24 research outputs found

    The Influence of Random Defect Density on the Thermal Stability of Kaolinites

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    The thermal stability of kaolinite and the microstructure of its thermal products strongly depend on random defects (R2) rather than crystalline defects (HI). Kaolinite with lower random defect density is more stable than that with higher defect density during dehydroxylation and the derived metakaolinite can be directly transformed into orthorhombic mullite (3/2-mullite). However, for kaolinite with higher random defect density, there is a cubic phase occurring in the transformation from metakaolinite to primary mullite. Primary mullite will be transformed into orthorhombic mullite as temperature increases. AlV is universally present in the metakaolinite and the relative amounts of AlVI, AlV and AlIV vary with the random defect density of the parent kaolinite

    Thermodynamic properties of chlorite and berthierine derived from calorimetric measurements

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    International audienceIn the context of the deep waste disposal, we have investigated the respective stabilities of two iron-bearing clay minerals: berthierine ISGS from Illinois [USA; (Al0.975FeIII0.182FeII1.422Mg0.157Li0.035Mn0.002)(Si1.332Al0.668)O-5(OH)(4)] and chlorite CCa-2 from Flagstaff Hill, California [USA; (Si2.633Al1.367)(Al1.116FeIII0.215Mg2.952FeII1.712Mn0.012Ca0.011)O-10(OH)(8)]. For berthierine, the complete thermodynamic dataset was determined at 1 bar and from 2 to 310 K, using calorimetric methods. The standard enthalpies of formation were obtained by solution-reaction calorimetry at 298.15 K, and the heat capacities were measured by heat-pulse calorimetry. For chlorite, the standard enthalpy of formation is measured by solution-reaction calorimetry at 298.15 K. This is completing the entropy and heat capacity obtained previously by Gailhanou et al. (Geochim Cosmochim Acta 73:4738-4749, 2009) between 2 and 520 K, by using low-temperature adiabatic calorimetry and differential scanning calorimetry. For both minerals, the standard entropies and the Gibbs free energies of formation at 298.15 K were then calculated. An assessment of the measured properties could be carried out with respect to literature data. Eventually, the thermodynamic dataset allowed realizing theoretical calculations concerning the berthierine to chlorite transition. The latter showed that, from a thermodynamic viewpoint, the main factor controlling this transition is probably the composition of the berthierine and chlorite minerals and the nature of the secondary minerals rather than temperatur
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