Phyllosilicate Distribution and Origin in Aridisols on a Granitic Pediment, Western Mojave Desert

Abstract

AbstractThere is considerable uncertainty about the extent of mineral weathering and neosynthesis in arid soils. Although researchers have speculated that smectite neosynthesis can occur in soils containing opaline silica 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 in soils with opaline silica. The clay fraction of both soils is dominated by Al‐rich, dioctahedral smectite, characterized by a Mg‐saturated d(001) spacing of 1.52 nm and a d(060) spacing of 0.149 to 0.150 nm. This smectite is also present in silt and sand fractions of deeper horizons, where it exists mainly as microagglomerates of clay‐sized crystals. Biotite is most abundant in silt and sand fractions of near‐surface 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 weathering of biotite to trioctahedral vermiculite. Clay‐ and silt‐sized 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 released by feldspar weathering in these Aridisols. Silica not consumed in smectite neosynthesis can cement microagglomerates. We further speculate that minimal physical weathering in the alkaline, high‐silica environment of deep horizons favors aggregation of smectites to silt‐ and sand‐sized microagglomerates.