On the growth of witherite and its replacement by the Mg-bearing double carbonate norsethite: Implications for the dolomite problem
On the growth of witherite and its replacement by the Mg-bearing double carbonate norsethite: Implications for the dolomite problem
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DOI：
10.2138/am-2018-6232
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发表时间：
2018-01
影响因子：
3.1
通讯作者：
M. Lindner;Guntram Jordan
中科院分区：
文献类型：
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作者：
M. Lindner;Guntram Jordan
Witherite [BaCO3]和norsethite [bang (CO3)2]分别被认为是文石[CaCO3]和白云石[CaMg(CO3)2]的化学和结构类似物。然而，与白云石不同的是，norsethite在环境条件下很容易从水溶液中析出。这是特别有趣的，因为Mg2+的脱水屏障可能是白云石生长抑制的一个可能原因。北石生长的易性表明白云岩的形成问题更为复杂。为了全面了解BaCO3-MgCO3模拟体系和有序无水含镁双碳酸盐的形成，我们研究了镁在辉石生长过程中的命运和行为。在50°C、不同mg浓度（0.25-2 mM Ba2+、0-20 mM Mg2+、pH 7.8-8.5、离子强度0.1 M）的混流反应器中对萎凋石种子进行生长实验。在Mg:Ba比小于6:1的溶液中，Mg2+对辉石生长动力学没有影响。未掺入大量的Mg2+。首次测定了赤铁矿生长的速率常数k和反应阶数n （k = 0.65±0.05 × 10-7 mol m-2 s-1; n = 1.3±0.1；过饱和度Ω = IAP/Ks = 1-4，其中IAP为离子活性产物，Ks为溶解度常数）。衰辉石生长动力学对这些Mg水平的不敏感与文石生长相似。然而，在整个BaCO3-MgCO3体系中普遍不存在固溶体的形成，而CaCO3-MgCO3体系却不存在这种情况，因为众所周知，六重配位阳离子位点的取代广泛发生。溶液中Mg:Ba比大于12:1导致威锡矿被诺西铁矿取代。这种替代也与CaCO3-MgCO3体系形成强烈对比，后者需要更高的温度和/或更长的时间尺度才能获得白云石。据估计，在50°C时，威辉石的替代速度比在60°C下观察到的白云石在7年内对文石的类似替代速度快约200倍（Usdowski 1989）。我们推测Mg2+和Ba2+离子半径的巨大差异促进了有序硝石在固溶体上的优先形成。由于固溶体的形成自由能可能非常高，排列成不同的Ba和mg层是在一个相内结合两种阳离子的唯一方法。在CaCO3-MgCO3体系中，固溶体的存在是常见的，并且在广泛的条件下有效地有助于抑制有序双碳酸盐白云岩的形成（cf. Arvidson and Mackenzie 1999）。
Abstract Witherite [BaCO3] and norsethite [BaMg(CO3)2] are perceived as chemical and structural analogs of aragonite [CaCO3] and dolomite [CaMg(CO3)2], respectively. However, norsethite, unlike dolomite, readily precipitates from aqueous solutions at ambient conditions. This is of special interest as the dehydration barrier of Mg2+ may be a likely cause of the dolomite growth inhibition. The easiness of norsethite growth shows that the problem of dolomite formation is more complex. To attain a comprehensive understanding of the analog BaCO3-MgCO3 system and of the formation of ordered anhydrous Mg-bearing double carbonates, we investigated the fate and behavior of aqueous magnesium during growth of witherite. Growth experiments were conducted on witherite seeds in mixed-flow reactors at 50 °C and various Mg-concentrations (0.25–2 mM Ba2+, 0–20 mM Mg2+, pH 7.8–8.5, ionic strength 0.1 M). At Mg:Ba ratios in solution smaller than 6:1, Mg2+ did not affect witherite growth kinetics. No significant amount of Mg2+ was incorporated. The rate constant k and reaction order n for witherite growth were determined for the first time (k = 0.65 ± 0.05 × 10-7 mol m-2 s-1; n = 1.3 ± 0.1; supersaturation Ω = IAP/Ks = 1–4, where IAP is the ionic activity product and Ks the solubility constant). The insensitivity of witherite growth kinetics to these levels of Mg is analogous to aragonite growth. The general absence of the formation of solid solutions in the entire BaCO3-MgCO3 system, however, is not shared by the CaCO3-MgCO3 system, for which it is well known that substitution in the sixfold-coordinated cation sites occurs extensively. Mg:Ba ratios in solution larger than 12:1 led to a replacement of witherite by norsethite. This replacement also is in strong contrast to the CaCO3-MgCO3 system, where higher temperatures and/or much longer timescales are necessary to obtain dolomite. The replacement rate of witherite at 50 °C was estimated to be ~200 times faster than the analogous replacement of aragonite by dolomite observed over 7 years at even 60 °C (Usdowski 1989). We speculate that the preferential formation of ordered norsethite over a solid solution is facilitated by the large difference in Mg2+ and Ba2+ ionic radii. Due to the presumably 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. In the CaCO3-MgCO3 system, solid solution occurrence is common and effectively contributes to the inhibition of the formation of the ordered double carbonate dolomite over a wide range of conditions (cf. Arvidson and Mackenzie 1999).