Solubility behavior of quartz and corundum in supercritical water: A quantitative thermodynamic interpretation

Dissolution reaction equilibria for {alpha}-quartz (SiO{sub 2}) and corundum ({alpha}-Al{sub 2}0{sub 3}) in pure, supercritical water are quantified using a density-dependent thermodynamic model. The database of existing solubility literature for {alpha}-quartz (0.2-10 kb, 200--575{degrees}C) is shown to be consistent with the presence of two hydrolyzed SI(IV) ion forms: Si(OH){sub 4}(aq) and Si{sub 2}O(OH){sub 6}(aq); the corundum database (1-20 kb, 400--700{degrees}C) is consistent with Al(OH){sub 3}(aq) and Al(OH){sub 4}{sup {minus}}. A third Si(IV) ion hydroxocomplex, Si{sub 2}O{sub 2}(OH){sub 5}{sup {minus}}, is indicated at lower pressures (0.03-0.10 kb). The characteristic sigmoidal nature of the solubility isobars is explained by dimerization of Si(OH){sub 4}(aq) (at high densities) or the formation of anionic hydrolysis products, Si{sub 2}0{sub 2}(OH){sub 5}{sup {minus}} and Al(OH){sub 4}{sup {minus}}, in the low density region (p < 0.01 gm/cc). By means of the evaluated equilibria, thermochemical properties of Si{sub 2}O(OH){sub 6}(aq) and Si{sub 2}O{sub 2}(OH){sub 5}{sup {minus}} are made available for the first time