Layer Charge Relations in Clay Minerals of Micaceous Soils and Sediments
Micaceous clays (“illites”) of a number of sediments, subjected to quantitative mineral-ogical analysis, ranged in different samples from 58 to 65% of a dioctahedral mica of muscovite type, from 3 to 8% vermiculite, from 0 to 10% montmorillonite, from 2 to 12% quartz, from 0 to 4% feldspars, from 1 to 2% rutile and anatase, from 0.3 to 1.2% free iron oxides, and from 16 to 30% chlorite. The soil clays contained less mica (13 to 35%) but more vermiculite (13 to 16%), montmorillonite (6 to 14%) and chlorite (30 to 33%), besides kaolinite (3 to 8%) and amorphous material (6 to 15%). The presence of the various minerals was verified by X-ray diffraction. When the interlayer K of these micaceous clays was replaced by hydrated Na ions through treatment with 0.2 n sodium tetraphenylboron in 1 n NaCl, the analytically determined vermiculite and montmorillonite contents increased. The freshly produced vermiculites had an interlayer charge of 177 to 198 me/100 g. During this replacement of interlayer K in a 1 N salt solution, the pH of the equilibrium matrix solution was observed to increase instead of decreasing as would be expected if oxonium (ONz+) ions were associated with K in the interlayer space, thus precluding the possibility of interlayer ONz+. The calculated stoichiometric equivalent of the pH rise established a significant uptake of protons by the lattice as K was released.
Thermal analyses of vermiculites show 0.5 to 0.7% higher OH water content than the parent micas. Oxidation of ferrous iron in the silicate structure, suggested as a possible mechanism for lowering of layer charge during weathering of mica to vermiculite, accounted for about 66% of this in samples high in iron (16.0% FeO) and only 30% of this in samples fairly low in iron. The rate of extraction of K from muscovite with no ferrous iron was found to be pH dependent, increasing at lower pH values. The data indicate proton incorporation with structural oxygens of the silicate sheet to form OH as a charge reduction mechanism in weathering of mica, independent of and additional to that involving the oxidation of ferrous iron.
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