Mineralogical and geochemical analyses of ultrafine (<0.1m) extracts of Pliocene clays from Olduvai Gorge reveal the complexities of clay diagenesis in saline, alkaline paleo-waters. Multiple authigenic phases are routinely present; these may be distinguished and quantified by decomposition of XRD (060) peaks coupled with geochemical (microprobe, HRTEM-AEM) and crystallographic (IR) investigation. By assigning geochemistry based on XRD results, we identify three principal phases by octahedral composition: dioctahedral Al-rich, dioctahedral relatively Fe-rich, and Mg-rich with a trioctahedral component. Average octahedral formulae are Al 1.30 Fe 0.57 Mg 0.13 (Al-rich), Al 0.56 Fe 0.80 Mg 0.62 (Fe-rich), and Al 0.09 Fe 0.23 Mg 2.09 (Mg-rich) per half formula unit. IR analyses support these octahedral structures. The Mg-rich phase either has coexisting dioctahedral and trioctahedral domains within individual sheets, or a homogeneous octahedral sheet of intermediate composition. This is, to our knowledge, the first report of an intermediate octahedral occupancy in a 2:1 phyllosilicate.HRTEM observations indicate both solid state octahedral and illitization reactions and dissolution-precipitation. Layer charge and interlayer cations suggest that during illitization, layer charge increase was due to interaction with Na-rich brines, but later K uptake may have been with either saline or fresher fluids with higher K/Na ratios.All three phases occur throughout the basin; relative proportions and octahedral compositions vary. Total Mg content of "bulk" <0.1m clay fractions, (a paleochemical indicator in this and other basins) is the product of both the relative abundance and the Mg content of the Mg-rich phase. In addition to shedding light on the crystal chemistry of authigenic 2:1 phyllosilicates, these results demonstrate the paleolimnologic importance of discriminating the presence of multiple authigenic clay phases in lacustrine deposits.

The mechanism for the kaolinization of smectite is extremely complex. The purpose of this study was to explore this mechanism by providing more microscopic information about kaolinite-smectite (K-S) intermediate phases. Crystal-chemical changes were investigated and integrated in a model of the transformation mechanism. Eight K-S samples from three localities, derived from volcanic ash beds, were studied using transmission and analytical electron microscopy (TEM, AEM) and high-resolution TEM (HRTEM). The study completes a previous investigation, using several analytical techniques. The samples cover the range of K-S composition available from the previously studied sample set. Analysis by TEM indicated the preservation of particle morphology throughout the process. Most K-S particles had anhedral, smectite-like morphology, and only the most kaolinitic specimen revealed the coexistence of anhedral and euhedral, hexagonal particles. Analytical electron microscopy showed large chemical variations within samples, corresponding to various degrees of smectite kaolinization. Comparison of chemical results (Si/Al) and d 060 values (proxy for octahedral composition) with the extent of kaolinization from thermogravimetry (TG) indicates that chemical changes in the octahedral sheet occur mainly when the proportion of kaolinite is 40À70%. The results above are consistent with kaolinization occurring via layerby-layer transformation through the progressive loss of individual tetrahedral sheets in smectite layers and subsequent chemical changes in the octahedral sheet. Such a mechanism would produce the results observed in this study: (1) most particles preserve their original morphology; (2) significant variation in terms of the extent of transformation of particles within samples, and (3) formation of crystal structures intermediate between those of smectite and kaolinite, with parts of the tetrahedral sheets missing (kaolinite-like patches). Such structures become least stable at kaolinite~50%, when the perimeter of the kaolinite-like patches is largest and chemical changes in the octahedral sheet can occur more easily. Kaolinite layers could not be resolved by HRTEM in most cases and showed lattice fringes corresponding to superstructures. A model was established to quantify kaolinite and smectite layers in the HRTEM images with results which matched TG-derived values.

Kaolinite-smectite mixed-layers have been found to have a complex structure with smectite and kaolinite domains within layers. Here we further investigate this structure in samples with 0–80% kaolinite layers, as determined by X-ray diffraction, by means of magic angle spinning nuclear magnetic resonance (MAS NMR) of 29Si and 6Li. The 29Si NMR experiments were carried out on two samples (55 and 80% kaolinite layers), before and after their exchange with Mn2+, a paramagnetic ion that causes NMR signal loss from neighbouring nuclei, in order to investigate the distance between Mn ions and Si atoms in kaolinite sites. The 29Si NMR intensity from such sites (at ~–91 ppm) was reduced upon Mn exchange, indicating that some Mn ions are located near kaolinite Si sites. The position of the 6Li peak changes slightly (–1.3 to –1.8 ppm) but progressively with increasing kaolinite content (0–80% kaolinite layers) of four K-S specimens, suggesting two slightly different chemical environments for interlayer Li, one related to smectite and the other to kaolinite. The two sets of experiments are consistent with a complex structure of kaolinite-smectite, including smectite and kaolinite domains within layers and/or interlayers of varying smectitic and kaolinitic character.