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

Abstract

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 [H 4SiO 4aq 0] 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 P 0, ( H T 0 - H 298 0) T , ( G T 0 - H 298 0) T , S T 0 - S 0 0, ΔH ƒ,298 0 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 P 0 = 1430.26 − 0.78850 T + 3.0340 × 10 −4 T 2 −1.85158 × 10 −4 T 2 1 2 + 8.3341 × 10 6 T −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.