The processes of clay mineral formation were studied in seven podzol profiles developed on granitic regoliths in the Polish part of the Tatra Mountains. The selected profiles have similar parent material and macroscopically represent different stages of soil development (from initial to advanced). Bulk soil material (<2 mm fraction) and separated clay fractions (<2 μm) were studied using a petrographic microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy-dispersive spectrometry methods. The mineral compositions of the bulk soil samples are more or less the same (quartz, feldspars, mica and minor amounts of other phyllosilicates). The clay fractions are composed of mica and mixed-layer minerals which contain hydrated interlayers of vermiculitic and/or smectitic type, and kaolinite. Smaller amounts of chlorite, feldspars and quartz were also identified. Chlorite is present almost exclusively (except for one profile) in lower soil horizons (C, B/C, B). The amount of minerals with hydrated interlayers increases up the profiles. Kaolinite is present in all the samples except for the lowermost soil horizons (C) of two of the profiles. In some of the B horizons, the formation of hydroxy interlayers within hydrated interlayers is observed. The main processes of clay mineral formation recognized in the soils studied are: inheritance from the parent rocks; crystallization of kaolinite from soil solutions; the formation of dioctahedral vermiculite at the expense of inherited dioctahedral mica, and the formation of dioctahedral smectite at the expense of vermiculite. The recognized sequence of transformation is as follows: M → R0 M-V (12 Å or 14 Å) → R0 M-12 Å V → R1 M-12 Å V → 12 Å V → V-S → S. Observed formation of hydroxy interlayers seems to be pH dependent, starting when the pH ≥4.4. The process of dissolution of primary silicates occurring simultaneously with the transformation is also documented.

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