Biotite dissolution rates were determined at 25°C, at pH 2-6, and as a function of mineral composition, grain size, and aqueous organic ligand concentration. Rates were measured using both open-and closed-system reactors in fluids of constant ionic strength. Element release was non-stoichiometric and followed the general trend of Fe, Mg > Al > Si. Biotite surface area normalised dissolution rates (r i ) in the acidic range, generated from Si release, are consistent with the empirical rate law:where k H,i refers to an apparent rate constant, a H þ designates the activity of protons, and x i stands for a reaction order with respect to protons. Rate constants range from 2.15 Â 10 À10 to 30.6 Â 10 À10 (moles biotite m À2 s À1 ) with reaction orders ranging from 0.31 to 0.58. At near-neutral pH in the closed-system experiments, the release of Al was stoichiometric compared to Si, but Fe was preferentially retained in the solid phase, possibly as a secondary phase. Biotite dissolution was highly spatially anisotropic with its edges being $120 times more reactive than its basal planes. Low organic ligand concentrations slightly enhanced biotite dissolution rates. These measured rates illuminate mineral-fluid-organism chemical interactions, which occur in the natural environment, and how organic exudates enhance nutrient mobilisation for microorganism acquisition.

Compared to most other planetary materials in the Solar System, some lunar rocks display high δ 37 Cl signatures. Loss of Cl in a H<