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),
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.