Chemical and mineralogical studies of forest soils from six sites in the northeastern and southeastern United States indicate that soil in the immediate vicinity of roots and fine root masses may show marked differences in physical characteristics, mineralogy and weathering compared to the bulk of the forest soil. Examination of rhizosphere and rhizoplane soils revealed that mineral grains within these zones are affected mechanically, chemically and mineralogically by the invading root bodies. In SEM/EDS analyses, phyllosilicate grains adjacent to roots commonly aligned with their long axis tangential to the root surface. Numerous mineral grains were also observed for which the edge abutting a root surface was significantly more fractured than the rest of the grain. Both the alignment and fracturing of mineral grains by growing roots may influence pedogenic processes within the rhizosphere by exposing more mineral surface to weathering in the root-zone microenvironment. Chemical interactions between roots and rhizosphere minerals included precipitation of amorphous aluminium oxides, opaline and amorphous silica, and calcium oxalate within the cells of mature roots and possible preferential dissolution of mineral grains adjacent to root bodies. Mineralogical analyses using X-ray diffraction (XRD) techniques indicated that kaolin minerals in some rhizosphere samples had a higher thermal stability than kaolin in the surrounding bulk forest soil. In addition, XRD analyses of clay minerals from one of the southeastern sites showed abundant muscovite in rhizoplane soil adhering to root surfaces whereas both muscovite and degraded mica were present in the immediately surrounding rhizosphere soil. This difference in mineral assemblages may be due to either K-enrichment in rhizoplane soil solutions or the preferential dissolution of biotite at the root-soil interface.

Abstract--Clay of apparent hydrothermal origin that fills amygdales in the Granby Basaltic Tuff (Lower Jurassic) of the Connecticut Valley was analyzed and found to consist of two exceptionally well-crystallized Fe-rich, trioctahedral 2:1 layer expandable phyllosilicates. Based on chemical and XRD analyses, the minerals were tentatively identified as saponite and vermiculite. The saponite exists predominately in the two-water hydration state, but also displays one-and three-water layer hydration states, which suggests heterogeneous layer charge distribution--a phenomenon not uncommon in smectites. The identity of the second clay remains equivocal, but XRD analyses, especially with regard to the swelling properties of the clay, indicate that it is a vermiculite. The well-crystallized nature of the Granby clay and the large size of the clay flakes (up to 1 mm) allowed us to use SEM/EDS X-ray imaging and spot analysis techniques in an attempt to detect chemical differences between the saponite and vermiculite. Results showed that the chemistry of individual crystals, within and among amygdales, was essentially uniform. This suggests that the saponite and vermiculite are chemically similar and that variations in their swelling properties result from other factors, such as crystal size, layer charge density, or charge localization within the unit layers. Crystal size differences in the Granby clay were observed with both the petrographic and scanning electron microscope. Changes in layer charge density or charge localization within unit layers could have been affected by the oxidation of Fe 2+ to Fe 3+, a transformation inferred from the green-to-brown color changes observed in the larger amygdales. The Granby clay is of special importance, because it is one of the few examples of a naturally occurring mixture of two well-crystallized, Fe-rich trioctahedral 2:1 layer expandable phyllosilicates with crystallochemieal and swelling properties that appear to bridge the operational definitions for the smectite and vermiculite groups.

A B S T R A C T : Clay mineral analysis of Spodosols collected from the Adirondack Mountains reveals that smectite is common in the forest floor and uppermost soil horizons (the O, A and E horizons) and probably forms from the transformation of vermiculite via a low-charge vermiculite intermediate. The conversion of vermiculite to smectite occurs in the upper part of the soil profile where organic acids and strong inorganic acids (derived from atmospheric deposition) combine to create an intense weathering environment. X-ray diffraction (XRD) and chemical data for the clay fraction indicate that both the smectite and the low-charge vermiculite are Al-rich and dioctahedral. The smectite appears to be a beidellite. Transformation of vermiculite to smectite may have progressed in these acidic horizons by net layer-charge reduction resulting from the progressive substitution of Si for Al. The parent material for the soil clays was probably biotite, but little remains in these soils.

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