Reversible control of aqueous aluminum and silica during the irreversible evolution of natural waters

Primary aluminosilicates are transformed at low temperature into a sequence of metastable and thermodynamically stable secondary minerals by an irreversible process. The aqueous concentrations in the associated solution may continuously change during the process or they may be maintained constant through hydrodynamic or chemical steady-state mechanisms or through chemical equilibrium with a reversible metastable solid. Disequilibrium indices calculated for 152 natural waters and experimental solutions show that the solutions are unsaturated with amorphous aluminum hydroxide, microcrystalline gibbsite, amorphous silica and amorphous aluminosilicate, and they are supersaturated with gibbsite and kaolinite. The disequilibrium index for halloysite varies widely from unsaturation to supersaturation. Only the index for the reversible metastable cryptocrystalline aluminosilicate whose composition is pH dependent is very close to zero indicating saturation. The index varies in a narrow range. This, supported by electron micrographs and the results of X-ray fluorescence spectroscopy presented by other authors, suggests that this metastable solid, and not the secondary aluminosilicate minerals, controls the concentrations of alumina and silica in natural waters.