Character of Kaolinite and Gibbsite

Because of its simple chemical composition, the role of kaolinite in clay mineral assemblages is peripheral to or a limiting case for chemiographic representation of clay mineral systems which contain free silica forms as a compositional pole. Most often during epi-metamorphism kaolinite is incorporated into other phases due to a displacement of the bulk 

Gibbsite, a mineral more and more frequently identified in the early stages of rock weathering (especially magmatic rocks) is usually not important in river sediments but has been noted in small quantities over large areas in deep sea sediments (Griffin, et al., 1968 Biscay, 1965). 

Although gibbsite and kaolinite are important in quantity in some soils and hydrothermal deposits, they have diminishing importance in argillaceous sediments and sedimentary rocks because of their peripheral chemical position. They form the limits of any chemical framework of a clay mineral assemblage and thus rarely become functionally involved in critical clay mineral reactions. This is especially true of systems where most chemical components are inert or extensive variables of the system. More important or characteristic relations will be observed in minerals with more chemical variability which respond readily to minor changes in the thermodynamic parameters of the system in which they are found. However, as the number of chemical components which are intensive variables (perfectly mobile components) increases the aluminous phases become more important because alumina is poorly soluble in aqueous solution, and becomes the inert component and the only extensive variable. 

Pyrophyllite is probably not stable below some 300°C at 1 Kb pressure. This temperature will be reduced at lower total water pressure but probably will remain at a substantial value (Velde and Kornprobst, 1969). Its existence in sedimentary rocks should be indicative of relatively high temperatures if it is stable. It is typically found with illite-chlorite or occasionally with allevardite (Dunoyer de Segonzac, 1969 Ehlmann and Sand, 1959). The reaction Kaolinite + quartz = pyrophyllite is an important marker in phyllosilicates parageneses when it can be observed. 

The formation of talc has been accomplished in the laboratory only at high pH for temperatures below 80°C (Siffert, 1962). Talc which is rarely found in terrigeneous sediments is most probably always of detrital origin as is that found in recent Gulf Coast sediments (Isphording, 1971). 

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