Transitional Vermiculite‐Smectite Phases in Aqualfs of Southwestern Ohio

Abstract

AbstractThe clay mineralogy of two Aqualfs occurring on the Illinoian till plain of southwestern Ohio was investigated in order to (i) determine the clay mineral distribution with depth, (ii) examine the clay mineral weathering sequence, and (iii) identify the 2:1 mineral species in which hydroxy interlayers had formed. Despite the presence of multiple parent materials, no obvious discontinuities in clay mineralogy were observed. A weathering sequence involving the transformation of clay mica to vermiculite to low charge vermiculite/high charge smectite occurred. Alkylammonium ion exchange data indicated that weathering created a heterogeneous charge distribution in the 1.4‐nm (expandable) mineral phase resulting in mixed low‐and high‐charge interlayers. The low‐charge interlayers readily formed bilayer complexes with ethylene glycol (1.7 nm), and also accommodated double layers of glycerol (1.8 nm) when the mean layer charge was <0.65. Reductions in layer charge were correlated with the oxidation of octahedral Fe(II) and the expulsion of K and Mg from the mineral structure. The expansion of 2:1 layer silicates in horizons near the soil surface was restricted as a result of Alhydroxy interlayering. After interlayer materials were removed with boiling 0.33 M Na citrate, the previously interlayered minerals exhibited x‐ray diffraction effects following Mg saturation and ethylene glycol and glycerol solvation that were indicative of vermiculite.