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Lindner, Michael; Saldi, Giuseppe D.; Jordan, Guntram; Schott, Jacques (June 2017): On the effect of aqueous barium on magnesite growth – A new route for the precipitation of the ordered anhydrous Mg-bearing double carbonate norsethite. In: Chemical Geology, Vol. 460: pp. 93-105
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Abstract

The easiness of norsethite [BaMg(CO3)2] precipitation at ambient conditions is in vast contrast to the growth problems of other anhydrous Mg containing carbonate minerals such as magnesite [MgCO3] and dolomite [CaMg(CO3)2]. The striking structural similarity of dolomite and norsethite makes the different growth behavior even more noteworthy. In order to investigate this remarkable discrepancy, this study focuses on the fate and behavior of aqueous barium during growth of magnesite as magnesium endmember in the BaCO3-MgCO3 system. Knowledge of barium partitioning between carbonate minerals and solution, moreover, has important implications for the assessment of the mobility of toxic elements such as radium. Growth experiments have been conducted on magnesite seeds in hydrothermal mixed-flow reactors (T = 100 °C, pH ~ 7.8, 0–100 μM Ba2+, supersaturations Ω with respect to magnesite: ~ 100–200) and by hydrothermal atomic force microscopy (T = 100 °C, pH ~ 8.2, 0–50 μM Ba2+, Ω ~ 60–90). The experiments showed that aqueous barium leaves magnesite growth rates unaffected but leads to norsethite precipitation. At the conditions of the experiments, norsethite growth rates were found to be controlled by the aqueous Ba2+ concentration. Given enough Ba2+, Mg2+ withdrawal from solution by norsethite clearly exceeded the withdrawal by magnesite growth. The logarithm of the solubility product of norsethite was found to be lower than − 18.2 at 100 °C. Microscopic investigations of the growth on the (104) surface of magnesite did not reveal any signs of Ba2+ incorporation yielding a partitioning coefficient of barium between magnesite and solution in the range of 10−2 or smaller. Preferential formation of the ordered phase norsethite over a solid solution is presumably facilitated by the large difference in Mg2+ and Ba2+ ionic radii. Due to the likely very high free energy of formation of the solid solution, ordering into distinct Ba- and Mg-layers is the only way to combine both cations within one phase. The lack of a solid solution, therefore, is one important prerequisite for the direct and unimpeded growth of an ordered double carbonate from additive free solution at ambient conditions. This behavior is in great contrast to the CaCO3-MgCO3 system where solid solution occurrence is common and growth of the ordered double carbonate (i.e. dolomite) is largely inhibited below 120 °C.