Estimation of Vermiculite Content Using Rubidium-Fixation Procedures in Four California Soils
2008
Murashkina, M. A. | Southard, R. J. | Shiraki, R.
Quantification of phyllosilicates in soils often depends on a combination of methods, including x-ray diffraction (XRD), cation exchange and fixation capacities, and microscopy. We examined a Rb fixation method to quantify vermiculite content in California soils derived from granitic and nongranitic alluvium. Whole soil (<2-mm) samples were saturated with Rb and heated to 110°C followed by Rb displacement with NH₄ (Rb₁₁₀°C). Fixed Rb was determined directly by instrumental neutron activation analysis. The amount of vermiculite (g kg⁻ ¹) was calculated as fixed Rb content (cmolc kg⁻¹)/0.154 (based on a cation exchange capacity for vermiculite of 154 cmolc kg⁻¹). For comparison, Rb fixation was also estimated after a 1-h incubation (Rb₁ₕᵣ) with 2 mmol L⁻¹ RbCl followed by a 30-min extraction with NH₄Cl. The more rigorous heating treatment (Rb₁₁₀°C) fixed almost 35 times more Rb than the 1-h method. The Rb₁₁₀°C fixation was positively and significantly correlated with soil clay content. The XRD data showed, however, that clay fractions were dominated by smectite, mica, and chlorite, while vermiculite, the major K- and Rb-fixing mineral, was a dominant mineral in most silt fractions. The Greene-Kelly test showed that, in soils with smectitic clays, some of the charge originated in the tetrahedral sheet. Silt content was correlated with Rb₁ₕᵣ fixation but was not correlated with Rb₁₁₀°C fixation. Our data suggest that Rb fixation by heating to 110°C overestimates the amount of vermiculite when tetrahedrally substituted smectites are dominant in clay fractions, and includes fixation by minerals in the silt fraction.
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