Gibbsite, free energy

In a later study (DeKimpe et al., 1964) they demonstrated that it was necessary that the Al be in six-fold coordination before it would combine with Si to form kaolinite. Starting with the Al in six-fold coordination presumably decreased the free-energy barrier necessary for the formation of kaolinite. Gibbsite, Al(OH)3, was used as a starting material but was too stable to be attacked by depolymerized silica it was necessary for the gibbsite to be in the process of reorganizing ( dynamic stage) into boehmite, AIO(OH). 

Barany, R., and Kelley, K. K. Heats and free energies of formation of gibbsite, kaolinite, halloysite, and dickite. 

One can speculate that other soil aluminum oxyhydroxide phases, diaspore, bayerite, and nordstrandite also exhibit low surface free energies, similar to those for gibbsite and boehmite. More calorimetric studies are needed to quantify these relations. 

In this paper, the procedures are summarized by which the solubilities of gibbsite, boehmite, and diapore in alkaline solution are evaluated and used to compute thermodynamic properties of boehmite and the aluminate ion. Published data are then used in conjunction with the derived properties of the aluminate ion given in this paper to calculate preliminary thermodynamic properties for bayerite, and the Gibbs free energy of formation of nordstrandite at 25°C. Aluminum hydroxide and oxyhydroxide, and corundum... 

The value of this constant comes from the free energies of formation of the reactants and products of Eq. 7.10. The equilibrium constant defines the soluble silica activity at which gibbsite and kaolinite are in equilibrium ... 

Comparison of the standard free energies of formation at 298 K of corundum, diaspore, boehmite, gibbsite, and bayerite leads to the following selected values for free energies of formation diaspore —219.5 ... 

Su, C. and Harsh, J.B. (1994) Gibbs free energies of formation at 298 K for imogolite and gibbsite from solubility measurements. Geochim. Cosmochim. Acta, 58, 1667-1677. 

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