Revised values for the Gibbs free energy of formation of [Al(OH)4 aq-], diaspore, boehmite and bayerite at 298.15 K and 1 bar, the thermodynamic properties of kaolinite to 800 K and 1 bar, and the heats of solution of several gibbsite samples

Solution calorimetric measurements compared with solubility determinations from the literature for the same samples of gibbsite have provided a direct thermochemical cycle through which the Gibbs free energy of formation of [Al(OH)_{4 aq}⁻] can be determined. The Gibbs free energy of formation of [Al(OH)_{4 aq}⁻] at 298.15 K is −1305 ± 1 kJ/mol. These heat-of-solution results show no significant difference in the thermodynamic properties of gibbsite particles in the range from 50 to 0.05 μm.

The Gibbs free energies of formation at 298.15 K and 1 bar pressure of diaspore, boehmite and bayerite are −9210 ± 5.0, −918.4 ± 2.1 and −1153 ± 2 kJ/mol based upon the Gibbs free energy of [A1(OH)_{4 aq}⁻] calculated in this paper and the acceptance of −1582.2 ± 1.3 and −1154.9 ± 1.2 kJ/mol for the Gibbs free energy of formation of corundum and gibbsite, respectively.

Values for the Gibbs free energy formation of [Al(OH)_{2 aq}⁺] and [AlO_{2 aq}⁻] were also calculated as −914.2 ± 2.1 and −830.9 ± 2.1 kJ/mol, respectively. The use of [AlC_{2 aq}⁻] as a chemical species is discouraged.

A revised Gibbs free energy of formation for [H₄SiO_4aq⁰] was recalculated from calorimetric data yielding a value of −1307.5 ± 1.7 kJ/mol which is in good agreement with the results obtained from several solubility studies.

Smoothed values for the thermodynamic functions C_P⁰, (HT0- H2980)T">HT0- H2980)T, (GT0- H2980)T">(GT0- H2980)T, S_T⁰ - S₀⁰, ΔHƒ,2980">ƒΔHƒ,2980 kaolinite are listed at integral temperatures between 298.15 and 800 K. The heat capacity of kaolinite at temperatures between 250 and 800 K may be calculated from the following equation: C_P⁰ = 1430.26 − 0.78850 T + 3.0340 × 10⁻⁴T² −1.85158 × 10⁻⁴T²12+ 8.3341 × 106T−2">12+ 8.3341 × 106T−2.

The thermodynamic properties of most of the geologically important Al-bearing phases have been referenced to the same reference state for Al, namely gibbsite.