Kaolinite formation and cation release from biotite in a tropical forest soil

¹Dept. of Geosciences, Penn State University, University Park, PA 16802, USA

²U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025-3591, USA

Abstract

Soil profile cores and pore water samples were collected from a site in the United States' only tropical rain forest, El Yunque, in the Luquillo Mountains of Puerto Rico. A thorough study of the soil profile with a focus on biotite-kaolinite transformation permits an estimation of the Mg, K, and Si release from biotite in this tropical soil. XRD studies reveal that most minerals weather rapidly at the granitic bedrock interface; the only minerals present higher in the profile are biotite, quartz, kaolinite and iron oxides. Biotite is therefore the only primary mineral releasing potassium and magnesium to the soil. This mica is visually detectable as 200-800 micron-wide "books" throughout most of the 10-meter soil/saprolite profile, and disappears at a depth of approximately 75 cm. XRD and SEM analyses reveal that the biotite books actually contain two phases, kaolinite and biotite, at every depth down to the soil-bedrock interface. The (060) peak of the mica is present at 1.55 angstroms, indicating a trioctahedral mica, and shows no significant shift toward a dioctahedral nature through the soil profile. XRF analysis of books indicates that iron, potassium and magnesium concentrations decrease toward the surface, with a sharp change at a depth of approximately 450 cm. ICP analysis of pore water samples collected from lysimeters reveals great fluctuation in the solute concentrations in the top meter of soil (probably due to vegetation cycling and evapotranspiration), then a steady increase in K, Mg and Si with depth. Na and Ca remain relatively constant below 1 meter, verifying that the only mineral contributing to cation release is biotite. Stability diagrams indicate kaolinite as the stable mineral phase, and the waters are almost all oversaturated with respect to quartz.

A mass balance calculation is used to estimate the magnesium, potassium, and silica release rates of the biotite. These rates are determined by the saturation state of the pore waters, and the modal volume, density, and surface area of the biotite at varying depths. The determination of these release rates may also allow an estimation of the rate of transformation from biotite to kaolinite.

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