CALCITE AND ARAGONITE PRECIPITATION UNDER CONTROLLED INSTANTANEOUS SUPERSATURATION: ELUCIDATING THE ROLE OF CACO3 SATURATION STATE AND Mg/Ca RATIO ON CALCIUM CARBONATE POLYMORPHISM
CALCITE AND ARAGONITE PRECIPITATION UNDER CONTROLLED INSTANTANEOUS SUPERSATURATION: ELUCIDATING THE ROLE OF CACO3 SATURATION STATE AND Mg/Ca RATIO ON CALCIUM CARBONATE POLYMORPHISM
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DOI：
10.2110/jsr.2009.043
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发表时间：
2009-05-01
影响因子：
2
通讯作者：
Gonzalez, Luis A.
中科院分区：
文献类型：
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作者：
De Choudens-Sanchez, Vionette;Gonzalez, Luis A.
几十年来，研究人员一直试图弄清控制碳酸钙多晶型沉淀的因素，以及在自然体系中方解石沉淀中镁的掺入。在这项研究中，一种新的实验方法描述了瞬时过饱和度和溶液镁/钙比在碳酸钙矿物学和成分中的作用。研究结果表明，在不同镁钙比的溶液中，文石与方解石+文石沉淀场之间的转变受溶液相对于碳酸钙的饱和状态和溶液中镁钙比的共同控制。结果表明，随着溶液镁钙比的增加，方解石的析出需要逐渐达到较高的过饱和度，高溶液镁钙比和低过饱和度时文石的优势归因于在较高溶液镁钙比下方解石晶格中镁离子的掺入增加而导致方解石生长速率的相对下降。大量的实验工作表明，增加溶液的镁钙比会降低方解石的生长速度，而文石的生长速度不受影响。我们的实验确实表明，尽管生长速度降低，但方解石中的碳酸镁含量随着溶液镁钙比的增加而增加。随着方解石生长速率的降低，文石的生长速率保持不变，在高镁钙比、低过饱和度的溶液中成为主要矿物相。这些发现与当今碳酸盐体系中的矿物学分布有关，可能是阐明导致矿物学随地质时代变化的环境条件的关键。碳酸钙矿物学对溶液组成和过饱和度的依赖关系表明，碳酸盐矿物学受生长动力学控制。
For many decades researchers have attempted to clarify the factors controlling the precipitation of calcium carbonate polymorphs and the incorporation of Mg into calcite precipitating in natural systems. In this study a novel experimental approach delineates the role of instantaneous supersaturation and solution Mg/Ca ratio in calcium carbonate mineralogy and composition. The results of this study show that in solutions with different Mg/Ca ratios, the transition between the aragonite and the calcite + aragonite precipitation fields is controlled by a combination of the saturation state of the solution with respect to CaCO3 and the Mg/Ca ratio in solution. The data show that for increasing solution Mg/Ca ratios, progressively higher supersaturation level is required for calcite precipitation.The predominance of aragonite at high solution Mg/Ca and low supersaturations is attributed to a relative decrease in calcite growth rates as a result of increasing incorporation of Mg2+ in the calcite lattice at higher solution Mg/Ca ratios. As suggested by abundant experimental work, increasing solution Mg/Ca decreases the growth rate of calcite while aragonite growth rates stay unaffected. Our experiments do demonstrate that despite decreased growth rate the percentage of MgCO3 in calcite increases with increasing solution Mg/Ca ratio. As calcite growth rates decrease, aragonite growth rates stay constant and it becomes the dominant mineral phase in solutions with high Mg/Ca ratio and low supersaturations.These findings are relevant to present-day mineralogical distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogic changes through geologic time. The observed dependence of calcium carbonate mineralogy on solution composition and supersaturation suggests that carbonate mineralogy is controlled by kinetics of growth.