Amorphous silica solubility and the thermodynamic properties of H4SiO4 degrees in the range of 0 degrees to 350 degrees C at P-sat

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 highe r temperatures. Our results are consistent with previous experiments betwee n 100 degrees and 200 degreesC, but at higher temperatures they indicate lo wer solubility. Below 100 degreesC our result are lower than the results of some researchers, but in good agreement with others. Our solubility data h ave been combined with previously reported data to retrieve a temperature e quation 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 l ogT logK(quartz) = -34.188 + 197.47 x T-1 - 5.851 x 10(-6) x T-2 + 12.245 x log T where T is in K. They are valid in the temperature range 0 degrees to 350 d egreesC at 1 bar below 100 degreesC and at P-sat at higher temperatures. Fr om the quartz solubility equation and the thermodynamic properties of quart z and liquid water, the standard partial molal Gibbs energy of formation an d 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 a pparent Gibbs energy of H(4)SiO(4)degrees as calculated from quartz solubil ity, on one hand, and amorphous silica solubility, on the other, is about t he same over the temperature range 0 degrees to 350 degreesC indicating tha t 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 quar tz 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 presen ted for this species. These new Gibbs energy values for H(4)SiO(4)degrees i ndicate that all silicate minerals are considerably more soluble under Eart h'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.