Magnesium and Silicon Activities in Matrix Solutions of Montmorillonite-Containing Soils in Relation to Clay Mineral Stability
1971
Weaver, R. M. | Jackson, M. L. | Syers, J. K.
Soils of varying geomorphological site characteristics were sampled in the southern Wisconsin catena for the preparation of soil matrix solutions used for solute activity determinations. The catena clays contained from 27 to 45% of montmorillonite and from 11 to 3% of kaolinite in the < 0.2µm fraction. The SiO₂/Al₂O₃ molar ratios of the reactive fraction of these < 0.2µm clay fractions ranged from 2.1 to 2.4 for the more acid horizons, indicating kaolinite or aluminous chlorite weathering products. Those of the reactive fraction from the calcareous and poorly-drained horizons ranged from 3.1 to 3.9, indicating montmorillonite or beidellite (dioctahedral smectite) stability. The solute activity functions, pH -½pMg²⁺ and pSi(OH)₄, for the field-fresh soil matrix solutions fell distinctly into areas indicative of either montmorillonite stability (poorly drained or calcareous horizons) or of kaolinite stability (moderately well- to well-drained, acid horizons) in a phase stability diagram developed to include a Mg-saturated montmorillonite phase. The solute activity functions of the matrix solutions from Mg-saturated soils or clays approached the montmorillonite-kaolinite phase join more closely, with less impress from the geomorphic and weathering environments of the sites. The solute activity functions for montmorillonite-free soils from northern Indiana, for montmorillonites of bentonites, and for kaolinites and chlorite (controls) fell in the appropriate areas of the diagram. The pH -½pMg²⁺ solute values of the kaolinite-forming soils ranged from 2.3 to 4.6; the pSi(OH)₄ values, from 3.2 to 3.9. The montmorillonite-forming soils had pH -½pMg²⁺ solute values ranging from 4.7 to 6.2 and pSi(OH)₄ from 3.0 to 4.0. Montmorillonite stability was indicated by higher values of the pH -½pMg²⁺ function and/or Si(OH)₄ activities.
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