Rates of Biotite Weathering, and Clay Mineral Transformation and Neoformation, Determined from Watershed Geochemical Mass-Balance Methods for the Coweeta Hydrologic Laboratory, Southern Blue Ridge Mountains, North Carolina, USA

Biotite is a common constituent of silicate bedrock. Its weathering releases plant nutrients and consumes atmospheric CO2. Because of its stoichiometric relationship with its transformational weathering product and sensitivity to botanical activity, calculating biotite weathering rates using watershed mass-balance methods has proven challenging. At Coweeta Hydrologic Laboratory the coupling of biotite to its transformational weathering product is only valid if the stoichiometric relationship for the two phases is known; this relationship is unlikely layer-for-layer. Rates of biotite weathering and transformation of its secondary weathering product at the Coweeta Hydrological Laboratory are comparable with other Appalachian watersheds. The magnitude and sign of the difference between fieldand laboratory-determined biotite weathering rates are similar to those of other silicate minerals. The influence of major-cation proportions in biomass on the rates of biotite weathering and transformational weathering product is greatest for watersheds with high biomass aggradation rates. The watershed with the lowest bedrock reactivity and highest flushing rate yielded the highest gibbsite formation rate of *500 mol ha year and lowest kaolin-group mineral formation rates of 4–78 mol ha year. The kaolin-group mineral formation rate increases as bedrock reactivity increases and flushing rate decreases to a maximum of *300 mol ha year, with a similar minimum gibbsite formation rate. The relative differences in bedrock reactivity and flux of water through Coweeta Hydrological Laboratory watersheds studied appear to be invariant over geologic timescales.