Tuesday, 7 October 2008
George R. Brown Convention Center, Exhibit Hall E
Occurrence, abundance, and distribution of clay-sized minerals in desert soils are strongly affected by parent material, eolian deposition, translocation, pedogenic transformation, vegetation, and several other factors. The complex mutual interactions of these factors and multiple processes make the assignment of mineral origin difficult. We studied the distribution of minerals of four desert soils in one chronological sequence of alluvial deposits in a small area (2 km by 3 km) near Cibola, Southwestern Arizona,
USA. These soils developed on alluvial fans originating from the same parent rocks under the same dry climate of less than 75 mm annual rain precipitation. Accumulation of pedogenic calcite, gypsum, soluble chloride and nitrate salts have been observed. The abundance and depth of accumulation of the carbonate and evaporites in the soils increase with age of the soils. Common minerals such as quartz, chlorite, illite, and kaolinite have been identified in the four soils. Moreover, palygorskite has been observed in the lower horizons of the four soil profiles. Palygorskite is absent in the vesicular surface horizons (Av) of all the soils. In each soil profile, the abundance of palygroskite is the highest in the argillic, calcic, or/and gypsic subsurface (Btk, Btkym) horizons. The abundance of palygorskite in these B horizons increases with the age of the soils. The distribution of palygorskite in each soil, and the abundance of the mineral in the four soils correlate well with the distribution and abundance of evaporites and calcite, suggesting the formation of palygorskite is strongly related to solution transport and evaporation in the profiles. Small amount of palygorskite also occurs in some parent material horizons (Cy or Cky) when evaporites accumulate. We conclude that palygorksites in these soils are pedogenic origin, they formed from high-salinity and high-alkalinity soil solutions after evaporation.