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Montmorillonites In Miocene Bentonites From Croatia

Smectites from bentonites at several locations in Croatia were analysed: Gornja Jelenska and Pakrac (Moslavina region), Bednja-Šaša (Hrvatsko Zagorje region), Medakovica and Kovačevići (Banovina region) and Sjeničak (Kordun region). Outcrops and deposits of studied clays occur in Miocene sediments. They were formed by alteration of primary pyroclastic rocks (dacite to andesite composition) in, depending to locality, marine to fresh-water conditions. Bentonitic clays and pyroclastic rocks from fresh water basins are associated with clastic layers while marine originated bentonites are associated with carbonate rocks. According to X-ray diffraction analyses and optical microscopic examinations mineral composition of bentonites (with some differences from location to location) is smectite, tridymite, quartz, plagioclase, micaceous minerals, calcite, dolomite, relicts of volcanic glass etc. Practically pure fraction of smectite was separated from each sample of bentonite and analysed by X-ray powder diffraction, thermal (DTA, TG and DTG) and chemical analyses. Diffraction patterns of randomly oriented smectite samples and of oriented samples (natural, saturated with Ca2+, Mg2+, Na+ and K+ ions and treated with ethylene glycol and glycerine) were analysed by X-ray powder diffraction. Diffraction patterns of smectite products developed by heating on different temperatures in range from 800-1300&ordm ; ; C were also taken. Depending on chemistry of original sample and heating temperature occurring products are cristobalite, spinel, high-temperature quartz, plagioclase, mullite, cordierite, hematite and tridymite. On the basis of performed investigations analysed smectites are determined, depending on locality, as montmorillonites of Cheto, Otay, Tatatilla and Wyoming-type.

montmorillonites; Miocene bentonites; heating products of montmorillonites; Croatia

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