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Abstract--The mineral and chemical composition of green glauconific grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast-Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and strafigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost A1 but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe 3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by > 1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3~ The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.

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)/A1 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 Fe 2+ and Mg-rich minerals and interstratified phases with 1:1 and 2:1 layers in varying proportions observed with the aid of hi~h-resolution transmission electron microscopy (HRTEM) imaging. Included among the newly formed 7-A 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 mixedlayer 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.