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New thermochemical evidence on the stability of dickite vs. kaolinite

Claire-Isabelle Fialips, Juraj Majzlan, Daniel Beaufort and Alexandra Navrotsky
New thermochemical evidence on the stability of dickite vs. kaolinite
American Mineralogist (June 2003) 88 (5-6): 837-845


The energetics of the Saint Claire dickite sample, a highly pure and low defect material, have been studied by high-temperature oxide-melt solution calorimetry. Data were corrected for adsorbed water, the only significant impurity to the ideal formula Al (sub 2) Si (sub 2) O (sub 5) (OH) (sub 4) . Enthalpies of dickite dissolution into molten lead borate at 702 degrees C and of dickite formation from the oxides and from the elements at 25 degrees C and 1 bar are 356.3+ or -3.3 kJ/mol, -31.1+ or -4.2 kJ/mol, and -4099.8+ or -4.8 kJ/mol, respectively. Using entropy data from the literature, the standard Gibbs free energy of dickite formation from the elements at 25 degrees C is -3777.5+ or -4.8 kJ/mol. This value is 7 to 25 kJ/mol more endothermic than literature values obtained for other dickites by calculations, solubility measurements, or calorimetry. Since the Saint Claire dickite is almost pure, the applied corrections are very small and these results are likely to be the most reliable of the available data. The Gibbs free energy of formation of the Saint Claire dickite at 25 degrees C and 1 bar is 12 to 25 kJ/mol less negative than data obtained for kaolinite using the same and different techniques. This trend is unlikely to reverse at higher temperature and/or pressure in the range of kaolin mineral occurrences. Thus, the new data further corroborate our earlier findings that dickite is thermodynamically metastable relative to kaolinite. This is in agreement with results obtained by mineral synthesis and with most of the observations of kaolinite and dickite occurrences in natural environments. However, further investigations are required to fully explain the transformation of kaolinite to dickite, which appears to occur during diagenesis.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 88
Serial Issue: 5-6
Title: New thermochemical evidence on the stability of dickite vs. kaolinite
Affiliation: Los Alamos National Laboratory, Department of Earth and Environmental Sciences, Los Alamos, NM, United States
Pages: 837-845
Published: 200306
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 59
Accession Number: 2003-051822
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of California at Davis, USA, United StatesUniversite de Poitiers, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200316
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