Abstract

The solubility in aqueous fluids of peralkaline alkali aluminosilicate components has been determined in the 0.8-2.0 GPa and 1000°-1300°C pressure and temperature range, respectively. Compositions were along the haploandesite join, Na2Si4O9 - Na2(NaAl)4O9, with 0, 3 and 6 mol % Al2O3, denoted NS4Ax, where x represents the mol % Al2O3. The silicate solubility in aqueous fluid is in the 3-20 mol% range for NS4, 2-13 mol % for NS4A3, and 1.5-8 mol % for NS4A6 with a linear and positive temperature-dependence and a nonlinear and positive pressure-dependence. The silicate solubility decreases with increasing Al2O3 content. From stepwise regression, the pressure-, temperature-, and Al2O3-dependence of the solubility can be described with the expression:

Xsilicate (mol%) = 1.9 - 1.3XAl2O3 (mol %) + 0.008T(°C) - 13P(GPa) + 7.3P2. Partial molar volume of H2O in the silicate-saturated fluids, , is in the range 17-25 cm3/mol. The was calculated with the assumption that the activity of dissolved silicate in aqueous fluid equals the mol fraction of dissolved silicate. Compared with the molar volume of pure H2O, V°H2O, the values are 10-15 % lower at 0.8 GPa and as much as 15 % higher than V°H2O at 2.0 GPa. This volume difference diminishes as the system become more aluminous.

For all compositions, the is a linear and negative function of pressure with b[=-1/Vo( /P)T] ~ 0.125 GPa-1 for NS4. The bNS4A3 increases from 0.141 to 0.172 in the 1000°-1300°C range, whereas bNS4A6 increases from 0.193 to 0.213 GPa-1.

The is a positive and linear function of temperature with thermal expansion coefficients in the range 3-8x10-4 K-1. This thermal expansivity resembles that of pure H2O although the exact values and the range in values for pure H2O differ from 1/Vo( /T)P of silicate-saturated aqueous solution.