DISSOLUTION OF SILICATES AND THE STABILITY OF POLYSILICIC ACID

Abstract

In several experiments albite, orthoclase, diopside, muscovite, sepiolite, magadiite, kenyaite, natrosilite, δ-Na2Si2O5, sodium metasilicate, kanemite, quartz, opal, amorphous silica, and silica gel were dissolved in sulfuric aqueous solutions at pH 3. The experimental results show that most of the solutions contain silicic acid as both polymeric and monomeric species. Polysilicic acid is measured as high- and as low-molecular-weight silica. The polymers may amount to ~50 mol.% of total dissolved silica. The proportion of polymers varies with reaction time of the dissolution experiment. As a function of time, polymers decompose into monomeric species, which is the stable silica species at experimental conditions. Therefore, the transfer of polysilicic acid into the solution represents a transition stage during the dissolution of silicates. The pH, the temperature, and the kind of the dissolved components of most natural waters provide high depolymerisation capacities. Polysilicic acid decomposes to monomer within a few hours or days in river and seawater. Therefore, it is not surprising that in most natural waters silicic acid consists of monomeric species. However, in natural environments with acid solutions rich in bivalent cations, polysilicic acid may exist as a metastable component over several months.