Clay mineralogy of the Boda Claystone Formation (Mecsek Mts., SW Hungary)

Németh, Tibor and Máthé, Zoltán and Pekker, Péter and Dódony, István and Kovács Kis, Viktória and Sipos, Péter and Cora, Ildikó and Kovács, Ivett (2015) Clay mineralogy of the Boda Claystone Formation (Mecsek Mts., SW Hungary). Open Geosciences. ISSN 2391-5447 (Submitted)

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Abstract

Boda Claystone Formation (BCF) is the host rock of the planned site for high level nuclear waste repository in Hungary. Samples representing the dominant rock types of BCF were studied: claystone with high illite content, albitic claystone and analcime bearing claystone. Clay minerals in these three rock types were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (DTA-TG), and the results were interpreted from the point of view of the radionuclide sorption properties being studied in the future. Mineral compositions of bulk BCF samples vary in wide ranges. In the albitic sample, besides the dominant illite, few percent of chlorite represents the layer silicates in the clay fraction. Illite is the dominating phase in the illitic sample, with a few percent of chlorite. HRTEM study revealed that the thickness of illite particles rarely reaches 10 layers, usually are of 5-6 TOT layer thick. Illite crystals are generally thicker in the albitic sample than in the illitic one. The significant difference between the clay mineral characterisitics of the analcimous and the other two samples is that the former contains 10-20 % regularly interstratified chlorite/smectite beside the dominant illite. Based on the structural and chemical data two illite type minerals are present in the BCF samples: 1M polytype containing octahedral Fe and Mg besides Al, 2M polytype illite generally is free of Fe and Mg. Close association of very thin illite plates and nanosized hematite crystals is typical textural feature for BCF. The goal of this study is to provide solid mineralogical basis for further studies focusing on radionuclide sorption properties.

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