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.

Kutnohorite with moderate and bright orange-red cathodoluminescence (CL) was studied by CL microscopy and spectroscopy. This mineral was found in fossiliferous concretions composed mainly of rhodochrosite from the Mn-carbonate mineralization at Úrkút, Hungary. The CL microscopy reveals that kutnohorite occurs as impregnations, layers and veinlets. X-ray diffraction, infrared spectroscopy and electron microprobe studies indicate that the luminescent kutnohorite has excess Ca (72.9–80.0 mol.% CaCO3, 16.3–20.5 mol.% MnCO3, 3.3–5.6 mol.% MgCO3 and 0.0–0.5 mol.% FeCO3). Transmission electron microscopy shows that the luminescent carbonate has a dolomite-type structure, with modulated and mosaic microstructures. The CL spectra of this Ca-rich kutnohorite have a single emission band at 630 nm that is characteristic of Mn2+ substitution in the structure. Our results provide evidence for moderate-to-bright cathodoluminescence of Mn-rich natural carbonates even at 8–10 wt.% Mn and up to 2400 ppm Fe. The self-quenching of Mn appears incomplete in the case of Ca-rich kutnohorite from Úrkút.

Rhodochrosite has been discovered as a minor component of two manganese deposits in Scotland. At Islay it occurs as coatings and veinlets on and within hollandite-pyrolusite-hematite-goethite mineralisation. Two foram microfossils were found within the carbonate which is considered to have formed from the interaction of organic-rich groundwater and seawater. The Dalroy rhodochrosite has the form of stromatactis-like cavity infill structures and veinlets in a mineralogically complex manganese-iron ore in which braunite, bustamite, manganoan ilmenite and pyrophanite have been identified. Intergrowths of rhodochrosite and native copper grains occur in a calcite matrix within the infill structures. These intergrowths are typically one micrometre across, but achieve diameters of up to 50 µm. The genesis of the Dalroy rhodochrosite is more problematical, although a hydrothermal source is most likely.

The high-pressure behaviour of manganese carbonate was investigated by in situ synchrotron X-ray powder diffraction up to 54 GPa with a laser-heated diamond anvil cell. A phase transition from rhodochrosite to a new structure form was observed at 50 GPa after laser heating. The diffraction pattern of the new high-pressure form was reasonably indexed with an orthorhombic unit-cell with a = 5.361 A, b = 8.591 A and c = 9.743 Å. The pressure-induced phase transition implies a unit-cell volume reduction of ∼5%. This result does not support the direct formation of diamond by dissociation of solid state MnCO3 reported in a previous study. Fitting the compression data of rhodochrosite to a second-order Birch-Murnaghan equation of state (Ko’ = 4) gives K0 = 126(±10) GPa. The c axis of the unit-cell parameter was more compressive than the a axis.

Specimens of honey-brown to pinkish-brown globular carbonates encrusting concretionary goethite–coronadite from the oxidized zone at Broken Hill, New South Wales, have compositions in the rhodochrosite–smithsonite series. This may be the first extensive natural occurrence of this solid-solution series. Growth of the carbonates occurred in zones which have near uniform composition. The ratio MnCO3/(MnCO3 + ZnCO3) for each zone bears a linear relationship to the measured d spacing for the 104 X-ray reflections. Because cerussite is the only other mineral associated with the Zn-Mn carbonates and because of an absence of detailed locality information, the paragenetic significance of these minerals cannot be determined. The solutions depositing them may have been derived from the near-surface equivalents of the Zinc Lode horizons.

Minute CaMg-bearing rhodochrosite inclusions were discovered in spessartine crystals of small coticule globules enclosed within redschists that form the country rock of the famous coticule bands of Belgium. This points to a former metamorphic reaction relationship between the two minerals, such as rhodochrosite + pyrophyllite = spessartine + quartz + CO2 + H2O. The coexistence of spessartine with Mg-chlorite in the coticule globule could also indicate more complex reactions such as rhodochrosite + sudoite + quartz = spessartine + Mg-chlorite + CO2 + H2O. Our finding casts some doubt on the suggestion by Kramm (1976) that spessartine formed from Mn-montmorillonite in previously glassy volcanic tufts.

The nucleation and growth behaviour of the (Mn,Ca)CO3 solid solution from aqueous solutions in silica hydrogel were studied at 25ºC. In order to obtain different levels of supersaturation at nucleation, experiments were carried out in diffusion columns of different lengths. Zoning phenomena, compositional heterogeneity and morphological modifications of the crystals obtained were examined by electron microprobe and scanning electron microscopy and microanalyses. The large difference between the solubility of calcite (CaCO3) and rhodochrosite (MnCO3) implies a significant preferential partitioning of Mn towards the solid. The distribution of Mn2+ and Ca2+ ions between the aqueous and solid phase demonstrates that at elevated supersaturation, this preferential partitioning is softened. The morphologies of experimentally grown individuals show a clear dependence on the composition of the parent solution. Crystals obtained from aqueous solutions with large Mn contents exhibit a relatively sharp concentric zoning along the equatorial sections. On these bases, the roles of multiple kinetic and thermodynamic factors during nucleation and growth are also discussed. Finally, an X-ray diffraction analysis of the precipitates provided no evidence of the formation of the dolomite-type ordered kutnahorite [CaMn(CO3)2] in any of the experimentalsets.