Clay Mineral Genesis of Some New York Spodosols

Some Spodosols in northern New York previously shown to have well crystallized montmorillonite in the albic horizon but only amorphous clay sized material in the spodic horizon were studied to determine the source or genesis of the montmorillonite. The soils used to test several hypotheses of the clay mineral genesis represented Orthods, Humods, and Aquods. These soils were also shown to be deficient in mica and chlorite in the sand, silt, and clay fractions. This allowed a test of the hypothesis that montmorillonite in albic horizons weathers from mica, a process which previously has been reported to be active. The presence of only traces of mica and chlorite in the sand and silt fractions would indicate that weathering of sand and silt sized minerals directly to clay sized minerals should not be important. However, the small amount (generally less than 3%) of clay sized materials in the pedons makes this assumption inconclusive. Soil solution studies, coupled with equilibrium diagrams, provide evidence that clay mineral synthesis from ions does not occur in these soils. The well crystallized clay sized minerals detected in the upper solum of the pedons studied may have (i) weathered directly from sand and silt sized minerals, (ii) been added as dust, or (iii) weathered from minerals added as dust. There is recent evidence that appreciable amounts of montmorillonite, illite, vermiculite, and kaolinite are deposited as dust throughout the northern United States. If projected figures are valid, dust could account for most of the montmorillonite in the albic horizons studied. Concomitant additions of chlorite and mica are not detected. Chlorite and mica added as dust, as well as any traces of these minerals present as sand or silt, must be weathered to some other species in the clay fraction, because they are not detected in the clays. It is proposed that in the Spodosols studied (i) any chlorite in the albic horizon is weathered to amorphous aluminosilicates and translocated to the spodic horizon, and (ii) mica is weathered to montmorillonite in the albic horizon, to vermiculite in the Bh horizon, and to “chloritized” vermiculite in the Bir horizon.