Feldspar in soils

The reporting of mica in soil clays depends somewhat on the method of detection. Jackson and Mackenzie [1964] state that some soil clays, which show no indication of mica based on X-ray diffraction, may contain from 5 to 20 % or more of micas based on chemical analysis and on the basis of 10% K2O in mica. According to Schuffelen and van der Marel [1955], soils high in allophane fix very considerable quantitities of potassium. Thus, potassium does not necessarily reside altogether in micas and feldspars in soils. Some of it may be in amorphous material. However, some of the potassium may be in micalike zones of particles, which are largely montmorillonite or vermiculite and have weathered from micas. Such zones may be too small to be detected by X-ray diffraction (Knibbe and Thomas [1972]). 

It is almost trite to add that these are the results to be expected when the position of the large K ion in the feldspar framework structures is considered in relation to the migration of K through the lattice, which would be necessary if there was to be any effective exchangeable K available from feldspars in soils. 

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