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Solubility of Hydroxy-Aluminum Interlayers and Imogolite in a Spodosol

USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Drive, Riverside, CA 92501

James B. Harsh and Jeffrey S. Boyle

Dep. of Crop and Soil Science, Washington State Univ., Pullman, WA 99164-6420

* Corresponding author.

ABSTRACT

Information on the thermodynamic stability of minerals is essential when describing dissolution-precipitation reactions. The solubility of imogolite in the Bs and BC horizons of a Spodosol influenced by volcanic ash in the Cascade Range, Washington, was studied using batch chemical equilibrium modeling at 25°C and 0.1 MPa pressure. Imogolite was found to be a major component of the clay fraction in the Bs and BC horizons of these soils using transmission electron microscopy (TEM) and selective dissolution techniques. Soil solutions were collected in 60-, 120-, and 180-d intervals from soils (<2 mm) equilibrated in 0.01 M NaCl at initial pH 5.7 or 3.5 under conditions of both under- and oversaturation. The pH, total and labile (inorganic monomeric) Al, and labile (monomeric) Si were determined. We used a computer speciation model to calculate the activities of Al, H₄SiO₄, F, SO₄, and other species including a soluble aluminosilicate complex. Soil solutions appeared to be in equilibrium with interlayer Al(OH)₃ of 2:1 phyllosilicates and natural imogolite in the Bs and BC horizon soils, based on the most recent free energy values for synthetic and natural imogolites. The equilibrium activity of H₂SiO₄ was 10^{–3.71±0.08}. Soil solutions were oversaturated with respect to gibbsite (3pH + log Al³⁺ = 8.73 ± 0.16). Interlayer Al(OH)₃ appeared to have a higher solubility than well-crystallized gibbsite. Assuming equilibrium with the soil solution, imogolite in this Spodosol was more soluble (log K_25C [equilibrium constant at 25°C and 0.1 MPa] = 13.76 ± 0.25) than its synthetic counterpart (log K_25C = 13.04 ± 0.14), but not different from a Japanese natural imogolite (log K_25C = 13.56 ± 0.18).

Received for publication April 8, 1994.

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