Wednesday, November 15, 2006 - 10:15 AM
301-7

Estimation of Vermiculite Content Using Rb-Fixation in Soils Derived from Granitic and Non-Granitic Alluvium of the San Joaquin Valley, California.

Mariya Murashkina1, Randal Southard1, and Ryoji Shiraki2. (1) LAWR, UC Davis, One Shields Ave, Davis, CA 95616-8627, (2) UC Davis McClellan Nuclear Radiation Center, 5335 Price Avenue, Bldg 258, McClellan, CA 95652

Quantification of phyllosilicates in soils often depends on a combination of methods, including x-ray diffraction (XRD), cation exchange and fixation capacity, and microscopy. We examined a Rb-fixation (Rb110°C) method to quantify vermiculite content in soils derived from granitic and non-granitic alluvium of the San Joaquin Valley (SJV), California. In this method a sample is saturated with Rb and heated to 110°C followed by Rb displacement with NH4. Any Rb remaining after NH4 treatment was determined directly by instrumental neutron activation analysis (INAA). In the INAA analysis the reaction 85Rb + n → 86Rb + g was quantified by measurement of the gamma radiation. The amount of vermiculite (g kg-1) was calculated as fixed Rb content (cmolc kg-1)/0.154. For comparison, Rb fixation was also estimated after a 1-hr incubation (Rb1hr) with 2 mmol RbCl followed by a 30-min extraction with NH4Cl. Fixed Rb was calculated as the difference between the initial Rb added and the extracted Rb

While the amount of Rb fixation in the 1 h technique was correlated with Rb fixed by heating (R 2 = 0.48**), the more rigorous heating treatment fixed almost 35 times more Rb. Data for Rb110°C had a highly significant correlation with soil clay content (R 2 = 0.85***). However, XRD data showed that clay fractions were dominated by smectite, chlorite, and mica while vermiculite, the major K- and Rb-fixing mineral, was a dominant mineral in most silt fractions. Silt content correlated with Rb1hr fixation (R 2 = 0.48**), but did not correlate with Rb110°C fixation (R 2 = 0.21ns).

For soils from the SJV the Rb110°C fixation method did not provide a reliable estimate of vermiculite content, relative to XRD results.  Our data suggest that Rb fixation with heating to 110°C method overestimates the amount of vermiculite if high-charge smectites are present.