Soils and soil clay mineral formation in the Virginia Blue Ridge and Piedmont provinces

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Virginia Polytechnic Institute


X-ray diffraction studies of 29 Virginia Blue Ridge and Piedmont soil profiles and petrographic examinations of the parent rocks reveal that even after prolonged chemical weathering, physical and mineralogical differences exist between soil types which are due largely to changes in the parent rock, and to a lesser extent changes in the topography.

Soils developed from different source rocks can be distinguished on the basis of color, texture, degree of contrast between horizons, and depth to fresh rock. The clay minerals of such soils differ in their distribution, amount and kind. The role of parent material in soil and clay mineral formation is a complex function of several variables: (1) chemical composition (2) mineralogical composition (3) rock texture and (4) rock structure. Soils developed from calcium-aluminum rich rocks or from basic rocks which have structures or textures not conducive to good drainage are characterized by kaolin and montmorillonite clay minerals and by dense plastic, poorly drained, clayey subsoils. In all other soils and in the surface horizon of these soils, the major clay minerals are kaolin and vermiculite.

Topography affects the degree of profile development, color, and soil texture and controls the rate of clay mineral accumulation and weathering. The climates of the Blue Ridge and Piedmont are most favorable to the formation of kaolin minerals. However, greater amounts of kaolinite appear to be formed in the warmer Piedmont climate and more vermiculite in the cooler Blue Ridge climate.

The most important mechanism for clay mineral formation is probably by direct alteration of the primarily silicate minerals. The distribution of these clays in the profile is determined by the weathering stability of the primary silicate. All ferromagnesium silicates weather to 14 A clay minerals. An intermediate 14 A chlorite phase was not observed. The most common product is vermiculite. The formation of montmorillonite is controlled by the four parent material factors listed above. Hornblende and augite are the chief source materials of montmorillonite. Biotite possibly weathers to montmorillonite if the internal drainage is strongly retarded. Montmorillonite weathers to vermiculite in the surface horizons. Kaolinite forms early in the profile during the weathering of feldspar. Medium- to coarse-grained muscovite is also a source of kaolinite. There was no evidence found to indicate that kaolinite is a weathering product of the 12 A clay minerals. Illite occurs in minor amounts in a few micaceous profiles but appears to be the result of mechanical break-down of larger mica particles.