Crystal properties and energetics of synthetic kaolinite
-
Claire-Isabelle Fialips
and
Sabine Petit
Abstract
Six kaolinite [Al2Si2O5(OH)4] samples were synthesized under different conditions of temperature, pressure, and pH from two different starting materials. Chemical composition and properties of the samples were characterized using classical methods (electron microprobe, atomic absorption spectrometry, X-ray diffraction, differential and thermal analyses, and Fourier transform infrared spectrometry). All synthetic kaolinite samples contained various amounts of a boehmite impurity. The defect density was different for each kaolinite, ranging from high to low. The enthalpy of formation of these kaolinites at 25 °C was investigated by drop solution calorimetry into molten lead borate at 700 °C. All data were corrected for impurities. Whatever the synthesis conditions and the kaolinite properties, the enthalpy of kaolinite dissolution into molten lead borate at 700 °C and the standard enthalpy of kaolinite formation from the oxides and from the elements at 25 °C 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 literature, the standard Gibbs free energy of kaolinite formation from the elements at 25 °C is -3793.9 ±4.1 kJ/mol. This value is in excellent agreement with most of the literature data obtained for natural kaolinites. Furthermore, the standard Gibbs free energy of kaolinite formation at 25 °C and 1 atm is very close to that obtained using the same method for the San Juanito dickite, which is commonly used as a standard mineral, the value for kaolinite being slightly more negative than the value for dickite. This trend is also true for all the temperature and pressure range of kaolin minerals occurrences. Thus, dickite is a metastable phase relative to kaolinite, and kaolinite seems to be thermodynamically more stable than dickite, as already proposed by DeLigny and Navrotsky (1999) and Anovitz et al. (1991). The natural occurrence of dickite must result from specific reaction paths and be controlled by kinetic factors.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Partitioning of Sr between coexisting minerals of the hollandite- and piemontite-groups in a quartz-rich schist from the Sanbagawa metamorphic belt, Japan
- Crystallization processes in migmatites
- Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts
- Reduction of water loss from gold-palladium capsules during piston-cylinder experiments by use of pyrophyllite powder
- Trace-element partitioning between immiscible lunar melts: An example from naturally occurring lunar melt inclusions
- Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part I. Theory
- Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part II. Application
- The stability of clinopyroxene in the system CaO-MgO-SiO2-TiO2 (CMST)
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- Al,Si order in the crystal structure of α-eucryptite (LiAlSiO4)
- TEM-EDX study of weathered layers on the surface of volcanic glass, bytownite, and hypersthene in volcanic ash from Sakurajima volcano, Japan
- Electron microscopic study of the dehydration of diaspore
- Crystal properties and energetics of synthetic kaolinite
- Structures and energies of AlOOH and FeOOH polymorphs from plane wave pseudopotential calculations
- XPS measurement of fivefold and sixfold coordinated sulfur in pyrrhotites and evidence for millerite and pyrrhotite surface species
- Structure of synthetic 6-line ferrihydrite by electron nanodiffraction
- Atomic structures of planar defects in oxybiotite
- Potassium hydrogen disilicate: A possible model compound for 17O NMR spectra of hydrous silicate glasses
- Strain analysis of phase transitions in (Ca,Sr)TiO3 perovskites
- Triclinic liddicoatite and elbaite in growth sectors of tourmaline from Madagascar
- Letter. A simple inorganic process for formation of carbonates, magnetite, and sulfides in martian meteorite ALH84001
