Thermodynamics of the goethite-diaspore solid solution
Thermodynamics of the goethite-diaspore solid solution
European Journal of Mineralogy (June 2003) 15 (3): 495-501
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 Delta H (sub mix) = WX (sub goethite) (1-X (sub goethite) ), where W = 79+ or -14 kJ/mol. Small positive vibrational excess entropy terms may exist in 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 Fe (super 3+) and Al (super 3+) . Despite large positive Delta H (sub mix) in these two solid solutions, a number of more complex structures with Fe (super 3+) /Al (super 3+) 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.