The Formation of Rhodochrosite-Smithsonite (MnCO3-ZnCO3) Solid-Solutions at 5°C

Michael E. Böttcher*
Geochemical Institute, Georg-August-University, Goldschmidtstr.1, D-37077 Göttingen, Germany
*Present address: Institute of Chemistry and Biology of the Marine Environment (ICBM), Microbiogeo-chemistry, Carl von Ossietzky University, P.O. Box 2503, D-26111 Oldenburg, Germany

Abstract: MnxZn(1−x)CO3 solid-solutions were prepared at 5°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, Zn4(OH)2(CO3)3·4H2O was precipitated from an aqueous solution with initially high Zn2+ concentration. The negative logarithm of the solubility product of Zn4(OH)2(CO3)3·4H2O was estimated theoretically to be 43.9 (25°C). Remaining saturation with respect to Zn4(OH)2(CO3)3·4H2O 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.

Keywords: rhodochrosite • smithsonite • solid-solutions • zinc hydroxycarbonate • synthesis

Mineralogical Magazine; September 1995 v. 59; no. 396; p. 481-488; DOI: 10.1180/minmag.1995.059.396.09
© 1995, The Mineralogical Society
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