Steady-state kinetics and equilibrium between ground water and granitic rock

A hypothesis is presented that the dissolution of albite includes the exchange of sodium for hydrogen ion in a surface layer of the mineral and the structural collapse of the residual anionic lattice of the layer. The ion exchange is described by the first law of diffusion (D_25°C = 3 × 10^?22 and 1.5 × 10^?20 cm²sec^?1 at P_CO2 = 0 and 26.2 atm, respectively). The surface residual layer reaches a steady-state thickness ranging from n × 10^?8 to n × 10^?5 cm according to the temperature and P_CO2. The increase in aqueous sodium with time in a continuous ground-water system is described by a simple exponential equation. The equation is used to estimate the percolation time of ground water from the data on the chemical composition of a water sample. The probable times range from 14 to 3840 days for various ground-water systems and are compared to the times of percolation calculated from the geothermal and hydraulic data. Both estimates are found to be in general agreement. The concentrations of Al and Si in cold water from granitic rocks are shown to be controlled by the chemical equilibrium with respect to an aged aluminosilicate. The aluminosilicate precipitates from ground water as an amorphous isoelectric solid. Its chemical composition is represented by a simplified stoichiometric formula [Al(OH)₃](_1?x)[SiO₂]_x and varies linearly with pH of the solution. The atoms of Al, O and H tend to occupy a fixed position in the solid given by the gibbsite structure upon aging in the field. The solubility product of the solid is estimated from the published data on experimental and field research into the dissolution of feldspars: logK = (1 ? x) × log [Al³⁺] + xlog [H₄SiO₄] ? (3 ? 3x) log [H⁺] = 8.56 ? 11.26x, where x is the molar fraction of silica in the aluminosilicate.