Crystal Chemistry of Layer Silicates of the Miocene Green Grain (Congo Basin) from Transmission Electron Microscopy (TEM) and Analytical Electron Microscopy (AEM) Observations

A. Wiewióra1, P. Giresse2, A. M. Jaunet2, A. Wilamowski1 and F. Elsass3
1 Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, 00-818, Warszawa, Poland
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

Abstract: Transmission electron microscopy (TEM) and analytical electron microscopy (AEM) methods were used to study the crystal chemistry of phyllosilicates occurring in green grains of Miocene sediments from the Congo continental shelf. Using diagrams based on wt. % K and the (Fe + Mg)/Al ratio, minerals were distinguished from mixed-layer phases. The most abundant detrital mineral is Fe-kaolinite. The morphology and composition identify this mineral as a component of ferralitic soils. This Fe-rich kaolinite has undergone a complex process of partial dissolution and recrystallization and further enrichment in Fe and, to a lesser extent, in Mg in the marine environment. The detrital mica observed with TEM retains the original morphology and chemistry of muscovite. Alteration processes resulted in the crystallization of 1:1 trioctahedral Fe2+ and Mg-rich minerals and interstratified phases with 1:1 and 2:1 layers in varying proportions observed with the aid of high-resolution transmission electron microscopy (HRTEM) imaging. Included among the newly formed 7-Å phases are those apparently containing excess Si. The smectites are apparently neoform, and chemical analyses showed that these marine K-smectites belong to the beidellite-nontronite series and have tetrahedral substitutions similar to muscovite. Their compositions are closer to beidellite than to nontronite, although the latter was observed in association with goethite. The TEM observations and crystallochemical data show that mineral alteration ceased after forming mixed-layer minerals, and alteration did not reach the glauconitization stage. Apparently, the Miocene assemblages experienced rapidly changing environmental conditions and high sedimentation rates that continue today.

Key Words: Alteration Markers • Congo Basin • Detrital Phyllosilicates • Green Grains • Neoformed Phyllosilicates

Clays and Clay Minerals; October 1999 v. 47; no. 5; p. 582-590; DOI: 10.1346/CCMN.1999.0470505
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)