Mineralogy and crystallochemical characteristics of heu-type minerals from zeolitic tuff deposits of Serbia

Abstract

The results of perennial research of several Serbian zeolitic tuffs enriched with HEU-type minerals are presented in this paper. There are several recognized zeolitic tuff deposits containing HEU-type minerals: Zlatokop, Igros, Beocin, Toponica and Slanci, but their comparative mineralogical and crystallochemical features have not been studied in detail so far. These zeolitic tuff deposits are spatially and genetically connected to volcanic and pyroclastic rocks of marine and lake environments of Senonian and Eocene, and Neogene age, respectively. As a result of devitrification and diagenesis process of volcanic glass within zeolitic tuffs hypocrystalline porphiry and vitroclastic textures occur. The studied zeolitic tuffs are mainly composed of heulandite occuring in a form of needle-to plate-like crystals of 0.1 do 100 mu m in length, associated with other silicates. Depending on the type and content of exchangeable cations as well as the thermal stability of these raw materials, clinoptilolite-Ca and heulandite-Ca can be distinguished. The values of cation exchange capacity and surface area capacity range from 96 to 166 meq/100 g, and from 8.0 to 10.5 meq/100 g, respectively. HEU-type minerals can be distinguished either by a Si/Al ratio or arrangement of extra framework cations within the crystal structure of these minerals.