Kaolinite thermochemical data

In order to construct the activity diagrams in a rigorous fashion, a certain amount of information must be available. Some experimental data for the mica-feldspar-kaolinite-gibbsite-montmorillonite relations are available. Data for the other minerals are often inferred from measurements of natural chemical parameters (K+, SiC, H+ concentrations in solutions) in situations where the different minerals are assumed to be stable. The relations between minerals can also be calculated as a function of K+, SiO and pH using thermochemical data for the participating phase (Hess, 1966) when they are known with precision. Frequently it is... 

The enthalpy change on formation of Portland cement clinker cannot be calculated with high precision, mainly because of uncertainties associated with the clay minerals in the raw material. Table 3.1 gives data for the main thermochemical components of the reaction, almost all of which have been calculated from a self-consistent set of standard enthalpies of formation, and which are therefore likely to be more reliable than other values in the literature. The conversion of the clay minerals into oxides is an imaginary reaction, but valid as a component in a Hess s law calculation. Few reliable thermochemical data exist for clay minerals those for pyrophyllite and kaolinite can probably be used with sufficient accuracy, on a weight basis. 

In addition to sparingly soluble metal (hydr)oxides, salt type materials involving two such oxides or more, and clay minerals, whose crystallographic and thermochemical data are presented in Chapter 2, the zero points of zeolites, clays, and glasses are listed (in this order) after mixed oxides. Soils and other complex and ill-defined materials are on the end of the list. It should be emphasized that the terms soil , sediment", etc. have somewhat different meanings in different scientific and technical disciplines. This may lead to confusion, e.g. terms kaolin (clay) and kaolinite (clay mineral) are treated as synonyms in some publications. The zero points obtained for composite materials with a layer structure (core covered by coating) are listed separately from those in which the distribution of components is more uniform. 

Gangue minerals and salinity give constraints on the pH range. The thermochemical stability field of adularia, sericite and kaolinite depends on temperature, ionic strength, pH and potassium ion concentration of the aqueous phase. The potassium ion concentration is estimated from the empirical relation of Na+/K+ obtained from analyses of geothermal waters (White, 1965 Ellis, 1969 Fournier and Truesdell, 1973), experimental data on rock-water interactions (e.g., Mottl and Holland, 1978) and assuming that salinity of inclusion fluids is equal to ffZNa+ -h m + in which m is molal concentration. From these data potassium ion concentration was assumed to be 0.1 and 0.2 mol/kg H2O for 200°C and 250°C. 

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