Calcium carbonate’s two common forms, calcite and aragonite, are widely found in nature—most notably, in the shells of mollusks and sea snails. Aragonite’s abundance is puzzling, though. Despite being only slightly less stable than calcite, aragonite almost never crystallizes from solution in ambient conditions. Researchers seeking the secret to aragonite biomineralization have examined several possibilities—the presence of proteins, scaffold molecules, and organic additives, for instance—all of which can influence nucleation processes. Fiona Meldrum and her colleagues at the University of Leeds in the UK have now shown that the secret may be much simpler: confinement. They found that aragonite crystallizes inside submicron-diameter pores of arbitrary depth without any special additives and in amounts that depend only on the diameter of the pore.
The finding emerged from dozens of experiments using pores created by accelerating heavy ions and shooting them through polycarbonate films. In different-sized pores the researchers precipitated calcium carbonate...
