TEM study of the mineralization processes within
A green grain of Miocene marine sediment of Congo

A. Wiewióra¹, P. Giresse², A.M. Jaunet³, A. Wilamowski¹, F. Elsass³

¹ Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, 00-818, Warszawa, Poland, wiewiora@twarda.pan.pl

² Laboratoire de Sédimentologie et Géochimie marine, URA CNRS 715, LEA Sciences de la Mer, Université de Perpignan, Avenue de Villeneuve, 66860 Perpignan, France

³ Sciences du Sol, INRA, Route de Saint Cyr, 78026 Versailles, France

Key words: Congo Basin, green grains compositions, detrital phyllosilicates, neoformed phyllosilicates, markers of transformation

Abstract

TEM and AEM methods were applied to study crystal-chemistry of phyllosilicates occurring in the green grains of the Miocene sediments on the Congo continental shelf.  Detrital mica has original morphology and chemistry common to muscovite. Another and more abundant detrital mineral is Fe-kaolinite. Its morphology and composition clearly identifies it as a component of the ferrallitic soils very characteristic for the tropical zone. Fe-kaolinite underwent in the marine environment the complex processes of the further enrichment in Fe and to lesser extent in Mg, coupled with uptake of K, leading to the formation of the 2:1 phase. Evidently neoformed nannostructure is shown by smectite, whose composition is closer to beidellite than to nontronite. Another neoformed phase is potassium-free. Its structure is not clear. Mixed-layer phyllosilicates show a very inhomogeneous constitution, resulting from various contents of 1:1 and 2:1 phases.