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Thermodynamics of the goethite-diaspore solid solution

Thermochemistry Facility and Department of Geology, University of California at Davis, Davis CA 95616, USA

^* e-mail: jmajzlan{at}princeton.edu. Current address: Department of Geosciences, Princeton University, Princeton NJ 08544, USA

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 H_mix = WX_goethite(1-X_goethite), where W = 79 ± 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 Fe³⁺ and Al³⁺. Despite large positive H_mix in these two solid solutions, a number of more complex structures with Fe³⁺/A1³⁺ 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.

Key-words: goethite, diaspore, solid solution, thermodynamics of mixing.

This article has been cited by other articles:

				A. Navrotsky, L. Mazeina, and J. Majzlan Size-Driven Structural and Thermodynamic Complexity in Iron Oxides Science, March 21, 2008; 319(5870): 1635 - 1638. [Abstract] [Full Text] [PDF]