₃.4H₂O were obtained by warming a solution of rubidium bicarbonate with magnesium carbonate in a current of carbon dioxide at 60° C.

(NH₄)₂CO₃.MgCO₃.4H₂O was obtained in rectangular prisms by the action of ammonium carbonate on magnesia or magnesium carbonate suspended in water. Deville obtained unstable (NH₄)HCO₃.MgCO₃.4H₂O by acting on magnesium salts with a large excess of ammonium bicarbonate, and at a low temperature unstable 2(NH₄HCO₃.MgCO)₃.11H₂O. The former loses ammonia, water, and carbon dioxide in the air, and is decomposed by water with the formation of MgCO₃.3H₂O. Engel only obtained (NH₄)₂CO₃.MgCO₃.4H₂O by saturating a mixture of ammonium and magnesium bicarbonates with carbon dioxide in the absence of air. (NH₄)₂CO₃.MgCO₃.6H₂O precipitates from a magnesium salt and normal ammonium carbonate when the solution contains half its volume of alcohol.

Dolomite, MgCO₃.CaCO₃, is widely distributed in nature. One thousand parts of water at 18° C., charged with carbon dioxide at 750 mm., dissolve 0.310 parts of normal dolomite, but the solubility in carbonated water varies with the condition of the mineral. Strong acids dissolve it, and its resistance to attack by dilute acids seems to increase with its iron content. When heated to 400° C. and treated with water it acts like a cement, though the loss of carbon dioxide is said to begin at 500° C.

Artificial dolomite has been prepared by the action of magnesium chloride and sodium carbonate solutions on calcium carbonate (vaterite) at 180°-200° C. in carbon dioxide at 50 atmospheres, and by many other methods.