Calcium carbonate, as one of the most abundant biominerals, has attracted considerable interest and therefore it has been deeply studied throughout the past years. Calcium carbonate exists in a variety of polymorphic forms, including three anhydrous crystalline polymorphs, calcite, aragonite and vaterite. Under ambient conditions, calcite is the most stable phase and forms rhombohedral carbonate structures. Although the orthorhombic aragonite phase is less stable than calcite, its formation is also common in natural mineralization processes. Vaterite is described by a hexagonal unit cell. It is the less stable anhydrous CaCO₃ phase, and preferably transforms into more stable phases. However, it is also considered as a key phase in biomineralization and biomimetic crystallization.¹

The common experimental pathway for biomimetic crystallization usually includes the presence of organic molecules, which has received an enormous amount of attention in the past.² However, biomimetic morphologies could also be achieved by pure inorganic crystallization. In fact, the crystallization of orthorhombic alkaline earth carbonates such as witherite (BaCO₃) and the strontianite (SrCO₃) in silica rich alkaline environment result a wide range of highly oriented self-assembled poly-nano-crystalline structures exhibiting non-crystallographic morphologies such as regular helicoids and filaments, which are reminiscent of the shape of living organisms. This kind of “life-like” orthorhombic (Ba or Sr) carbonates structures forming in pure inorganic silica-carbonate environments was called “silica biomorphs”.^3-6 However, in the case of calcium carbonate, the phase forming under classical alkaline conditions is calcite, the rhombohedral polymorph. This difference makes a critical change on the morphology of the structures obtained that creates dendrites and fractal like formations, while witherite and strontianite exhibit complex biomorphic formation under the same conditions. Biomorphic structures with complex curvilinear forms of silica-calcium carbonate were only detected when the formation of orthorhombic aragonite was induced by temperature and using strontium as additive.^4,7

In this study, the precipitation of calcium carbonate in silica gel under specific initial concentration and pH formed the biomorphic curvilinear structures at room temperature without the presence of any other additive. The crystallization procedure was performed by counter diffusion method in a lab made crystallization cell. The resulting biomorphic formation was characterized by X-ray diffraction (European Synchrotron Radiation Facility) and Raman microspectroscopy, and the results revealed that aragonite was the only crystalline phase. The growth behavior and morphological study was followed by optical microscopy and field emission scanning electron microscopy (FESEM). The crystal exhibited a petal-like structure with continuous smooth curvature and in some cases, regular helicoids were also observed. The detailed study by FESEM show that the biomorphic aragonite is a polycrystalline aggregates made of highly co-oriented nanorods with a size of hundreds of nanometer, thus similar to the case of barium and strontium biomorphs⁵. Notably, the biomorphic aragonite only occurred within a very specific narrow zone in the crystallization cell, inserted among the layers of elongated calcite and sheaf of wheat calcite.

1. F. C. Meldrum, Int. Mater. Rev., 2003, 48, 187-224.

2. F. C. Meldrum and H. Cölfen, Chem. Rev., 2008, 108, 4332-4432.

3. J. M. García-Ruiz, S. T. Hyde, A. M. Carnerup, A. G. Christy, M. J. Van Kranendonk and N. J. Welham, Science, 2003, 302, 1194-1197.

4. A. E. Voinescu, M. Kellermeier, B. Bartel, A. M. Carnerup, A.-K. Larsson, D. Touraud, W. Kunz, L. Kienle, A. Pfitzner and S. T. Hyde, Cryst. Growth Des., 2008, 8, 1515-1521.

5. M. Kellermeier, H. Cölfen and J. M. García-Ruiz, European Journal of Inorganic Chemistry, 2012, 2012, 5123-5144.

6. A. E. Voinescu, M. Kellermeier, A. M. Carnerup, A.-K. Larsson, D. Touraud, S. T. Hyde and W. Kunz, J. Cryst. Growth, 2007, 306, 152-158.

7. H. Imai, T. Terada, T. Miura and S. Yamabi, J. Cryst. Growth, 2002, 244, 200-205.

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