Gibbsite growth kinetics on gibbsite, kaolinite, and muscovite substrates:Atomic force microscopy evidence for epitaxy and an assessment of reactivesurface area

New experimental data for gibbsite growth on powdered kaolinite and single crystal muscovite and published data for gibbsite growth on gibbsite powder s at 80 degrees C in pH3 solutions show that all growth rates obey the same linear function of saturation slate provided that reactive surface area is evaluated for each mineral substrate. Growth rate (mol m(-2) s(-1)) is exp ressed by Rate(ppt) = (1.9 +/- 0.2) X 10(-10)\Delta G(r)\/ RT(0.90+/-0.01), which applies to the range of saturation states from Delta G(r) = 0 to 8.8 kJ mol(-1), where Delta G(r) = RT[ln(Q/K)] for the reaction Al3+ + 3H(2)O = Al(OH)(3) + 3H(+), and equilibrium defined as Delta G(r) = 0 was previous ly determined. Identification of the growth phase as gibbsite was confirmed by rotating anode powder x-ray diffraction. Rates an kaolinite were determ ined using steady-state measured changes between inlet and outlet solutions in single-pass stirred-flow experiments. Rates on muscovite were determine d by measuring the volume of precipitated crystals in images obtained by Ta pping Mode(TM) atomic force microscopy (TMAFM). In deriving the single grow th rate law, reactive surface area was evaluated for each substrate mineral . Total BET surface area was used for normalizing rates of gibbsite growth onto powdered gibbsite. Eight percent of the BET surface area, representing the approximate amount occupied by the aluminum octahedral sheet exposed a t crystal edges, was used for powdered kaolinite. The x - y area of the TMA FM images of the basal surface was used for single crystal muscovite. Linea rity of growth rates with saturation state suggests that the dominant nucle ation and growth mechanisms are two dimensional. Such mechanisms are suppor ted by observations of the morphologies of gibbsite crystals grown on musco vite at Delta G(r) = 8.8 kJ mol(-1). The morphologies include (1) apparent epitaxial films as determined by hexagonal outlines of edges and thicknesse s of 30 to 40 Angstrom, (2) elongate crystals 30 to 40 Angstrom thick align ed with the structure of the distorted Si-tetrahedral sheet of the 2M, musc ovite, and (3) micrometer-scale three-dimensional clumps of intergrown crys tals. Reactive surface area as defined now for heterogeneous crystal growth in reactive-transport models must be modified to include substrates other than that of the growing mineral and to account for the role of structural and chemical controls on epitaxial nucleation and growth. Copyright (C) 199 9 Elsevier Science Ltd.