Silica in Duric Soils: I. A Depositional Model

Abstract

Many alluvial Argids in central Nevada are cemented by illuvial silica and CaCO3. 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 CaCO3. 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 (SiO2). The resulting SiO2 polymers form bonds between adjacent Si(OH)4 absorbing soil particles without necessarily plugging the voids between them. In contrast, CaCO3 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.

Cemented Horizons and Hardpans in the Coastal Tablelands of Northeastern Brazil

“…Observation of micromorphological evidence for silica cementation, even in duripans, can be impeded by the close relationship of silica with ferriargillans and clays of these horizons (Torrent et al, 1980). However, this evidence is commonly described in micrographs and in electron microscopy images of horizons with duripans or with some degree of silica cementation (Chadwick et al, 1987; Boettinger and Southard, 1990;Creutzberg et al, 1990;Hollingsworth and Fitzpatrick, 1994;Moody and Graham, 1997;Gutiérrez-Castorena and Effland, 2010).…”

“…Such a process predominantly occurs in the small pores and in the grain-to-grain contacts, without a total filling of the spaces between grains (Chadwick et al, 1987).…”

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“…Los cambios climáticos o hidrogeológicos afectan directamente a las silcretas en constitución (Summerfield, 1983;Thiry y Milnes, 1990;Armenteros et al, 1995). e) La participación de una actividad orgánica, ya que ciertos tipos de encostramientos silíceos se realizan sobre suelos (Wilding et al, 1977;Chadwick et al, 1987a). d) Edad de formación de las silcretas.…”

An Electron Optical investigation of Aluminosilicate Cements in Silcretes

“…Most research on silcretes has been directed to their field distribution (Stephens, 1971), geomorphology (Langford-Smith, 1978;Callen, 1983), petrology (Butt, 1983(Butt, , 1985 and geochemical characteristics (Milnes andHutton, 1974, Hutton et al, 1978). Some recent studies (Chadwick et al, 1987a(Chadwick et al, , 1987bThiry and Milnes, 1990) have investigated silcretes from a field scale down to the resolution of conventional scanning electron microscopy (SEM). Milnes and Thiry (1992) reviewed current work on silcretes and emphasized the need for use of high resolution techniques to recognize the mechanisms of formation of these materials.…”

“…The complexity and fine-grained sizes of the cementing phases in these materials warrant the use of high resolution techniques to resolve the composition and to suggest genetic processes involved in the formation of various silcretes. Chadwick et al (1987a) proposed models for the deposition of various types of silica in soils. According to their model, in loamy soils Si(OH)4 is concentrated in solution due to the high activation energy of Si-O bond breakage.…”