PHYLLOSILICATE DISTRIBUTION AND ORIGIN IN ARIDISOLS ON A GRANITIC PEDIMENT, WESTERN MOJAVE DESERT

There is considerable uncertainty about the extent of mineral weatheri ng and neosynthesis in arid soils. Although researchers have speculate d that smectite neosynthesis can occur in soils containing opaline sil ica cement, no evidence has been presented to support this hypothesis. We investigated the mineralogy of two Aridisols in the western Mojave Desert, California, to study phyllosilicate distribution and origin i n sons with opaline silica. The clay fraction of both soils is dominat ed by Al-rich, dioctahedral smectite, characterized by a Mg-saturated d(001) spacing of 1.52 mn and a d(060) spacing of 0.149 to 0.150 mm. T his smectite is also present in silt and sand fractions of deeper hori zons, where it exists mainly as microagglomerates of clay-sized crysta ls. Biotite is most abundant in silt and sand fractions of near-surfac e horizons of both pedons, where physical weathering is greatest. Deep , alkaline, silica-rich horizons of both pedons contain more silt- and sand-sized vermiculite than biotite, probably due to rapid chemical w eathering of biotite to trioctahedral vermiculite. Clay- and silt-size d hydroxy-interlayered 2:1 minerals are present in the upper horizons of these soils. Given the lack of gibbsite and substantial amounts of kaolinite and AL hydroxy-interlayered 2:1 phyllosilicates, we propose that neosynthetic dioctahedral smectite was the dominant sink for Al r eleased by feldspar weathering in these Aridisols. Silica not consumed in smectite neosynthesis can cement microagglomerates. We further spe culate that minimal physical weathering in the alkaline, high-silica e nvironment of deep horizons favors aggregation of smectites to silt- a nd sand-sized microagglomerates.