THE FORMATION OF RHODOCHROSITE SMITHSONITE (MNCO3-ZNCO3) SOLID-SOLUTIONS AT 5-DEGREES-C

MnXZn((1-X))CO3 solid-solutions were prepared at 5 degrees C by precipitation from metal-bearing bicarbonate solutions. The solids were identified by X-ray powder diffraction and infrared spectroscopy. Zn2+ ions substitute extensively for Mn2+ ions in the crystal lattice of anhydrous rhombohedral carbonates. Throughout the 24 h during which the experiments were conducted, the aqueous solutions remained undersaturated with respect to pure oxides, sulphates, hydroxides and hydroxysulphates. The solutions, however, were supersaturated with MnXZn(1-X)CO3 of any given composition. Besides the anhydrous rhombohedral carbonates, Zn-4(OH)(2)(CO3)(3) . 4H(2)O was precipitated from an aqueous solution with initially high Zn2+ concentration. The negative logarithm of the solubility product of Zn-4(OH)(2)(CO3)(3) . 4H(2)O was estimated theoretically to be 43.9 (25 degrees C). Remaining saturation with respect to Zn-4(OH)(2)(CO3)(3) . 4H(2)O was calculated accordingly. The suggestion is made that hydrated zinc hydroxycarbonate is metastable under the experimental conditions used here, but that it should transform into anhydrous carbonates.