Free energy aluminosilicate

A complete description of any groundwater system necessitates consideration of reactions between rock forming minerals and the aqueous phase. This cannot be achieved without accurate thermodynamic properties of both the participating aluminosilicate minerals and aqueous aluminum species. Most computer codes used to calculate the distribution of species in the aqueous phase utilize the "reaction constant" approach as opposed to the "Gibbs free energy minimization" approach (3). In the former, aluminosilicate dissolution constants are usually written in terms of the aqueous aluminum species, Al, which is related to other aqueous aluminum species by appropriate dissociation reactions. 

Table IV summarizes log Kj4 values for aluminum hydroxides and oxyhydroxides, and corundum between 0 and 350 C. They were calculated using the modified H.K.F. equation of state (70) together with the data given in Tables I and II. These values are suitable for incorporation into distribution of species codes such as EQ3 (67), provided that A1(0H)4 (or AIO2) is made a basis species. Calculation of dissolution constants for other aluminosilicates can be made using the Gibbs free energy data for Al(OH)4 or AIO2 Provided in Table II.

Clay particles occur abundantly in the soil. They are mostly colloidal aluminosilicates. To learn how their extensive surfaces react with water, we have studied the following properties of water associated with montmorillonite and other clay minerals threshold gradient [1], thermal expansibility [2, 3], isothermal compressibility [4], frequency of O—H stretching [5, 6], molar absorptivity [7], freezing point depression [8, 9] specific volume [10], specific heat capacity [11], heat of compression [12], viscosity [13], and free energy, enthalpy and entropy [6,14,15]. Not all of these properties will discussed here. Instead, we will discuss only a few of them to illustrate the kind of results obtained. 

They introduced a solvent coefficient (Henry s law constant) for each Eu species to account for the energy difference between the ideal gas and the particular liquid in question. They found, for a particular choice of major cation (i.e., Ca or Mg) in the solvent, 2EuO to be more soluble than EU2O3 in going from the ortho-or metasilicate end member of the mixture to the aluminosilicate end member. This result was not expected because the aluminosilicate end member has a higher fraction of its oxygen bound into polymer chains. This would be expected to decrease the activity of free oxide ion in the aluminosilicate melts, which according to eq. (21.4) would favor formation of Eu(III) at the expense of Eu(II), just opposite to what was observed. 

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