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Thermodynamics of the goethite-diaspore solid solution

Juraj Majzlan and Alexandra Navrotsky
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.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 15
Serial Issue: 3
Title: Thermodynamics of the goethite-diaspore solid solution
Affiliation: University of California at Davis, Department of Geology, Davis, CA, United States
Pages: 495-501
Published: 200306
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 35
Accession Number: 2003-055034
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200318
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