Silica in Duric Soils: I. A Depositional Model

Many alluvial Argids in central Nevada are cemented by illuvial silica and CaCO 3 . In loamy soils, microsite deposition of these authigenic components tends to be mutually exclusive, with silica being finely distributed throughout the plasma phase and calcite plugging packing voids formed by skeleton grains and root channels. The following model is proposed to explain the differences in depositional locations of silica and CaCO 3 . As soils dry, calcite precipitates rapidly in an ionic, diffusion‐controlled reaction while monosolicic acid, which requires greater activation energy for Si‐O bond breakage prior to precipitation, is concentrated in the solution phase. Monosilicic acid [Si(OH) 4 ] can diffuse away from the evaporation front into smaller pores where, in contact with higher surface areas, it is absorbed onto clay, sesquioxide, and weathered primary mineral surfaces. Adsorbed silica is a template for further adsorption, and on drying, the precipitation of opaline silica (SiO 2 ). The resulting SiO 2 polymers form bonds between adjacent Si(OH) 4 absorbing soil particles without necessarily plugging the voids between them. In contrast, CaCO 3 plugs large voids by preferentially precipitating on previously deposited calcite crystals. Calcite is more easily redissolved than opaline silica and tends to move lower in the soil profile during wet years. Differences in precipitation processes result in silica‐rich, calcite‐poor argillic and upper duric horizons that grade to more strongly cemented duric/calcic horizons. The latter are characterized by horizontally distributed low calcite durinodes interspersed by more calcareous internodal areas.