Solution composition smectites

Smectite Number of WD Cycles Solution Composition Measurements of 6180 Average S180... 

Figure 7 Mineral mass transfer coefficients versus smectite solid solution composition for Wyman Creek mass balances, Inyo Mountains (Bowser and Jones, 2002). Upper and lower bounds on possible smectite compositions are where goethite and K-feldspar mass transfers are equal to zero.

FIG. 8 Cd sorption onto SWy-1 smectite as a ftmction of solution composition. (From Ref. 

Chlorite is another mineral that is commonly associated with mixed-layered clays. Complete soHd solutions of chlorite mixed-layer minerals have not been identified. In contrast to iUite—smectite mixed-layer minerals, chlorite mixed-layer minerals occur either as nearly equal proportions of end-member minerals (Rl) or dominated by one end member (RO) (142). Mixed-layer chlorite may consist of any of the di—tri combinations of chlorite and chlorite mixed-layering occurs with serpentine, kaolinite, talc, vermicuhte, smectite, and mica. References of specific chlorite mixed-layer minerals of varied chemical compositions are available (142,156). 

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

A recent contribution to mass-balance modeling of weathering (Bowser and Jones, 2002) utilizes a spreadsheet graphical method to interpret mass balance in watershed systems in place of the strictly numerical solution methods. Key to the approach is to solve the mass-balance equation for a 10 X 10 matrix as a function of mineral composition (specifically in terms of plagioclase feldspar and smectite compositions). Exploration by means of models that cover mineral compositional space limited the range of possible compositions and restricted the range of possible mass-balance solutions. Figure 1 (after Bowser and Jones, 2002) is an example of the spreadsheet approach applied to the Sierra Nevada ephemeral... 

The transformation of smectite to mixed layer smectite-illite, and ultimately to illite, with increasing temperature is an extremely important reaction in many sedimentary basins, including the northern Gulf of Mexico Basin (Hower et al., 1976 Boles and Franks, 1979 Kharaka and Thordsen, 1992). The water and solutes released and consumed by this transformation are major factors in the hydrogeochemistry of these basins, because of the enormous quantities of clays involved. Several reactions conserving aluminum or maintaining a constant volume have been proposed for this transformation (Hower et al., 1976 Boles and Franks, 1979). The reaction proposed below (Equation (4)) conserves aluminum and magnesium, and is probably a closer approximation based on the composition of formation waters in these systems ... 

Ransom, B., and H. C. Helgeson. 1993. Compositional end members and thermodynamic components of illite and dioctahedral aluminous smectite solid solutions. Clays Clay Minerals 41(5) S37-50. 

Isotopic composition of smectites in bentonites from Ishirini indicate an alteration of the pyroclastic rocks predominantely by heated meteoric water at temperatures ranging from 30 to 90 °C. Alteration by fluids similar to the present day hydrothermal solutions from the deep geothermal reservoir can be excluded. Geothermometric calculations based on 5 0 and 6D ratios of the smectites can only provide a temperature range for the bentonite formation. In addition, several uncertainties have to be considered, e.g. the reability of the fractionation factors, deviations in the climatic conditions or isotopic exchange with fluids percolating through the newly formed bentonites. 

In the first method, commercial, partially neutralized AP solutions are modified by addition of La + or Ce +, and the pillaring process of a smectite clay mineral is performed under reflux or under hydrothermal conditions. A material is obtained that is, at least partially, expanded to a t/ooi spacing of 2.6 nm. The structure and composition of the pillaring oligomer is unknown, nor is it known to what extent the La or Ce cations are incorporated into the pillar. In view of the pH conditions of the pillaring process, only traces are expected to be incorporated in the pillar some may be ion exchanged or adsorbed on the pillar. Anyhow, the data of Table 2 clearly show the increase in surface area and pore volume with respect to the regularly pillared materials. 

---