Local autocatalytic co-precipitation phenomena in self-assembled silica-carbonate materials.

Under certain conditions, precipitation of barium carbonate in alkaline silica-rich environments affords unusual polycrystalline aggregates exhibiting curved shapes and hierarchical structuring, very much reminiscent of biogenic mineral tissues. The formation of these so-called silica biomorphs is thought to rely on a coupling of chemical equilibria in solution, which drives concerted co-mineralization and self-assembly of components. In the present work, we have studied the effect of fluid motion on morphogenesis by conducting syntheses in media stirred at different rates and exposed to an ultrasonic field, respectively. The traced growth behavior is discussed on the basis of statistical analyses of the occurring morphologies as well as in terms of expected changes in the supersaturation of the system. It is shown that the observed complex architectures arise as a consequence of an autocatalytic precipitation cycle, during which evolving aggregates synthesize and organize their constituents on their own upon growth. These processes occur only at a local scale within a certain active region that appears to extend over some microns beyond the growth front. Our findings provide concrete experimental evidence supporting the proposed formation mechanism and suggest that growth of silica biomorphs is independent of mass transport from the bulk, unless forced convection becomes strong enough to affect the active region and hence interfere with autocatalysis.