Thermodynamics and mechanisms of mineral growth: baseline for understanding biologically mediated processes.
Researchers H. Henry Teng and Patricia M. Dove of Georgia Institute of Technology, with Christine A. Orme and James J. DeYoreo of Lawrence Livermore National Laboratory, have investigated the effects of organic constituents in controlling energetic factors governing the precipitation and growth of calcium carbonate (as calcite) from aqueous solutions (Science, 282:5389, 724, 1998) . With atomic force microscopy, they measured the dependence of spiral growth step length and speed on the degree of supersaturation, as a test of the classic Gibbs-Thomson theory for equilibrium crystal growth. Because the step lengths follow predicted inverse dependence on supersaturation, transport of the components along the steps must be sufficiently rapid to replenish the fluid adjacent to the growing crystal. However, the measured speed of step growth exceeds that predicted by classical theory, consistent with rate control by anisotropy in step-edge energies and in the step cornering angles. The investigators have further shown that the surface-energy of the steps is modified by aspartic acid, a common component of biomineralizing systems, which modifies both the kinetics of step growth and the equilibrium morphology of calcite growth hillocks. These factors are important for understanding the presence and stabilization of metastable crystal morphologies, and metastable calcium carbonate minerals, that grow in biologically mediated systems. Teng and Dove's preliminary work to measure effects of aspartic acid on dissolution of calcite, published in the American Mineralogist, 82:878, 1997; was honored at the recent 1998 Annual Meeting of the Geological Society of America with the inaugural Best Paper Award by the Mineralogical Society of America.
