On the effect of aqueous barium on magnesite growth – A new route for the precipitation of the ordered anhydrous Mg-bearing double carbonate norsethite
On the effect of aqueous barium on magnesite growth – A new route for the precipitation of the ordered anhydrous Mg-bearing double carbonate norsethite
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DOI：
10.1016/j.chemgeo.2017.04.019
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发表时间：
2017-06
期刊：
影响因子：
3.9
通讯作者：
M. Lindner;G. Saldi;Guntram Jordan;J. Schott
中科院分区：
文献类型：
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作者：
M. Lindner;G. Saldi;Guntram Jordan;J. Schott
与其他含镁碳酸盐矿物，如碳酸镁和白云石[CaMG(CO3)2]的生长问题形成鲜明对比的是，它在环境条件下很容易析出。白云石和诺森石在结构上的惊人相似性使其不同的生长行为更加值得注意。为了研究这种显著的差异，本研究重点研究了BaCO3-MgCO3体系中作为镁端元的菱镁矿生长过程中，水中Ba2+的去向和行为。在水热混流反应器中对菱镁矿种子进行了生长实验(T=约100℃，pH=7.8，0~100μM Ba2+，相对于菱镁矿的过饱和Ω：~100~200)，并用水热原子力显微镜(T=约100°C，pH=~8.2，0~50μM Ba2+，Ω=~60~90)进行了生长实验。实验表明，含水的钡叶对菱镁矿的生长速率没有影响，但导致了菱镁矿的析出。在实验条件下，水相Ba2+的浓度控制着诺森石的生长速度。在有足够Ba2+的情况下，镁铁矿从溶液中的析出量明显大于菱镁矿的析出量。在100℃时，钠镁矿的溶度积的对数小于−的18.2。对菱镁矿(104)表面生长的微观研究没有发现任何Ba2+掺入的迹象，使Ba2+在菱镁矿和溶液之间的分配系数在10−2或更小的范围内。镁铁矿比固溶体更有利于有序相的形成，可能是由于镁离子和Ba2+离子半径的巨大差异。由于固溶体可能有很高的形成自由能，有序成不同的Ba层和Mg层是将这两个阳离子结合在一个相中的唯一途径。因此，缺少固溶体是在环境条件下从无添加剂溶液中直接和畅通无阻地生长有序双碳酸酯的一个重要先决条件。这一行为与CaCO3-MgCO3体系形成鲜明对比，在该体系中，固溶体普遍存在，在120℃以下，有序双碳酸盐(即白云石)的生长受到很大程度的抑制。
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-MgCO3system. 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 10−2or 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-MgCO3system where solid solution occurrence is common and growth of the ordered double carbonate (i.e. dolomite) is largely inhibited below 120 °C.