SOLUBILITY OF HYDROXYALUMINUM INTERLAYERS AND IMOGOLITE IN A SPODOSOL

Information on the thermodynamic stability of minerals is essential wh en describing dissolution-precipitation reactions. The solubility of i mogolite in the Bs and BC horizons of a Spodosol influenced by volcani c ash in the Cascade Range, Washington, was studied using batch chemic al equilibrium modeling at 25-degrees-C and 0.1 MPa pressure. Imogolit e was found to be a major component of the clay fraction in the Bs and BC horizons of these soils using transmission electron microscopy (TE M) 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- a nd oversaturation. The pH, total and labile (inorganic monomeric) Al, and labile (monomeric) Si were determined. We used a computer speciati on model to calculate the activities of Al, H4SiO4, F, SO4, and other species including a soluble aluminosilicate complex. Soil solutions ap peared to be in equilibrium with interlayer Al(OH)3 of 2:1 phyllosilic ates and natural imogolite in the Bs and BC horizon soils, based on th e most recent free energy values for synthetic and natural imogolites. The equilibrium activity of H2SiO4 was 10(-3.71+/-0.08). Soil solutio ns were oversaturated with respect to gibbsite (3pH + log Al3+ = 8.73 +/- 0.16). Interlayer Al(OH)3 appeared to have a higher solubility tha n well-crystallized gibbsite. Assuming equilibrium with the soil solut ion, imogolite in this Spodosol was more soluble (log K25C [equilibriu m constant at 25-degrees-C and 0.1 MPa] = 13.76 +/- 0.25) than its syn thetic counterpart (log K25C = 13.04 +/- 0.14), but not different from a Japanese natural imogolite (log K25C = 13.56 +/- 0.18).