CHEMICAL-WEATHERING IN A TROPICAL WATERSHED, LUQUILLO MOUNTAINS, PUERTO-RICO - II - RATE AND MECHANISM OF BIOTITE WEATHERING

Samples of soil, saprolite, bedrock, and porewater from a lower montan e wet forest, the Luquillo Experimental Forest (LEF) in Puerto Rico, w ere studied to investigate the rates and mechanisms of biotite weather ing. The soil profile, at the top of a ridge in the Rio Icacos watersh ed, consists of a 50-100-cm thick layer of unstructured soil above a 6 00-800 cm thick saprolite developed on quartz diorite. The only minera ls present in significant concentration within the soil and saprolite are biotite, quartz, kaolinite, and iron oxides. Biotite is the only p rimary silicate releasing significant K and Mg to porewaters. Although biotite in samples of the quartz diorite bedrock is extensively chlor itized, chlorite is almost entirely absent in the saprolite phyllosili cates. Phyllosilicate grains are present as 200-1000 mu m wide books b elow about 50 cm depth. X-ray diffraction (XRD) and electron microprob e analyses indicate that the phyllosilicate grains contain a core of b iotite surrounded by variable amounts of kaolinite. Lattice fringe ima ges under transmission electron microscope (TEM) show single layers of biotite altering to two layers of kaolinite, suggesting dissolution o f biotite and precipitation of kaolinite at discrete boundaries. Some single 14-Angstrom layers are also observed in the biotite under TEM. The degree of kaolinitization of individual phyllosilicate grains as o bserved by TEM decreases with depth in the saprolite. This TEM work is the first such microstructural evidence of epitaxial growth of kaolin ite onto biotite during alteration in low-temperature environments. Th e rate of release of Mg in the profile, calculated as a flux through t he soil normalized per watershed land area, is approximately 500 mol h ectare(-1) yr(-1) (1.6 x 10(-9) mol(Mg) M-soil(-2) s(-1)). This rate i s similar to the flux estimated from Mg discharge out the Rio Icacos ( 1000 mol hectare(-1) yr(-1), or 3.5 x 10(-9) mol(Mg) m(soil)(-2) s(-1) ), indicating that scaling up from the soil to the watershed is possib le for Mg release. The rate of Mg release from biotite, normalized to Brunauer-Emmett-Teller (BET) surface area, is calculated using a mass balance equation which includes the density and volume of phyllosilica te grains, porewater chemistry and flux, and soil porosity. The mean r ates of biotite weathering calculated from K and Mg release rates are approximately 6 and 11 x 10(-16) mol(biotite) m(biotite)(-2) s(-1) res pectively, significantly slower than laboratory rates (10(-12) to 10(- 11) mol(biotite) m(biotite)(-2) s(-1)). The discrepancy in scaling dow n from the soil to the laboratory is probably explained by (1) differe nces in weathering mechanism between the two environments, (2) higher solute concentrations in soil porewaters, (3) loss of reactive surface area of biotite in the saprolite due to kaolinite and iron oxide coat ings, and/or (4) unaccounted-for heterogeneities in flow path through the soil. Copyright (C) 1998 Elsevier Science Ltd.