Minerały ilaste osadów serii poznańskiej z profilu Mastki

Ryszard Wyrwicki, Andrzej Wiewióra




A bore made at near Łowicz Lowland) pierced, under the 2,5 m thick Quaternary the Poznań series deposits (Pliocene). The thickness of the Poznań series amounts here to 64 m. It consists of five sedimentary cycles. Cycle I the lowermost, one, corresponds to the horizon of grey clays, whereas cycles II–V – to the horizon of green clays. The section at Mastki lacks the uppermost part of the Poznań series, i.e. the horizon of veriegated clays. Each cycle reveals grey and green-grey clays, clayey silts, and silts sands at the bottom. 10 samples for granulometric, chemical, thermal and X-ray studies minerals have been taken from the clay deposits considered. The knowledge of the minerals has been based on the clay fraction below 2 mm, separated from the deposits. Chemical composition of this fraction is given in Tab. 2, DTA curves – and X-ray diagrams – in Figs. 2–5. The following results have been obtained. The part of the clay minerals in the deposits of the individual cycles and within the individual cycles is differentiated, ranging from 23% to 71% by weight. A mineral of the beidellite-montronite series, disclosing a predominance of the beidellite particle, is the main clay mineral in all the cycles. In the above positions it includes mainly divalent or monovalent cations, depending upon the stratigraphical position of the deposit. The least amount this mineral is noted to appear in c1ay minerals of the I cycle, i.e. the oldest one. The greatest percentage, more than 90% , is observed in the youngest cyc1e V.
Kaolinite is found to occur in all the cycles of these deposits. The highest amount of this minerals is in the I cycle, the lowest amount in the V cyc1e.
Illite occurs in the deposits of the cycles I–IV. Its predominance, about 30%, is observed in the sample of the cycle, whereas in the II cycle, whereas in the I cycle it does appear at all.

In addition to the three clay minerals mentioned above, part of the analysed samples from the lower portion of the section reveals the presence of another phase: muscovite-beidellite.
Thus, three assemblages of clay minerals occur in the section considered: beidellite-kaolinite (samples 1–4); beidellite-kaolinite-illite with traces of mixed structure (samples 5, 6, 8); beidellite-illite-kaolinite and mixed structures (samples 7, 9, 10).
The differentiation of clay mineral composition, and, in consequence of this, a differentiation of chemical composition of the clay fraction have been a basis to subdivide the horizon of green clays into two parts. The upper part of the horizon of the green clays, represented by the deposits of the V cycle (samples 1–4), contains almost pure beidellite with divalent cations in exchangeable positions. Kaolinite is here minor component.
The lower part of the horizon of the green clays, cycles IV–II, includes all the three clay minerals, i.e. beidellite, kaolinite and illite, at places with an admixture of mixed-packet phase.
The horizon of the grey clays, cycle I, is characterized by the occurrence of the three clay minerals, with the predominance of beidellite. In this sedimentary cycle the kaolinite is found in greater amounts than in the previous cycles.
The differentiation of mineral composition of Poznań clays allows to explain the differentation in technological properties of clays used as ceramic materials. On account of the lack of fossils it seems to be possible to perform a horizontal correlation of the strata of the Poznań series, too.


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