Dissolution kinetics and the weathering of mafic minerals

Fayalite, hypersthene, basalt, and obsidian were dissolved in buffered solutions (25°C; pH 4.5 and 5.5) under air, N₂ or O₂ atmospheres, in order to follow the kinetics of dissolution. Each dissolved more rapidly at lower pH values, dissolving most rapidly in the initial few days, followed by slower dissolution for periods up to six months. Dissolution was more rapid when air was excluded. In oxygen atmospheres an Fe(OH)₃ precipitate armors mineral surfaces, thus inhibiting further dissolution, and further affects the solution by scavenging dissolved silica and cations. Dissolution reactions include initial exchange between cations and H⁺, incongruent dissolution of silicate structures, oxidation of Fe²⁺ in solution, precipitation of Fe(OH)₃, and scavenging of dissolved silica and cations by Fe(OH)₃. Dissolution kinetics may explain weathering of mafic rocks and minerals at the Earth's surface, the formation of Fe-oxide coatings on mineral grains, weathering of submarine mafic rocks and intrastratal solution of mafic minerals in buried sandstones. Early Precambrian weathering would have been more rapid before the appearance of large amounts of oxygen in the atmosphere, and continental denudation rates may have been higher than at present because of this effect and the predominance of mafic igneous rocks at an early stage of continent formation and growth.