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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

Geochimica et Cosmochimica Acta

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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 ??m. 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 [AlO2 aq-] were also calculated as -914.2 ?? 2.1 and -830.9 ?? 2.1 kJ/mol, respectively. The use of [AlC2 aq-] as a chemical species is discouraged. A revised Gibbs free energy of formation for [H4SiO4aq0] 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 CP0, ( HT0 - H2980) T, ( GT0 - H2980) T, ST0 - S00, ??Hf{hook},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: CP0 = 1430.26 - 0.78850 T + 3.0340 ?? 10-4 T2 -1.85158 ?? 10-4 T2 1 2 + 8.3341 ?? 106 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. ?? 1978.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
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
Series title:
Geochimica et Cosmochimica Acta
Volume
42
Issue:
10
Year Published:
1978
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
First page:
1533
Last page:
1543
Number of Pages:
11