Highly-weathered clay mineralogical suites of southeastern (USA) soils (portions of Piedmont and Coastal Plain) typically consist of kaolinite, hydroxy-interlayered vermiculite, gibbsite, quartz and iron oxides. The development of these mineral assemblages is often coincident with Ultisol formation in these landscapes. Using thermodynamic evaluations, depth trends, and chronosequences, researchers have developed a more thorough understanding of mineral transformations in these systems.  For example, we evaluated a chronosequence of Quaternary terraces of the Tallapoosa River of central Alabama (USA), with landscape ages ranging from contemporary floodplains to mid-Pleistocene terraces.  Six pedons were evaluated that ranged from Typic Udifluvents and Fluventic Dystrudepts on the floodplain to (subactive) Typic Paleudults on relatively higher terraces.  The ECEC and CEC/100 g clay, oxalate extractable Fe, oxalate extractable Fe/dithionite extractable Fe, and total K/total Ti (p<0.05) were relatively lower in soils located on higher (older) levels.  Although somewhat confounded by sediment source, results support a progression of higher to lower activity mineralogy as soils develop in this environment.  In addition, clay mineral stability of six highly weathered Kandi- and Kanhapludults of the Alabama Piedmont and Coastal Plain was evaluated using a thermodynamic approach.  Solubility products suggest hydroxy-interlayered vermiculite is relatively more stable in near surface compared with subsoil environments.  These results generally agree with observations of depth trends and past findings, however, mineral quantification provides challenges.  The aggregate of findings indicate that given sufficient time, highly weathered mineralogical suites form from a variety of parent materials in this environment.