Weathering of biotite in a tropical forest soil, Luquillo mountains, Puerto Rico

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

Abstract

The tropical environment comprises only 25% of the Earth's surface, but contributes 38% of the river ionic load, 65% of dissolved silica, and 48% of dissolved oxygen entering the world's oceans (Meybeck, 1979; Milliman and Meade, 1983). Despite the area's obvious importance in understanding global biogeochemical cycles, little is known about its weathering processes and rates. Knowledge of the rate and mechanism of weathering of minerals and the release of cations is required to understand such cycling.

To investigate the rates and mechanisms of biotite weathering, we analyzed soil and saprolite profile cores and pore water samples from the United States' only tropical rain forest, El Yunque, in the Luquillo Mountains of Puerto Rico. The profile, sampled at the top of a ridge in El Yunque, consists of a thin (100 cm) layer of unstructured soil above a 620-cm thick layer of saprolite, which developed on a Tertiary quartz dioritic intrusion. X-Ray Diffraction (XRD) of soil and saprolite reveals that few minerals remain above the bedrock interface. The only minerals identified above the bedrock interface are biotite, quartz, kaolinite and iron oxides.

Biotite is therefore the only primary mineral releasing potassium and magnesium to the soil. This mica is visibly detectable in hand sample as 200-1000 µm-wide phyllosilicate "books" throughout most of the 720 cm soil/saprolite profile, and disappears at a depth from the soil-air interface of approximately 75 cm. XRD and electron microprobe analyses reveal that the mica grains actually contain two phases, kaolinite and biotite. Electron microprobe analyses also reveal that the mica grains often consist of a core of biotite surrounded by a thick rim of kaolinite. Lattice fringe images show single layers of biotite splitting into two layers of kaolinite, suggesting dissolution of biotite and precipitation of kaolinite at linear boundaries. The degree of kaolinization as observed under TEM decreases with depth in the saprolite. This TEM evidence represents the first such microstructural evidence to document epitaxial growth of kaolinite directly following alteration of biotite in low-temperature environments.

Below a depth of 120 cm in the profile, the concentrations of dissolved K, Mg, and Si in soil pore waters increase steadily with depth. Dissolved Na and Ca in pore waters remain relatively constant in concentration throughout the profile. The rates of release of Mg, K, and Si from biotite weathering were calculated from these concentrations with a mass balance equation which included density, volume, and surface area of the mica grains, pore water chemistry and flux, and soil porosity. The rate of release of Mg from the biotite is between 10^-15 and 10^-16 mol_Mg m^-2 _biotite s^-1, which is much slower than the average release of Mg observed in laboratory studies (10^-12 to 10^-13 mol_Mg m^-2 biotite s^-1). Biot ite dissolution is slowed in the studied system due to pore waters saturated or near-saturated with respect to kaolinite, low Mg concentration in the biotite octahedral sheet, and a thick rim of kaolinite surrounding the core of biotite.

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