Me. Bottcher, THE FORMATION OF RHODOCHROSITE SMITHSONITE (MNCO3-ZNCO3) SOLID-SOLUTIONS AT 5-DEGREES-C, Mineralogical Magazine, 59(396), 1995, pp. 481-488
MnXZn((1-X))CO3 solid-solutions were prepared at 5 degrees C by precip
itation from metal-bearing bicarbonate solutions. The solids were iden
tified by X-ray powder diffraction and infrared spectroscopy. Zn2+ ion
s substitute extensively for Mn2+ ions in the crystal lattice of anhyd
rous rhombohedral carbonates. Throughout the 24 h during which the exp
eriments were conducted, the aqueous solutions remained undersaturated
with respect to pure oxides, sulphates, hydroxides and hydroxysulphat
es. 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 theo
retically 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 sugge
stion is made that hydrated zinc hydroxycarbonate is metastable under
the experimental conditions used here, but that it should transform in
to anhydrous carbonates.