Crystal properties and energetics of synthetic kaolinite

Six kaolinite [Al2Si2O5(OH)(4)] samples were synthesized under different co nditions of temperature, pressure, and pH from two different starting mater ials. Chemical composition and properties of the samples were characterized using classical methods (electron microprobe, atomic absorption spectromet ry, X-ray diffraction, differential and thermal analyses, and Fourier trans form infrared spectrometry). All synthetic kaolinite samples contained vari ous amounts of a boehmite impurity. The defect density was different for ea ch kaolinite, ranging from high to low. The enthalpy of formation of these kaolinites at 25 degreesC was investigated by drop solution calorimetry int o molten lead berate at 700 degreesC. All data were corrected for impuritie s. Whatever the synthesis conditions and the kaolinite properties, the enth alpy of kaolinite dissolution into molten lead borate at 700 degreesC and t he standard enthalpy of kaolinite formation from the oxides and from the el ements at 25 degreesC are constant: 372.3 +/- 1.0 kJ/mol, -46.6 +/- 2.6 kJ/ mol, and -4115.3 +/- 4.1 kJ/mol respectively. Using entropy data from the l iterature, the standard Gibbs free energy of kaolinite formation from the e lements at 25 degreesC is -3793.9 +/- 4.1 kJ/mol. This value is in excellen t agreement with most of the literature data obtained for natural kaolinite s. Furthermore, the standard Gibbs free energy of kaolinite formation at 25 degreesC and 1 atm is very close to that obtained using the same method fo r the San Juanito dickite, which is commonly used as a standard mineral, th e value for kaolinite being slightly more negative than the value for dicki te. This trend is also true for all the temperature and pressure range of k aolin minerals occurrences. Thus, dickite is a metastable phase relative to kaolinite, and kaolinite seems to be thermodynamically more stable than di ckite, as already proposed by DeLigny and Navrotsky (1999) and Anovitz et a l. (1991). The natural occurrence of dickite must result from specific reac tion paths and be controlled by kinetic factors.