Kaolinite, Pyrophyllite, Gibbsite and Talc

Gibbsite and the neutral lattice minerals, 1 1 or 2 1 represent the extremes of chemical variat.on in the clay minerals. Gibbsite is a hydrated form of alumina. Kaolinite and pyrophyllite can be considered to be strictly aluminum-silicates, i.e., no ions other than Al, Si, 0, H are present in appreciable quantities in these minerals. This is not as 

Fe and Al are found in magnesian talc and it also has a ferrous form minnesotaite. However, the talc found in sedimentary rocks in sediments is predominantly hydrated magnesium-silicate. 

For various reasons, gibbsite and kaolinite are really the only minerals of the neutral lattice type which occur with any frequency in non-metamorphic rocks. These minerals are formed in soils, most noticeably from granitic rocks but are also commonly found forming from basic rocks during the weathering process (Millot, 1964 Tardy, 1969). The 

Kaolinite is a frequent product of hydrothermal alteration, commonly associated with mica or expanding phases in the argilic type alterations 

The above use of stable coexisting minerals is of course based upon the fundamental consideration that the chemical system is closed that is, the chemical components K, Si and OH are inert , their relative proportions, mass, in the system determines the phases formed. This can be assumed valid for many argillaceous sediments and rocks. However, in some geological environments, aqueous solutions containing alkalis and hydrogen ions in various concentrations (whose activities, therefore, are variables but constant throughout a given system) react with kaolinite or other minerals to influence its stability under otherwise constant physical and chemical parameters. 

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