Curving crystal clusters

Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaflike sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how. Juan Manuel García-Ruiz , his postdoc Emilio Melero-García (both at the University of Granada), and Stephen Hyde (Australian National University) now propose a mechanism for the morphogenesis. As the carbonate crystallizes, it lowers the pH of the local environment and triggers the precipitation of silica. The silica precipitation, in turn, raises the local pH, which prompts another round of carbonate formation. The sensitivity of silica and carbonate species to opposite trends in pH fluctuation creates a chemical feedback that eventually produces rodlike, carbonate particles, each coated with silica. Freed from the hexagonal symmetry restrictions imposed by carbonate growth, the silica-coated nanoparticles form clusters that can adopt various shapes. For reasons unexplained by their mechanism, the clusters align themselves on the micron scale and grow as two-dimensional sheets. The edges of those sheets can then curl like a scroll to create the sort of curved and twisted filaments captured here by optical microscopy. (J. M. García-Ruiz, E. Melero-García, S. T. Hyde, Science 323, 362, 2009 .) — R. Mark Wilson