Materials Science

Confined Stability

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Science  04 Apr 2014:
Vol. 344, Issue 6179, pp. 10-11
DOI: 10.1126/science.344.6179.10-d

Of the three polymorphs of calcium carbonate, vaterite is less stable than either calcite or aragonite and is difficult to obtain in single-crystal form without the use of additives. It is generally not seen in geologic materials, but there are a few cases where it appears as a biomineral. Schenk et al. synthesized CaCO3 crystals inside track-etched coated polycarbonate membranes under a range of conditions. Under high concentrations of Ca2+, precipitation of vaterite occurred both on the surface of the membranes and within the pores, but it was polycrystalline, whereas at low Ca2+, little infiltration of the pores was observed. At intermediate values, however, the authors observed primarily single-crystal rods of vaterite within the pores and predominately calcite on the membrane surfaces. The vaterite crystals also proved to be stable for several days, even when exposed to the reaction solutions. The results are somewhat complicated, because vaterite crystallization was not observed in the pores of similarly treated membranes from a different supplier. The authors speculate that there probably are subtle differences in the microenviroments of the pores in the two different membranes and, in particular, that the vaterite-growing membranes possess a surface coating that better collects and organizes the Ca2+ ions, thus stabilizing the formation of vaterite.

Chem. Commun. 10.1039/C4CC01093K (2014).

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