Kaolinization Members

Examples are the mica group (e.g., KMg3(OF[)2Si3A10io) of which biotite, (K(Mg,Fe)3(OH)2Sl3A10io) (Figure 1.55) and talc (Mg3(OFl)2Si40io) are members, and contains a sandwich of two layers with octahedrally coordinated cations between the layers and clay minerals such as kaolin, Al4(OH)8Sl4C3io. 

The formula of the solid product (Al2Si205(0H)4) is that for kaolinite, an important member of the serpentine-kaolin group of clay minerals (Section 4.5.1). This reaction demonstrates incongruent dissolution of feldspar, i.e. dissolution with in situ reprecipitation of some compounds from the weathered mineral. 

So far, everything that has been stated with regard to structure applies equally to all the members of the kaolin group the distinguishing feature is the way in which the units are stacked one upon another. In Figure 13 the stacking is approximately that of... 

Probably the most ubiquitous silicate minerals in soils throughout the world are the layer silicates known as the kaolin minerals. The group includes the dioctahedral minerals kaolinite, halloysite, dickite, and nacrite, and the trioctahedral minerals chrysotile, antigorite, chamosite, and cronstedite. Halloysite and disordered forms of kaolinite seem to be the only members of... 

Halloysite is probably the most common authigenic kaolin mineral in soils and the least understood. It commonly occurs in tubes or laths and occasionally as blocks and usually shows evidence of much structural disorder. Single crystals suitable for X-ray diffraction studies have not yet been found, but powder patterns and its chemical formulas clearly identify it as a member of the dioctahedral kaolin group of minerals. 

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