Chemical Weathering of Layer Silicate Clays in Loess-Derived Tama Silt Loam of Southwestern Wisconsin
Chemical weathering of layer silicate clay minerals in the major horizons of the Tama silt loam Prairie soil profile was followed down to 10 ft, where the calcareous parent loess was encountered, and on down to a depth of 13 ft. The predominant constituents of the clay fraction (— 2 μ) at all depths in the soil profile were montmorillonite (42 percent) and illite (20 percent), although vermiculite (12 percent), kaolinite (5 percent) and amorphous material (15 percent) were also in fair abundance. Deposition of secondary inter-layer alumina was appreciable in the uppermost soil horizons but became less in amount in the deeper soil horizons. Quartz was not found in the clay fractions of less than 0.2 μ in diameter at any depth in the soil profile, although both quartz and feldspars occurred in the coarse clay.
Vermiculite decreased from about 18 percent in the coarse clay and 5 percent in the fine clay of the deeper calcareous horizons to small amounts toward the soil surface, disappearing altogether in the fine clay ( — 0.08 μ). Little variation was found in the montmorillonite percentage of the clay in the subsoil and lower horizons (64 to 75 percent of the fine clay), but a decrease in the 18Å diffraction intensity of montmorillonite concurrent with an increase in amorphous silica and alumina (15 to 27 percent of the fine clay) was observed in the upper, acid, soil horizons, suggesting that montmorillonite is not a stable weathering product in the latter. Elemental analyses of the fine clays showed about 1.1 out of 4 octahedral positions to be Fe, 2.5 to be Al, and 0.4 to be Mg. A slight decrease in Fe and increase in Al in the montmorillonite octahedral structure occurred progressively in approaching the more intensely weathered zones near the soil surface.
The results suggest that dioctahedral montmorillonite has formed to considerable extent in both the calcareous and noncalcareous horizons by chemical weathering of tri-octahedral ferromagnesian mica, through a vermiculite intermediate stage. The decrease in amount of the silt fractions of density greater than 2.66 g/cm3 (found to be composed mainly of biotite, chlorite, and amphiboles) in going from the calcareous horizons toward the soil surface approximately equalled the increase in montmorillonite percentage of the soil, suggesting that montmorillonite of the loess originated by chemical weathering of the ferromagnesian minerals of the silt. The genesis of the loessial montmorillonite begins long before the free CaCO3 has been leached out (6 percent montmorillonite in the loess at 13 ft) and continues to a maximum (14 percent) in the B horizon. Some eluviation of montmorillonite from the A horizon into the B horizon was evident.
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