Highly weathered mineralogy of select soils from Southeastern U.S. Coastal Plain and Piedmont landscapes

Highly-weathered clay mineralogical suites of Southeastern (U.S.) soils, which are exemplified in portions of upland, well-drained environments of the Piedmont and Atlantic and Gulf Coastal Plain, consist of kaolinite, hydroxy-interlayered vermiculite, gibbsite, iron oxides, and to a lesser extent, quartz, halloysite and mica. The development of these mineral assemblages is often coincident with Ultisol formation in these landscapes. By Soil Taxonomy, these soils often have kandic horizons or are in subactive CEC activity classes. Hydroxy-interlayered vermiculite, a mineral considered to be pedogenic and ubiquitous within these systems, has been evaluated in past studies through depth distribution trends, thermodynamics-solubility, and alluvial chronosequences, to develop a more thorough understanding of mineral transformations and pedogenesis in these landscapes. The average formula for hydroxy-interlayered vermiculite developed from several regional studies (M+0.28(Si3.46Al0.54)(Al1.38Fe0.31Mg0.31)[(Al1.31(OH)3.35)]O10(OH)2) suggests the 1/2 cell layer charge to be approximately 0.85, with approximately 67% of the charge offset by non-exchangeable Al-hydroxy interlayers. Thermodynamic evaluations indicate a relatively more stable pedogenic environment for hydroxy-interlayered vermiculite in near-surface and eluvial environments (dependent on the degree of hydroxy-Al interlayer infilling), with a more stable environment for kaolinite in illuvial subsoils. This corresponds with observations of mineral distribution with depth in representative soils of the region, although other mechanisms and pathways, which have been elucidated through several studies, likely play a larger role. Quantification of mineral constituents in these systems faces challenges, but satisfactory results can be obtained using a variety of techniques. Deconvolution of thermal (DSC and TGA) output shows promise for quantifying discrete minerals that have overlapping temperatures of reactions. Chronosequence studies in the region have improved our understanding of the temporal state factor associated with the development of this mineralogy. For example, a study on the Tallapoosa River of Central Alabama illustrated the presence of vermiculite and smectite and the relative absence of hydroxy-interlayered vermiculite within the solum on Holocene terraces, but its presence on significantly older (Pleistocene) terraces. Another study evaluated 29 pedons on seven Alabama River terrace levels (late Pliocene to late Holocene) in Central and South Alabama, and found clay mineralogy on the oldest terrace levels was similar to adjacent upland Coastal Plain landscapes. The aggregate of findings indicate that given sufficient time in a leaching environment, highly weathered mineralogical suites form in a variety of landscapes in this region.