Место издания: Florence , Год издания: 2004

Upper Jurassic sediments of the Georgiev Formation are widely spread over the West Siberia Geosyneclise (WSG). The presence of phosphorite concretions and glauconite are the essential features of the Georgiev sediments. Georgiev Formation g1auconite from different parts of WSG was investigated by different methods (XRD, EPMA, SEM, TEM, AEM ICP-MS and XRF). There are three types of glauconite as observed in thin-sections: 1) green well rounded, almost without cracks; 2) brown of irregular shape and almost without cracks; 3) grains with green core and brown rim, with cracks filled by hematites and Fe-rich clays. Type 1 and type 2 grains are sometimes mixed together. Green grains differ from brown ones in their lower content of SiO2, Al2O3, CaO and Na2O and in their higher Fe-total and K2O contents. Type 3 grains are characterized by similar features: their green cores have lower content of SiO2, Al2O3, CaO and Na2O in comparison with their brown rims. At the same time, if green grains are characterized by higher content of MgO and TiO2 in comparison with brown grains, the green cores and brown rims have an inverse negative relationship. We interpret the change from green to brown colors as a secondary alteration process of green mature glauconite grains through leaching of Fe2+ and K+, and formation of a "less mature" glauconite together with Fe-rich clays and hematite within cracks and rims. These secondary processes are mainly observed in the western and northern parts of the WSG. It is supposed that glauconite in the marginal parts of the WSG was formed on the lower depths, then in the central, central-eastern and northern parts, and was more accessible to the free oxygen exposure as the result of the wave activity (storm), temporary lowering of sea level. This research was made in the framework of the INTAS grant YSF 2002-108