We carried out a dissolution experiment to determine the solubility and the Gibbs free energy of formation of natural imogolite at 25°C and 1 atm. Purified gelatinous film of natural imogolite, synthetic gibbsite, and imogolite plus gibbsite were equilibrated in 1 mmol L-1 HCI solutions and Si, Al concentrations and pH were monitored during 700 d. Imogolite plus gibbsite and gibbsite systems reached an equilibrium after 120 d. The logarithmic values of the equilibrium constants of dissolution reactions were 12.10±0.01 for imogolite and 8.01±0.01 for gibbsite. The calculated Gibbs free energy of formation was -2929.19±3.28 kJ mol-1 for imogolite and -1155.11 ± 1.42 kJ mol-1 for gibbsite. These values predict that the silicic acid concentration at the imogolite-gibbsite equilibrium would be 0.120 mmol L-1.
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