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The 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 MNa citrate, the previously interlayered minerals exhibited x‐ray diffraction effects following Mg saturation and ethylene glycol and glycerol solvation that were indicative of vermiculite.

A polygenetic soil from southern Ohio, formed by loess deposition on a pre‐existing Sangamon paleosol, was examined to determine if the lithologic discontinuity has affected subsequent clay‐mineral weathering or formation. Analyses of the <0.2 µm clays indicated that cation exchange capacities (CECs) and total Mg, K, and Fe contents were lowest near the interface between the loess and underlying paleosol at a depth of approximately 65 cm; in contrast, total Al contents were highest in this zone. An interstratified 1:1–2:1 mineral was identified in materials surrounding the loess‐paleosol contact and was concentrated in the 2.03 g/cm3‐density fraction of the <0.2 µm clay using density‐gradient centrifugation. Alkylammonium exchange of the 2.03 g/cm3‐density fraction demonstrated that the 2:1 component of the interstratified mineral was heterogeneous in charge and included layers characteristic of both smectite and vermiculite. Soil properties and depth distribution of the interstratified mineral suggest a pedogenic origin.

High speed centrifugation with a large‐scale zonal rotor and a linear, tetrabromoethane‐ethanol density gradient (1.0–2.8 Mg m−3) was utilized to fractionate the <0.2µm clay materials from three soils with differing Fe‐oxide mineralogy. Hematite, goethite, and lepidocrocite were concentrated 4‐ to 10‐fold in the high density range (>2.4 Mg m−3) while the low density fractions (<2.2 Mg m−3) were depleted in reductant‐soluble Fe. Density gradient centrifugation also produced useful mineralogical separations of the soil phyllosilicates. In particular, smectite was concentrated in the 1.8 to 2.1 Mg m−3density range whereas the 2.1 to 2.3 Mg m−3density range was enriched with kaolinite, vermiculite, and clay mica. Electron microscopy and x‐ray diffraction analyses indicated that Fe‐oxides were present in the materials studied as discrete particles rather than as “coatings” on associated minerals.

The acicular (needle‐like) clay minerals, sepiolite and palygorskite, were found to be widespread in late Cenozoic sediments of the Texas High Plains. Five soils developed in these sediments, or in materials derived thereform, were analyzed to determine if sepiolite and palygorskite were stable under soilforming conditions. X‐ray diffraction, cation exchange capacity, and electron optical data indicated that degradation of these minerals accompanied soil formation. Soil reaction values and water‐soluble Mg were found to decrease with proximity to the soil surfaces and total chemical analyses of carbonate‐free clays (< 0.2 µm) from two profiles showed similar losses of structural Mg and Si. Since sedimentary sepiolite and palygorskite are Mg‐silicates formed under alkaline conditions, reduced OH and Mg activities probably contributed to the instability of these minerals within the specified soil environments.