Clays in soils, sediments, and sedimentary rocks in many cases form by heterogeneous nucleation and growth on other mineral substrates. Primary and detrital sheet silicates provide perfect substrates for secondary clay growth because of the similarity in structure and sheet-cation composition. For example, gibbsite replacement reactions in lateritic soils, excess Al in fibrous illites from sandstones, and growth of vermicular kaolinite in soils and sedimentary rocks all may be a result of monolayer or polylayer growth of crystalline phases on sheet silicate substrates. Addition of brucite sheets to mica-type 2:1 layers must be a step in the formation of chlorite. We have undertaken a study to measure experimentally rates and mechanisms of clay nucleation and growth on various substrates, including sheet silicates. Our first results on hetero-epitaxial growth of gibbsite and brucite on muscovite are presented here. We hope to identify reactive surface sites and extrapolate to reactive surface areas for the substrate phases. This work should help address two questions that have arisen in previous studies. First, Nagy and Lasaga (1993) suggested that simultaneous precipitation of kaolinite and gibbsite on mixed powder substrates containing the two phases may be controlled by the individual substrate phases, e.g., gibbsite nucleated and grew only on gibbsite, and likewise for kaolinite. However, insufficient analytical tests of the substrate/overgrowth relationships could be performed in that study. Second, a reanalysis of the kaolinite dissolution and growth rate data from Nagy and Lasaga (1993) suggests that growth is a linear function of supersaturation state. Extrapolating both the dissolution and precipitation rate laws as separate curves results in an overlap along the DGr or solution saturation state axis. This may be because the growing phase is "more perfect" than the dissolving phase, although both are kaolinite. Analysis of nucleation and growth patterns could be used to evaluate the crystal defect structure of sheet silicate overgrowths.
Growth of gibbsite and brucite on single crystal muscovite sheets was measured in mixed-flow reactors at 80°C, pH 3 (gibbsite) and pH 9 (brucite). The muscovite sheets were held in a stationary position within the cell and the constantly stirred-fluid flowed past the surface at a constant rate. Solutions were supersaturated with respect to the two phases to a factor of about two in DGr. Gibbsite and brucite growth rates on powdered muscovite were measured for comparison. Because of the excellent cleavage of muscovite, distinct areas of atomically-smooth and stepped tetrahedral sheet surfaces could be obtained. We examined the effects of reacting only the basal surface vs. the basal plus edge surfaces of the single crystals by coating the edges with epoxy in one set of experiments. We freshly-cleaved the muscovite surface seconds prior to immersion in the experimental solution. The mirror surface and other freshly-cleaved surfaces of the same starting crystal were examined using AFM. X-ray scattering techniques using rotating anode radiation were applied to the unreacted and reacted surface. Semi-empirical ionic modeling of the substrate and growth phases was used to determine the relative binding energies of gibbsite and brucite to the muscovite surface.
Comparison of the growth rates of gibbsite and brucite on muscovite are presented by normalizing to total BET surface area, geometric surface area, and "reactive" surface area. We define "reactive" surface area by quantifying surface roughness and nucleation sites (as determined from AFM and X-ray scattering analysis). We compared the growth rate of gibbsite on muscovite to that of gibbsite on gibbsite from published data in order to understand effects of surface structure and composition on nucleation. The results obtained here have implications for understanding mechanisms and rates of nucleation and growth of more complex phases such as kaolinite and chlorite. In addition, a better approach to understanding reactive surface area can be obtained by combining data from single crystal and powder mineral kinetic experiments, both from a solution chemistry and surface analytical point of view.
This work was supported by the U. S. Department of Energy Office of Basic Energy Sciences/Geoscience and the U. S. Nuclear Regulatory Commission, under contract DE-AC04-94AL85000 to Sandia National Laboratories and by the U. S. Department of Energy Office of Basic Energy Sciences/Geoscience, under contract W-31-109-Eng-38 to Argonne National Laboratory.
Nagy, K.L. & Lasaga, A.C. Geochim. Cosmochim. Acta 57, 4329-4335 (1993).