Solubility of Hydroxy-Aluminum Interlayers and Imogolite in a Spodosol
- Chunming Su ,
- James B. Harsh and
- Jeffrey S. Boyle
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 K25C [equilibrium constant at 25°C and 0.1 MPa] = 13.76 ± 0.25) than its synthetic counterpart (log K25C = 13.04 ± 0.14), but not different from a Japanese natural imogolite (log K25C = 13.56 ± 0.18).Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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