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 < x < 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+ (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.sedimentary geology | carbon sequestration | nonaqueous solvent R arely is there a geological challenge that has endured as long a search for answers as the "dolomite problem" has. Identified more than 220 y ago by the French mineralogist Déodat de Dolomieu, the calcium magnesium carbonate mineral known as dolomite [CaMg(CO 3 ) 2 ] has since been synthesized repeatedly but exclusively at high-temperature and, in some instances, highpressure conditions (1-4). However, in many cases other than igneous-originated carbonatite and deep-burial dolomitization, the formation of dolomitic phases (e.g., microbial and cave deposits) are certainly not associated with such high-temperature and/or -pressure environments (5-8). This obvious discrepancy, compounded by the sharp contrast of massive ancient dolomite formations to the scarcity of modern observations, prompted an arduous search for answers for the past two centuries and, because of a lack of success, has been referred to as the dolomite problem (6, 9-16). Our inability to form dolomite at ambient conditions is hardly the only challenge in mineralogy; a similar situation exists in the case of pure magnesium carbonate [magnesite (MgCO 3 )], which has posed the same level of difficulty to nucleate and grow at room temperature and atmospheric pressure in laboratories but frequently occurs in natural sedimentary settings at earth (sub)surfaces. Interestingly, both dolomite and magnesite belong to the mineral class of anhydrous carbonates. Thus, it becomes logical to surmise that the dolomite problem and the magnesite problem probably share the same root, that is, the difficulty to incorporate unhydrated magnesium ions i...

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