engleski

Layer charge of bentonites from Croatia and neighbouring countries

Layer charge is one of the most prominent characteristics of clay minerals ; however, accurate determination of its amount and location remains problematic. Building upon previous research, 13 bentonite samples, for which the chemical and mineralogical composition was already known were chosen and their swelling behaviour after saturation with potassium ions was analysed by X-ray diffraction. The samples were collected from Croatia (Bednja, Poljanska Luka, Divoselo, Draga, Bunarić, Sjeničak, Paripovac and Lončarski Vis), Bosnia and Herzegovina (Šipovo), Serbia (Vranjska Banja) and Slovenia (Zaloška Gorica). The investigated bentonites were formed by alteration of volcanic material ; however, they differ in age, mineralogical and chemical composition. Three of these samples (Divoselo 1 and 2 and Šipovo 1) showed beidellite as the main mineral constituent on the Greene-Kelly test. Chemical formulae were calculated from the available data on the chemical and mineralogical composition of <2 µm fractions and the layer charge was deduced from the resulting formulae. As both of these results are indicative and could be inaccurate due to structural and compositional imperfections of clay minerals, the layer charge estimations were also performed by testing the K-saturated samples' swelling behaviour, using some of the principles postulated in the works of Sato et al. (1992) and Christidis and Eberl (2003). Clay-size fractions of the samples were saturated with K+ ions using a 1M chloride solution. Oriented K-saturated samples on glass slides were treated with ethylene glycol (~40 hours exposure to EG vapour in 40°C) and analysed using XRD. The results showed that the samples can be divided into three groups according to their swelling behaviour and chemical formula: high-charge, low-charge and medium-charge smectites. Both high- and low-charge smectites when solvated with EG showed a rational series of 00l reflexes, unlike the samples that could be characterized as medium-charge, where non integral order indicates random interstratification of monolayered and bilayered EG-smectite complexes. Moreover, observed XRD patterns of low-charge smectites show clearly visible higher orders of maxima, while those of high-charge smectites do not show all of the maxima and some of them are present only as “shoulders”. Some disparities between analysed swelling behaviour and calculated structural formulae exist, especially for samples that contain volcanic glass or mineral impurities. We believe that in those cases calculations based on chemical composition are not accurate enough and complementary methods in determining layer charge are needed and can be more informative.

bentonites, layer charge, formula calculation, XRD

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