Original paper

Thermodynamics of the goethite-diaspore solid solution

Majzlan, Juraj; Navrotsky, Alexandra

Abstract

Enthalpies of mixing and lattice parameters were determined for a series of Al-substituted synthetic goethite samples by high-temperature oxide-melt calorimetry and Rietveld refinement of X-ray diffraction patterns, respectively. Lattice parameters of Al-goethite deviate from Vegard's law (ideal mixing). The enthalpies of mixing can be fitted by a regular solution polynomial ΔHmix = WXgoethite (1-X ), where W = 79 ± 14 kJ/mol. Small positive vibrational excess entropy terms may exist in goethite Al-goethite but, regardless of the excess entropy, the equilibrium solubility of Al in goethite is negligible, and Al-goethite is metastable with respect to goethite and diaspore. The large positive enthalpies of mixing in Al-hematite (Majzlan et al. , 2002) and Al-goethite can be rationalized on the basis of ionic size mismatch between Fe3+ and Al3+. Despite large positive ΔHmix in these two solid solutions, a number of more complex structures with Fe3+/Al3+ substitution show ideal mixing, suggesting that more complex structures cope with substitution better, as suggested by the systematics provided by Davies & Navrotsky (1983). Aluminum substituted iron oxides are an example of metastable compounds that can persist for geological time because of small energy difference between the stable and metastable phases.

Keywords

goethitediasporesolid solutionthermodynamics of mixing