『Abstract
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.
Keywords: Ultisols; Hydroxy-interlayered vermiculite; Kandic horizons;
Chronosequence』
1. Introduction
2. Mineralogical assessment
2.1. Qualitative/quantitative analyses
3. Pedogenesis and highly weathered mineralogy
3.1. Thermodynamic approaches
3.2. Soil formation factors - time
3.2.1. Example - Tallapoosa River terraces (central Alabama)
3.2.2. Example - Alabama River terraces (central and south Alabama)
4. Taxonomic and soil survey considerations
References