Zinc Carbonate, ZnCO₃

The normal carbonate, ZnCO₃, occurs naturally as smithsonite, which is also known as zinc spar or calamine. A semi-crystalline scale of basic zinc carbonate, closely agreeing with the formula ZnCO₃.3Zn(OH)₂, has been found on rusted zinc. Natural basic zinc carbonates, usually named hydrozincite, often approximate to the composition ZnCO₃.2Zn(OH)₂, but their constitution varies. It has been suggested that smithsonite originated in interaction between calcium carbonate and zinc sulphate resulting from the oxidation of zinc blende.

When solutions of zinc salts are precipitated by alkali carbonates or bicarbonates the product varies greatly in composition. According to Kraut, normal amorphous zinc carbonate is the first product when the carbonate solutions are poured into the solution of zinc sulphate. This is then converted into the stable crystalline monohydrate, ZnCO₃.H₂O, or into the basic carbonate, 5ZnO.2CO₂.4H₂O. All other compounds described by other workers were mixtures of these two. Mikusch, from a study of the system ZnO-CO₂-H₂O, decided that 5ZnO.2CO₂. 4H₂O is the only chemically individual basic salt.

The basic carbonate only is formed when the zinc sulphate solution is added to the carbonate solution.

The monohydrate, ZnCO₃.H₂O, was prepared in small quadratic optically active crystals, mechanically mixed with crystals of Na₂O.3ZnO. 4CO₂.3H₂O, by allowing the precipitate from mixed solutions containing ZnSO₄: 4NaHCO₃ to become crystalline under the mother-liquor. The crystalline salt has also been obtained from a solution of zinc carbonate in ammonium carbonate.

The hemihydrate, 2ZnCO₃.H₂O, was obtained by precipitating the solution of a zinc salt with ammonium bicarbonate and digesting the gelatinous precipitate in excess of the precipitant till it became crystalline.

The basic zinc carbonate, 5ZnO.2CO₂.4H₂O, can be prepared by the direct hydrolysis of the normal carbonate, or by boiling a solution of zinc, zinc hydroxide, or zinc carbonate in an excess of aqueous carbonic acid.

Basic zinc carbonates can be converted into the normal carbonate by contact with water charged with carbon dioxide under pressure. The normal carbonate is itself somewhat soluble in water containing carbon dioxide - presumably forming the bicarbonate.

Crystals of the monammoniate, ZnCO₃.NH₄, are deposited from a solution of zinc carbonate in ammonium carbonate in rectangular prisms or stellate masses of needles.