Monohydrocalcite (MHC, CaCO₃•H₂O) is a metastable calcium carbonate crystalline phase, which can be rarely found in nature compared to other anhydrous crystalline phases such as calcite and aragonite or even vaterite.¹ In laboratory, most of the experiments reported that MHC has been synthesized by the reaction of Ca²⁺ and CO₃^2- in the presence of Mg²⁺ when the Mg/Ca ratio was higher than 1, the results suggesting that Mg²⁺ played a key role in MHC crystallization.² Although there were a few exceptions found that MHC can be synthesized without the presence of Mg²⁺, the MHC crystal in these experiments behaved as an intermediated product and transformed into stable anhydrous calcium carbonate (calcite and aragonite) soon.^{3, 4}

In this study, we show that the precipitation of calcium carbonate in silica rich alkaline solution by the counterdiffusion method results in the formation of spherical multi-layers structures made only of MHC crystals. The in situ analysis by X-ray diffraction (XRD) and Raman spectroscopy showed unequivocally that MHC is the only phase forming these structures and that the as formed MHC can be stabilized in the absence of Mg²⁺ at least for the duration of our experiments (months). Video optical microscopy showed that the MHC start growing as  peanut-like crystals during the first week crystallization. Then, they develop into spherical structures with size of hundreds meters after 4 weeks growth. Further studies by field emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis (EDX) reveal that the multi-layers structure consisted of plenty of nano-rod and nano-sphere particles in different layers, the outer layer contains more silica than the former. These results suggested that silica played a key role in the formation of MHC multi-layer structure. The variation in time of pH and Ca²⁺ concentration during the crystallization was followed by pH microprobe and ICP-OE spectrometry, respectively. The time variation was correlated with the morphological evolution, as the crystallization of MHC can be divided into 4 stages according to the pH changes, namely: the initial MHC crystallization, the development from peanut-like structure to spherical structure, the formation of the multi-layers structure and the formation of silica outer skin.

According to our results, the formation of the multi-layers structure is thought to be driven by the different level of coupling between calcium carbonate and the coprecipitated silica in different stages of the crystallization, as well as the different silica content in nano-rod and nano-sphere particles. We propose that the precipitation of calcium carbonate decreased the local pH and thus triggered the coprecipitation of silica on the surface of the crystal. On the other hand, the precipitation of silica would increase the local pH again, which provoke the new nucleation and growth of calcium carbonate crystals. From the iteration of these two consecutive steps a multi-layer structure would result. Noticeably, we fail to get MHC precipitation in parallel experiments by either counter diffusion method without silica or classical mixing experiments with silica, which suggested: a) the key role of silica in the formation of MHC and b) the importance of the supersaturation rate in CaCO₃ in selecting the precipitation of the MHC polymorph.

1. T. Kimura and N. Koga, The Journal of Physical Chemistry A, 2011, 115, 10491-10501.
2. F. Keisuke, M. Takashi, S. Minoru and Y. Shintaro, Sci. Tech. Adv. Mater., 2011, 12, 064702.
3. E. T. Stepkowska, J. Therm. Anal. Calorim., 2005, 80, 727-733.
4. C. Jiménez-López, E. Caballero, F. J. Huertas and C. S. Romanek, Geochim. Cosmochim. Acta, 2001, 65, 3219-3231.

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