Compositional Variation in Component Layers in Natural Illite/Smectite
Published chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O10(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per O10(OH)2 unit. Cation-exchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed.
Significance of the depth-related transition montmorillonite-beidellite in the Bouillante geothermal field (Guadeloupe, Lesser Antilles)
Abstract The crystal structure and crystal chemistry of dioctahedral smectites in high-enthalpy geothermal systems were investigated through samples collected in two wells drilled in the Bouillante geothermal area to understand the factors that control their vertical variation. Smectites were examined by optical and scanning electron microscopy, electron microprobe analysis, X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and oxygen-isotope analysis. Smectites predominate within the upper part of the drill holes (up to 260 m depth; temperature range: 67-163 °C). The XRD, FTIR, and chemical microanalyses clearly demonstrate a transition from montmorillonite to beidellite with increasing depth and temperature that proceeded through interstratification of beidellite-like and montmorillonite-like layers. Montmorillonite predominates at temperatures below 100 °C, whereas beidellite predominates between 110 and 163 °C. However, this transition is not explained by a thermally controlled beidellitization process but appears to be related to hydrothermal fluids from which these smectites precipitated. The δ18O values of the fluids that equilibrated with smectites (-3.3 to 0.4‰) indicate that beidellitic smectite precipitated from the hot geothermal fluid associated with minor amounts of residual solutions resulting from local boiling. In the same way, montmorillonitic smectite precipitated from reacting solutions whose origin lies in the phreatic water table (±seawater contribution) associated with minor amounts of liquids resulting from the condensation of vapors escaped from the boiling zones. The mixing rate of geothermal fluid with meteoric waters exerted a major influence on the montmorillonite vs. beidellite ratio of the smectite material as underlined by the irregular depth-related smectite transition.
Beidellite crystallization from plagioclase and amphibole precursors: local and long-range equilibrium during weathering
Abstract A comprehensive study of weathering in plagioclase-hornblende rock (amphibolite) from La Roche-l'Abeille, Massif Central, using optical and classical methods of clay mineralogy and electron microprobe determinations demonstrates the mineral reactions which can occur as amphibole and plagioclase weather. Both minerals decompose ultimately into beidellitic smectites but of different lattice compositions. It appears, depending on the host mineral, that an aluminous ‘intergrade’ mineral, similar to a hydroxy-complexed vermiculite, forms first, recrystallizing into the beidellites. Exchangeable ion composition of the beidellites suggests that all smectites were in equilibrium with waters of similar chemical activity for Ca and Mg ions at equivalent depths in the profile. The activities of the exchangeable interlayer ions of the newly-formed smectites are similar, regardless of the host mineral. Very local chemical potentials evidently dictate the lattice composition of the phases, ferribeidellite formed from amphiboles, aluminous beidellite formed from plagioclases, but longer range equilibria (centimetric) dictate the type of phase which will crystallize, i.e. smectite.