Crystal growth patterns linked to geological erosion

LIVERMORE, Calif. — Observing crystal growth has allowed scientists to get a better look into the world of geological erosion.

By considering how the mechanisms of classical crystal growth theory could be reversed to describe crystal dissolution, scientists from Lawrence Livermore National Laboratory and Virginia Polytechnic Institute and State University have used that theory to explain how quartz and other silicates erode during geochemical weathering.

For more than 50 years, scientists have studied mineral dissolution to find out what role it plays in the influence of mineral weathering rates on biogeochemical systems (the interface between the geochemistry of a region and the animal and plant life in that region).

In a recent study that appears in the Proceedings of the National Academy of Sciences online edition for the week of Oct. 17-21, James De Yoreo of Lawrence Livermore and Patricia Dove and Nizhou Han of Virginia Polytechnic conclude that mineral dissolution can be understood through the same mechanisms by which minerals grow.

By generalizing crystal growth rate equations to include dissolution, the trio created a model that predicts how the mechanism of quartz dissolution evolves as the surrounding fluid becomes less concentrated in dissolved silicon dioxide (SIO 2 ). As the solution becomes increasingly undersaturated, the process changes from retreat of atomic steps across the surface to the nucleation of pits in the surface, first at defect formations and then uniformly across the surface. The result is a characteristic behavior which the authors showed can be observed in many mineral systems.

Using bulk rectors to measure dissolution rates and atomic force microscopy (AFM) to characterize surface morphologies, the scientists documented dissolution of the natural prismatic surfaces of quartz exposed to four different solution chemistries. They compared the results to those measured for other silicates. In each case, the dissolution processes differed depending on the temperature and solution composition, but could always be tied to the dissolution mechanism that created the greatest step density.

"The rate at which a crystal dissolves is controlled by the density of steps on the surface," De Yoreo said. "These steps can be pre-existing on the initial crystal surface or at the crystal edges, or emerge from other processes. Whichever source creates the greatest step density will dominate the dissolution process."

The findings have broad implications for other families of crystalline materials. For example, Dove reports that the model also may explain the rapid dissolution or "demineralization" of biological materials under some conditions.

The team believes that their results resolve serious discrepancies in the geochemical literature on mineral dissolution, and that future work should focus on understanding the underlying thermodynamic and kinetic factors that determine the mechanism controlling step density.

Founded in 1952, Lawrence Livermore National Laboratory has a mission to ensure national security and apply science and technology to the important issues of our time. Lawrence Livermore National Laboratory is managed by the University of California for the U.S. Department of Energy's National Nuclear Security Administration.