The formation of rhodochrosite-smithsonite (MnCO3-ZnCO3) solid-solutions at 5~ MICHAEL E. BOTTCHER* Geochemical Institute, Georg-August-University, Goldschmidtstr.1, D-37077 G6ttingen, Germany Abstract MnxZn(l_x)CO3 solid-solutions were prepared at 5~ by precipitation from metal-beating 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 MnxZno_x)CO3 of any given composition. Besides the anhydrous rhombohedral carbonates, Zn4(OH)2(CO3)3.4H20 was precipitated from an aqueous solution with initially high Zn2+ concentration. The negative logarithm of the solubility product of Zn4(OH)2(CO3)3.4H20 was estimated theoretically to be 43.9 (25~ Remaining saturation with respect to Zn4(OH)z(CO3)3-4H20 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. KEYwoems: rhodochrosite, smithsonite, solid-solutions, zinc hydroxycarbonate, synthesis. Introduction quantitative significance has probably previously been underestimated. However, although B6ttcher A nearly complete natural rhodochrosite-smithsonite et al. (1993) applied a tentative thermodynamic (MnCO3-ZnCO3) solid-solution series has been model to estimate the metal-activity ratios of the found in the oxidized zone of the ore body at aqueous solutions from which the Broken Hill Broken Hill, N.S.W., Australia (Birch, 1986; carbonates were precipitated, less is known on the B6ttcher et al., 1993; B6ttcher et al., in prep.). The formation conditions of rhodochrosite-smithsonite series was studied by X-ray powder diffraction, solid-solutions. Apart from its pure end-members, the electron microprobe, stable isotope techniques, system MnCO3-ZnCO3-H20 has not been hitherto infrared and Raman spectroscopy. Given their habit investigated experimentally. In the study described and their encrustation of concretionary goethite here, preliminary experiments were conducted to (~-FeOOH) and coronadite (Pb2Mn8Ol6), a low- synthesize rhodochrosite-smithsonite solid-solutions temperature formation of these carbonate phases was from aqueous solutions at 5~ proposed and confirmed by oxygen isotope measure- ments (B6ttcher et al., 1993). Methods The formation of rhodochrosite-smithsonite solid- solutions may be important for supergene reactions in Rhodochrosite-smithsonite solid-solutions were ore deposits and for the purification processes of synthesized by the addition of 100 ml of a 0.4 M industrial waters containing transition elements. Its KHCO3 solution (presaturated at 22~ with CO2 from a gas tank (99.995 %; Messer Griesheim)) to 100 ml of a 0.1 M (Mn,Zn)SO4 solution at 5~ The * Present address: Institute of Chemistry and Biology of the Marine Environment (ICBM), Microbiogeo- sulphate solutions were prepared under N2 by the chemistry, Carl von Ossietzky University, P.O. Box dissolution of reagent grade chemicals (Merck) in 2503, D-26111 Oldenburg, Germany deionized water previously degassed with N2, Mineralogical Magazine, September 1995, Vol. 59, pp. 481-488 Copyright the Mineralogical Society 482 M. E. BOTTCHER The precipitates were allowed to age undisturbed ZnCO3 (mol%) = 1097.7-(2.839 - d(1014)(A)) (1) in contact with the mother liquid at 5 _+ I~ in closed Duran | glass bottles (500 cm 3) for about The d(10T.4) values of rhodochrosite and smith- 24 h. At the end of each run, the pH of the solution sonite were calculated from data given by was measured under air-tight conditions by an ion- Effenberger et al. (1981). Based on the results of selective combination micro-electrode (Ingold 10 BOttcher et al. (1993), a linear variation of the v4 402 3522 U402-M3-S7/60) and a digital pH-meter absorption maximum with increasing substitution of (Knick Portamess 645). The accuracy was _ 0.02 Mn 2§ by Zn 2§ was assumed to obey the following pH units. The electrode had been calibrated with relationship: buffers 4.00 and 6.88 (Riedel de H~ien). Immediately ZnCO3 (mol%) = 5.88-(v4(cm -1) - 725.5) (2) after the pH measurements, the solids were separated from the solutions by membrane filtration using a The v4 data for rhodochrosite and smithsonite Teflon| N2-pressure filtration-apparatus and were taken from B6ttcher et al. (1992) and BOttcher polyacetate filters (0.45 pm porosity; Sartorius). et al. (1993), respectively. The estimated composi- Finally they were washed with methanol and dried tions are believed to be accurate to within ___ 5 mol%. at 60~ After appropriate dilution and the addition of CsC1 The mineral phases in the precipitates were as an ionization buffer, the final aqueous solutions identified by X-ray powder diffraction (Philips- were analysed for Mn 2§ and Zn 2+ by atomic XRD goniometer and Ni-filtered Cu-Ket radiation) absorption spectrophotometry (Perkin Elmer 400) and