Investigations concerning selective sorption and fixation of K and similar cations by clay minerals and soil clays and the mechanisms of these reactions are reviewed. In particular, recent observations on selective sorption of these ions in dilute solutions by weathered micas and vermiculite in relation to the interlayer structures are discussed in detail. Also, implications of the resistance to weathering of small mica particles to cation selectivity by soils are described. Despite the increased understanding of sorption and fixation reactions, the following aspects remain unclear. First, the mechanism of the collapse of alternate layers in vermiculite on K or Cs sorption has not been unequivocally established. Second, factors that impart stability to the central core of mica particles so that K extraction becomes progressively difficult are not known. Third, inability of Ca or Mg ions to expand interlayers of Cs-saturated vermiculite in contrast to K-saturated vermiculite is not completely understood.

Determination of the Orientation of OH Bond Axes in Layer Silicates by Infrared Absorption

Selective sorption and fixation of cations by clay minerals : A review

The Layer Silicates

The infra-red spectra of layer silicates

Publisher Summary This chapter presents exchange studies concerned with pure systems—namely, the suspensions of clay minerals obtained from known sources and the clay fraction of soils known to contain predominantly a particular clay mineral. Partly because of its high exchange capacity, montmorillonite plays a major role in obtaining information about the mechanism of cation adsorption, covering almost complete ranges of composition for different ion pairs. The clay minerals vermiculite, kaolinite and illite (hydrous mica) have been used less frequently. The information about cation exchange involving the organic fraction of soils is scarce because of the difficulties of isolation and identification. In some cases, organic soils (peat soils) have been used. Cation-exchange properties of soil-borne oxides have recently received attention because of the increasing interest in the adsorption of heavy metals. Apart from investigations on specific soil constituents, samples of natural soils have also been used to obtain an estimate of the expected behavior in a field.

Chapter 5: Survey of Experimental Information on Cation Exchange in Soil Systems

Cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length/strength in these minerals. These data therefore provide a useful crystal-chemical perspective on relative mass-transfer kinetics of radiogenic isotopes, and account for the observation that biotite is generally less retentive of 40Ar and 87Sr than coexisting muscovite. Partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite. First, the preferences of large (Rb, Cs)+ in biotite and of small La3+ and Na+ in muscovite indicate a relatively spacious interlayer volume in biotite (suggesting a longer mean K−O bond). Second, the preference of interlayer vacancies in biotite (with some/all possibly H2O/H3O+-filled) suggests that its adjacent 2:1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite. Third, the relative exclusion of large Ba2+ from biotite despite its large interlayer sites is attributed to O−H bonds pointing into the interlayer cavity sub-normal to (001); (K+, Ba2+)-H+ repulsion thereby induced by the bare proton both destabilizes Ba2+ and weakens K−O bonds. In contrast, muscovite offers a more favorable electrostatic environment for Ba2+ substitution since its O−H bonds are directed into the vacant M1 octahedral site sub-parallel to (001). This hypothesis is supported by the observation that progressive F(OH)−1 exchange enhances Ba2+ partitioning into biotite/phlogopite relative to coexisting muscovite. These crystal-chemical differences between biotite and muscovite are mirrored in calculated values of “ionic porosity”, Z i , defined here as the percentage of their interlayer unit-cell volume not occupied by ions. A monitor of ionic packing density and geometry, Z i is inversely correlated with K−O bond strength, which appears to be the rate-determining “kinetic common denominator” for a variety of processes affecting micas – including those responsible for loss of radiogenic isotopes in biotite and muscovite. Accordingly, the relatively longer/weaker K−O bonds of biotite are envisioned as being more easily stretched (during volume diffusion) or broken (during recrystallization or retrograde alteration). This in turn accounts for common observations of enhanced radiogenic Ar/Sr loss and younger 40Ar/39Ar and Rb/Sr ages in natural biotite (high Z i ) relative to coexisting muscovite (lower Z i ). Significantly, this pattern may arise irrespective of isotopic loss mechanism (diffusion or recrystallization, etc.), and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field, petrographic, and petrologic constraints. Extension of this partitioning/porosity-based synthesis leads to prediction of corollary age-retentivity-composition effects among chemically diverse trioctahedral and dioctahedral micas, which are best field tested in terranes that cooled slowly under dry, static conditions. Pressure effects on argon retention are also inferred from the porosity model.

Determination of the Orientation of OH Bond Axes in Layer Silicates by Infrared Absorption

Citations

Selective sorption and fixation of cations by clay minerals : A review

The Layer Silicates

The infra-red spectra of layer silicates

Chapter 5: Survey of Experimental Information on Cation Exchange in Soil Systems

The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology

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The Layer Silicates

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