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.
AbstmcL Following the melhod of Gibbs, lhe equilibrium equations for a solid and various fluids in mnMcI, including apillarily and elasticity, are written for the general case. They are then applied to the example of a lhin plate in conlac1 with a drop of fluid. ?he classical Young", equalion is modified.
The thermodynamics and mechanics of the surface of a deformable body are studied here, following and refining the general approach of Gibbs. It is first shown that the 'local' thermodynamic variables of the state of the surface are only the temperature, the chemical potentials and the surface strain tensor (true thermodynamic variables, for a viscoelastic solid or a viscous fluid). A new definition of the surface stress is given and the corresponding surface thermodynamics equations are presented. The mechanical equilibrium equation at the surface is then obtained. It involves the surface stress and is similar to the Cauchy equation for the volume. Its normal component is a generalization of the Laplace equation. At a (body-fluid-fluid) triple contact line, two equations are obtained, which represent: (i) the equilibrium of the forces (surface stresses) for a triple line fixed on the body; (ii) the equilibrium relative to the motion of the line with respect to the body. This last equation leads to a strong modification of Young's classical capillary equation.
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