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Testing the cation-hydration effect on the crystallization of Ca–Mg–CO 3 systems

Jie Xu, Chao Yan, Fangfu Zhang, Hiromi Konishi, Huifang Xu and H. Henry Teng
Proceedings of the National Academy of Sciences of the United States of America
Vol. 110, No. 44 (October 29, 2013), pp. 17750-17755
Stable URL: http://www.jstor.org/stable/23754384
Page Count: 6
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Abstract

Dolomite and magnesite are simple anhydrous calcium and/or magnesium carbonate minerals occurring mostly at Earth surfaces. However, laboratory synthesis of neither species at ambient temperature and pressure conditions has been proven practically possible, and the lack of success was assumed to be related to the strong solvation shells of magnesium ions in aqueous media. Here, we report the synthesis of MgCO 3 and Mg x Ca (1–x) CO 3 (0 < × < 1) solid phases at ambient conditions in the absence of water. Experiments were carried out in dry organic solvent, and the results showed that, although anhydrous phases were readily precipitated in the water-free environment, the precipitates' crystallinity was highly dependent on the Mg molar percentage content in the solution. In specific, magnesian calcite dominated in low [Mg 2+ ]/[Ca 2+ ] solutions but gave way to exclusive formation of amorphous Mg x Ca (1–x) CO 3 and MgCO 3 in high-[Mg 2+ ]/[Ca 2+ ] and pure-Mg solutions. At conditions of [Mg 2+ ]/[Ca 2+ ] = 1, both nanocrystals of Ca-rich protodolomite and amorphous phase of Mg-rich Mg x Ca (1–x) CO 3 were formed. These findings exposed a previously unrecognized intrinsic barrier for Mg 2+ and CO 3 2− to develop long-range orders at ambient conditions and suggested that the long-held belief of cation-hydration inhibition on dolomite and magnesite mineralization needed to be reevaluated. Our study provides significant insight into the long-standing "dolomite problem" in geochemistry and mineralogy and may promote a better understanding of the fundamental chemistry in biomineralization and mineral-carbonation processes.

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