Interpretation of Alkylammonium Characterization of Soil Clays

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Abstract
The alkylammonium method of characterizing the charge of layer silicates has recently been applied to soil clays containing mixed mineral assemblages. Various sharp, low‐angle x‐ray diffraction (XRD) peaks (e.g., 2.5 nm for tetradecylammonium‐treated samples) have been attributed to vermiculite or high‐charge smectite. To the degree that alkylammonium cations can replace interlayer K+, however, the observed low‐angle peaks may represent K+‐depleted soil illite as well as vermiculite. This hypothesis was tested by treating two reference illites with alkylamine hydrochlorides. The treated illites exhibited low‐angle XRD peaks similar to those observed in the soil clays. Moreover, the occurrence of these low‐angle peaks coincided with the release of K+ from the illites. Similarly, the alkylamine hydrochloride treatments for layer‐charge determinations released as much as 49% of the K+ from a soil clay. These data clearly show that the alkylammonium method can expand soil illites to give low‐angle XRD peaks, which may be misinterpreted as evidence of soil vermiculite.

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