Clay Minerals

Palygorskite-bearing claystones and mudstones were deposited in a salt lake in the middle and lower parts of the Neogene Baiyanghe Formation in the Yangtaiwatan Basin, China. The petrological, mineralogical and geochemical characteristics of the sediments were investigated to determine the factors controlling palygorskite formation. The palygorskite claystones and mudstones have distinctly varying mineral compositions. The claystones are composed of detrital minerals, palygorskite and illite, whereas the mudstones consist mainly of mixed-layer illite/smectite and illite. The palygorskite crystals were intact with sharp edges and interwoven with other minerals, indicating an authigenic origin. The chemical characteristics indicate that the palygorskite claystones in the middle part of the Baiyanghe Formation were deposited in a salt lake environment in an arid and hot climate. As the salinity of the lake gradually increased, the detrital minerals such as quartz, feldspar, dolomite and detrital clay minerals dissolved in the alkaline medium, thus providing Si⁴⁺, Mg²⁺and Al³⁺for the crystallization of palygorskite. The palygorskite coexists with certain amounts of detrital quartz and feldspar with limited roundness and sorting, indicating that the shallow lake of the basin under an oxidation environment may represent a favourable environment for the crystallization of palygorskite.

The adsorption of Rhodamine 6G in aqueous suspension on Laponite B was investigated by electronic absorption and emission spectroscopies. Fluorescence spectra suggest that the monomer is adsorbed at two different surfaces, the external and the internal. A monomer is intercalated in the interlamellar space at low loading of dye (<3% CEC), whereas the monomeric state of the dye seems to be at the solid-aqueous interface in suspensions with high loading (>12% CEC). The metachromatic effect observed in the absorption spectra, for the loading interval between 1% and 15% CEC of Laponite B, is attributed to the dimerization of the dye, which seems, from X-ray diffraction measurements, to be formed at the clay interlayer. The formation constant and the absorption spectrum of the aggregate were obtained and the dimer was structurally characterized by applying the Exciton Theory. The observed fluorescence quenching for loadings lower than 15% CEC is attributed to energy transfer from monomer to the dimer, which obeys the Perrin model.

Solid solutions of fluoromicas WMg₂Li(Si_4−x Ge_xO₁₀)F₂ (W = K, Na, Li. 0 ≦ x ≦ 4), which were closely related to taeniolite, were synthesized and the effect of the isomorphic substitution Ge for Si and also the interlayer cations Li, Na and K respectively upon hydration and dehydration were studied by X-ray diffraction, infrared and thermal analysis. The Na- and Li-series exhibit swelling with water, whereas the K-series does not. The increase of tetrahedral germanium substitution reduces hydration and also lowers the dehydration temperature of hydrates in both the Na- and Li-series. The reasons for such tendencies are discussed. Repulsion between interlayer cations and tetrahedral cations seems to play the dominant role. K in opposition to the smaller interlayer cations Li and Na respectively fits in the interlayer sites of the mica structure and lowers repulsion. By increasing the substitution Ge for Si interionic distances become greater and repulsion also decreases.

Absorption and fluorescence spectroscopies were applied to study the adsorption of rhodamine 6G on several smectite-type clays in aqueous suspension. The dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. The presence of these species and their evolution with the stirring time of the sample and the relative dye/clay concentration depend on the nature of the smectite, the clay concentration and the particle size, factors that also affect the dispersion degree of the clay particles in water.

The distribution of rare earth elements (REE) in two different sediments, a marine clay and a till, was found to be dependent both on grain size and mineralogical composition. In the marine clay the REE content was highest in the coarsest fraction; in the till REE were markedly enriched in the finest fraction. Detrital and authigenic origins for different size fractions have been attributed on the basis of REE contents.

X-ray diffraction is used widely for quantitative analysis of geological samples but studies which document the accuracy of the methods employed are not numerous. Synthetic sandstones of known composition are used to compare a ‘routine application’ of a Rietveld and a reference intensity ratio (RIR) method of quantitative phase analysis. Both methods give similar results accurate to within ~±3 wt.% at the 95% confidence level. The high degree of accuracy obtained is believed to depend to a large extent on the spray-drying method of sample preparation used to eliminate preferred orientation.

Sediments from a Quaternary beach ridge complex along the coastal plain of north-west Egypt have been examined by X-ray diffraction and optical and scanning electron microscopy. Palygorskite was identified in the caliche capping the beach ridges and in the gypsiferous marls between the ridges; petrographic evidence indicated that this had formed authigenically in both environments. Examples are thus provided of the pedogenic and evaporative in situ formation of palygorskite.