The stability, free energy and heat of formation of imogolite

Abstract

From equilibrium silica concentrations over imogolite and boehmite at 100–155°C, the heat and free energy of the reaction (HO)₃Al₂O₃SiOH + H₂O ⇌ 2AlOOH + Si(OH)₄ have been obtained: ΔH°_r (298·15 K)=38·6 ± 4·1, ΔG°_r (298·15 K) = 26.8 ± 1·1 kJ mol⁻¹, and hence the heat and free energy of formation of imogolite: ΔH°_f (298·15 K) = −3189·6 ± 4·1, ΔG°_f (298·15 K) = − 2926·7 ± 1·1 kJ mol⁻¹ These results are consistent with observations indicating that imogolite, halloysite and gibbsite can co-exist in soils, but that imogolite is metastable relative to either halloysite or gibbsite in the long term. At temperatures above 25°C there is a widening range of silica concentrations in which imogolite is more stable than halloysite, although both are metastable relative to kaolinite.