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This study reports the nature of the nesquehonite-to-hydromagnesite transition at 52°C in an aqueous medium hosting magnesian calcite and nesquehonite. The latter mineral occurs with abundant calcite at the floor of the experimental chamber (substrate) and as a film of needles at the interface between the mother liquor and atmosphere (surface film). The experimental vessel was held at 52°C for 336 h and at 60°C for a further 192 h. Precipitates were analyzed by Fourier transform (FT)-Raman, augmented by FT-infrared and x-ray diffraction. At 52°C, hydromagnesite and dypingite occur with abundant quantities of a hitherto unreported transitory magnesium hydrate carbonate (TMHC), together with huntite, magnesian calcite, and traces of nesquehonite and monohydrocalcite. The FT-Raman spectra of the first-formed hydromagnesite crystals contain the Raman forbidden v 2 mode, interpreted to indicate a relaxation in selection rules, caused by rapid precipitation. Hydromagnesite growth at the expense of TMHC was more advanced in the substrate than in the coexisting surface film. Additional heating at 60°C resulted in the loss of TMHC and emergence of a dypingite-and hydromagnesite-rich assemblage, with associated strengthening of selection rules. Transitory magnesium hydrate and hydroxyl carbonates and huntite formed during CO 2 degassing, fueled by the thermally driven decrease in solubility of CO 2 in water and the progressive dissolution of metastable phases. Advancement of the N→HM transition in the substrate most likely reflects greater Ca 2þ ðaqÞ availability to promote acid generation through calcite precipitation, thereby accelerating transitory-phase dissolution.

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