infrared spectroscopy (683 Perkin Elmer spectro- using an air-acetylene flame. K + and Mn 2§ or Zn 2+ photometer; KBr-pellet technique). The composition AAS-standard solutions (Titrisol | Merck) were of the synthetic MnxZn(l_x)CO3 solid-solutions was added to the blanks and aqueous standards to match estimated from the position of the d(10i4) peak in the the matrix composition of the diluted run solutions. X-ray diffractogram and the v4 absorption band The accuracy of the analyses was estimated to be position in the infrared spectra. The d(1014) spacing better than ___ 5 %. The sum of the dissolved was assumed to bear a linear relationship with the carbonate species (EC) and the partitioning of the chemical composition according to the following individual species were calculated from the measured expression: pH values and the cation concentrations, based on an TABLE 1. Chemical compositions of aqueous solutions and final precipitates. X~n, XMn: mole fraction of the aqueous solution and MnxZn(l_x)CO 3 solid-solution, respectively Run #1 #2 #3 Initial solution: pH 7.41 7.41 7.41 K § (mmol/1) 200 200 200 Mn 2+ (mmol/1) 50.0 37.5 25.0 Zn 2+ (mmol/l) 0 12.5 25.0 EC (mmol/1) 215 215 215 SO2- (mmol/l) 50 50 50 X~n 100 0.75 0.50 Final solution: pH 6.78 6.81 6.79 Mn 2+ (mmol/1) 5.6 4.5 3.3 Zn 2+ (mmol/l) 0 1.5 2.7 EC (mmol/1) 154 152 153 X~an 1.00 0.75 0.55 Final precipitate: XMn 1.00 0.68 (-t-0.2) 0.51 (+0.I)* *: Minor Zn4(OH)2(CO3)3.4H20 was identified by XRD and IR. RHODOCHROSITE-SMITHSONITE SERIES 483 electron balance approach described by Plummer and zinc and 48% of the total dissolved manganese Busenberg (1982). The distribution and activities of existed as free Zn2+ and Mn2+, respectively. The the dissolved species were calculated with the concentration of dissociated species is therefore Solmineq.88 PC/Shell computer program (Kharaka slightly increased with respect to the initial solutions. et al., 1988; Wiwchar et al., 1988) based on an ion- This is due to the influence of decreasing pH and the pair model described by Kharaka et al. (1988). The total ion's concentrations on the partitioning of the dissociation constants at 5~ of MnHCO~ and dissolved species. At the end of the experiments and ZnHCO~ are from Lesht and Bauman (1978) and after a reaction time of 24 h, the aqueous solutions Ryan and Bauman (1978). For ZnSO~ and the zinc were still supersaturated with the anhydrous carbo- hydroxy complexes they are from the Solmineq.88 nates, but saturation (run #3) and even under- database. The values for MnOH§ MnSO~ were saturation (run #2) with hydrozincite and calculated according to an isocoulombic approach Zn4(OH)2(COa)3.4H20, was achieved for the Zn2+ (B6ttcher and Usdowski, 1990). The equilibrium bearing solutions (Table 2; Fig.l). constants for the CO2-H20 system are from Plummer It should be noted that the solubility product of and Busenberg (1982). hydrozincite used in the present calculations was measured on synthetic material by Schindler et al. Results and discussion (1969). The results should be valid for the present experiments. Natural hydrozincite specimens were The experimental conditions and analytical results found experimentally to be less soluble than synthetic are compiled in Table 1. materials (Zachara et al., 1989). These differences have been attributed to structural characteristics, the degree of hydration and/or solids stoichiometric Saturation calculations A thermodynamic analysis of the initial run solutions -6 indicated the presence of significant supersaturations #3 #2 #1 with respect to pure rhodochrosite (runs #1-#3), and smithsonite, Zn4(OH)2(CO3)3.4H20 (see below) and -7 hydrozincite (runs #2 and 3). The solution however, was undersaturated with respect to pure oxides, sulphates, hydroxides and hydoxysulphates (Table 2). -8 - C~ A-:::, :""~ ,,,,/,,,~ According to the calculated species distribution, 39% / of the total dissolved zinc and 43% of the total 0 n //- dissolved manganese concentrations (Table 1) were -9 initially present as Zn2§ and Mn2+, respectively. From the Lippmann phase diagram in Fig. 1 it can be seen that the initial aqueous solutions were also -tO _-..... 23~::~ ....... <So.d.. [ supersaturated with respect to MnxZno_x)CO3 solid- Solutus ......... ...... .-'.7~>_~ solutions of any given composition. The phase I diagram was constructed according to Glynn and -ll .... I ,,,, i,,, , I, ,,,I,,,,) Reardon (1990), and represents the equilibrium 0.0 0.2 0.4 0.6 0.8 1.0 conditions for the system MnCO3-ZnCO3-H20 at XMn; XMn,aq 5~ assuming ideal mixing behavior for the rhodochrosite-smithsonite solid-solutions. As a result of the precipitation of carbonates and FIG.