K‐fixing phyllosilicates in soils: the role of inherited components

SUMMARY K‐fixing potential of 29 illite/smectite mixed‐layer minerals, covering a broad range in layer ratios and stacking arrangements, has been investigated utilizing X‐ray diffraction as the principal investigative tool. Samples were derived from a borehole core transecting Permian sedimentary rocks. In 93% of all samples, K + was retained against replacement with Mg 2+ from a 1 M MgCI 2 solution in an average of 33% of the smectitic interlayers, defined via expansion to 1.7 nm upon ethylene glycol solvation after Mg‐saturation. This non‐ideal smectite behaviour points to a vermiculite‐type layer charge. Ordered structures fixed on average 20% more K than did random ones, but results were widely scattered. The Greene‐Kelly test revealed predominantly tetrahedral substitution, intercalation with dodecylammonium chloride molecules a high interlayer charge density in samples with a high K‐fixing capacity. The findings have implications in two fields: in pedology, soils developed from thermally altered sedimentary rocks may inherit a considerable potential for K–fixation; in mineral identification, interlayers in illite/smectite interstratifications fully expanded to 1.7 nm upon ethylene glycol solvation despite their vermiculite–type layer charge. Glycerol solvation provided equally inconsistent results, while dodecylammonium spacings seemed to correlate reasonably well with interlayer charge density characteristics.