Amorphous silica solubility and the thermodynamic properties of H4SiO4° in the range of 0° to 350°C at Psat

The solubility of amorphous silica was determined in the temperature range 8 degrees to 310 degreesC at 1 bar below 100 degreesC and at P-sat at higher temperatures. Our results are consistent with previous experiments between 100 degrees and 200 degreesC, but at higher temperatures they indicate lower solubility. Below 100 degreesC our result are lower than the results of some researchers, but in good agreement with others. Our solubility data have been combined with previously reported data to retrieve a temperature equation describing amorphous silica solubility. Quartz solubility data have also been assessed. The solubility equations for the reaction SiO2,s + 2H(2)O = H(4)SiO(4)degrees are: logK(am,silica) = - 8.476 - 485.24 x T-1 - 2.268 x 10(-6) x T-2 + 3.068 x logT logK(quartz) = -34.188 + 197.47 x T-1 - 5.851 x 10(-6) x T-2 + 12.245 x logT where T is in K. They are valid in the temperature range 0 degrees to 350 degreesC at 1 bar below 100 degreesC and at P-sat at higher temperatures. From the quartz solubility equation and the thermodynamic properties of quartz and liquid water, the standard partial molal Gibbs energy of formation and the third law entropy of H(4)SiO(4)degrees were calculated as -1,309,181 J/mole and 178.85 J/mole/K at 25 degreesC. The difference in the standard apparent Gibbs energy of H(4)SiO(4)degrees as calculated from quartz solubility, on one hand, and amorphous silica solubility, on the other, is about the same over the temperature range 0 degrees to 350 degreesC indicating that the solubility temperature equations obtained fur the two solids in this study are internally consistent. This indicates that the quartz solubility data of Rimstidt (1997), which were used in this study to retrieve the quartz solubility equation, are valid and also our data on Delta(G) over bar (1)degrees and (S) over bar degrees for H(4)SiO(4)degrees, at 25 degreesC and 1 bar as well as the Delta(G) over bar degrees temperature equation presented for this species. These new Gibbs energy values for H(4)SiO(4)degrees indicate that all silicate minerals are considerably more soluble under Earth's surface conditions than generally accepted to date, or by about 0.6 log K units at 0 degreesC per silicon atom in the unit formula. Copyright (C) 2000 Elsevier Science Ltd.