Wednesday, November 15, 2006 - 10:15 AM

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 (Rb110C) 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 110C 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 Rb110C 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 Rb110C fixation (R 2 = 0.21ns).

For soils from the SJV the Rb110C fixation method did not provide a reliable estimate of vermiculite content, relative to XRD results. Our data suggest that Rb fixation with heating to 110C method overestimates the amount of vermiculite if high-charge smectites are present.