Wietrzenie biotytu w obrębie pokrywy zwietrzelinowej z rejonu Strzelina na Dolnym Śląsku

Irena Kossowska




Biotites occurring granites and gneisses from Strzelin (Lower Silesia) and overlying the wearthering cover were subjected to petrographic and mineralogical studies. Biotites extracted from these rocks originated in the course of preliminary stage of alteration as well as the first, second and third stage of kaolinization; that is they represent all the lithological units differentiated in borehole profiles E/8, H/5 and 4/W. The presence of 2 to 5 phases, that is, biotite, kaolinite, decolorized biotite, quartz, muscovite and montmorillonite phases, was found in macroscopically uniform biotites. Biotite and kaolinite predominate quantitatively in these phases.

Biotites may be divided into: slightly weathered, moderately weathered, and declorized and heavily weathered. The predominance of Fe3+ on Fe2+ is marked in moderately and heavily weathered biotites only. In crystallochemical formulae of biotites the content of iron is overestimated as the formulae do not cover so-called inter-domaine iron, i.e. iron mechanically entrapped between plates of weathered biotite and post-biotite kaolinite, as well as iron presumably occurring in the structure of kaolinite originating at the expense of biotite.

Titanium, similarly as iron, is freed from the biotite structure in the course of weathering process. It does not leave the weathering environment but rather remains in the form of leucoxene in the space between biotite and post-biotite kaolinite plates.

Magnesium and potassium are freed from biotite structure and do not leave the environment until the third stage of kaolinitization.

Biotite is subjected to both slow and rapid transformations. Slow transformation begins at preliminary stage of alterations and it ends during the third kaolinization stage. It leads to weathering of biotite and, subsequently, to its transformation decolorized biotite, that is, muscovite or illite. Illite is subsequently alternated into montmorillonite, and montmorillonite – into kaolinite. During the preliminary stage only a small part of biotite was alternated into montmorillonite which, in turn, changed into kaolinite in upper parts of the weathering profile.

Rapid transformation of biotite take place at the preliminary stage of alterations and in the first and at the turn of the second and third stages of kaolinization. A low concentration of ions K+ and Mg2+ in solutions from these parts of the weathering profile facilitates direct alteration of biotite into kaolinite.

Montmorillonite phase was found in unkaolinitized biotite (sample 6–E/8). It may assumed, therefore, that montmorillonitization preceded kaolinization of biotite granites and gneisses.

The content of trace elements in mica samples was determined. The analysis of shown that Cr and Sn do not migrate from biotite structure in the course of weathering, whilst Ni, Co, V, Rb, Sr, and Ba are leached but not before the third kaolinization stage. Migration of Li from biotite structure takes place at second third kaolinization stage.


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