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SOIL MINERALOGY

Estimation of Vermiculite Content Using Rubidium-Fixation Procedures in Four California Soils

^a School of Earth and Environmental Sciences, Washington State Univ.–Vancouver, 14204 NE Salmon Creek Ave., Vancouver, WA 98686
^b Dep. of Land, Air, and Water Resources, Univ. of California, One Shields Ave., Davis, CA 95616
^c McClellan Nuclear Radiation Center, Univ. of California, Davis, CA 95616

^* Corresponding author (mmurashkina{at}yahoo.com).

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 (Rb_110°C). Fixed Rb was determined directly by instrumental neutron activation analysis. The amount of vermiculite (g kg^–1) was calculated as fixed Rb content (cmol_c kg^–1)/0.154 (based on a cation exchange capacity for vermiculite of 154 cmol_c kg^–1). For comparison, Rb fixation was also estimated after a 1-h incubation (Rb_1hr) with 2 mmol L^–1 RbCl followed by a 30-min extraction with NH₄Cl. The more rigorous heating treatment (Rb_110°C) fixed almost 35 times more Rb than the 1-h method. The Rb_110°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 Rb_1hr fixation but was not correlated with Rb_110°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.

Abbreviations: CEC, cation exchange capacity • Rb_110°C, Rb fixation by heating to 110°C • Rb_1hr, Rb fixation by 1-h incubation • XRD, x-ray diffraction