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Phase transformation between calcite and aragonite isan importantissue in biomineralization. To shed more light on the mechanism ofthis process at the molecular level, we employ solid-state 43Ca NMR to study the phase transformation from calcite to aragoniteas regulated by magnesium ions, with 43Ca enrichment ata level of 6%. Using the gas diffusion approach, the phase of Mg-calciteis formed initially and the system subsequently transforms to aragoniteas the reaction time proceeds. Our 43Ca solid-state NMRdata support the dissolution-recrystallization mechanism for the calciteto aragonite transition. We find that the 43Ca NMR parametersof Mg-calcite are very similar to those of pure calcite. Under thehigh-resolution condition provided by magic-angle spinning at 4 kHz,we can monitor the variation of the 43Ca NMR parametersof the aragonite signals for the samples obtained at different reactiontimes. Our data suggest that in the presence of a significant amountof Mg2+ions, aragonite is the most stable polymorph ofcalcium carbonate. The initial precipitated crystallites of aragonitehave spine-like morphology, for which the 43Ca spin–latticerelaxation data indicate that the ions in the lattice have considerablemotional dynamics. As the crystallinity of aragonite improves further,the 43Ca T1parameter of thearagonite phase changes considerably and becomes very similar to thatobtained for pure aragonite. For the first time, the difference incrystal morphologies and crystallinity of the aragonite phase hasbeen traced down to the subtle difference in the motional dynamicsat the molecular level. [ABSTRACT FROM AUTHOR]