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The Drmno coal deposit is located at the eastern part of the Kostolac coal basin (Serbia). Bentonite at the Drmno locality appears as light-gray interlayers in the oldest third coal layer (with average thickness of about 17 m) and its immediate substratum. Its characteristics were investigated with the X-ray diffraction (XRD), chemical and atomic absorption spectrophotometer (AAS) analysis. Complete samples composed from clay minerals which dominate over quartz and goethite, and with feldspars and calcite which are of very little content. Clayey fraction composed from smectite which dominate and very little content of kaolinite and illite. Bentonite is montmorillonite clay from the smectite group which formatted from the devitrification and chemical alteration of the volcanic glass. Its origin depends of various conditions (lithologic, geomorphologic, climate variations, etc.), but the contents of some micro elements (Cr and Ni) are very important because they could indicate to the composition of the starting volcano material.
The investigated bentonite sample has higher content of Cr (135 ppm) in comparison to the average in the Earth's core and in clays (100 ppm) and somewhat lower content of Ni (68 ppm) in comparison to the average in the Earth's core (75 ppm) and in clays (80 ppm). Therefore, this indicates that the protolith from which this bentonite formatted was most probably of the basalt type. Lower Ni content indicates that it was most probably precipitated with the Fe-hydroxide, i.e. goethite. Other investigated micro elements, such as Li and Cu have higher, while Mn, Co, Ba, Rb and Cs have lower contents than average in the Earth's core and clays.
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