Chemical weathering of the mafic minerals serpentine and olivine in natural environments

Chemical weathering is a critical process that plays a key role in controlling the chemistry of natural waters, the cycling of atmospheric carbon dioxide, and the formation of soil. The weathering of silicate minerals is important because the Earth's crust is composed of approximately 92 weight percent silicate minerals. Despite an extensive literature on mineral dissolution and kinetics, relatively few studies have examined weathering of the mafic minerals serpentine and olivine, especially in a natural environment. The first chapter of this thesis provides an introduction to the minerals serpentine and olivine. The second chapter of this thesis examines the important factors affecting incipient weathering of serpentinite at two sites in the Klamath Mountains, California. Serpentinite rock cores and serpentinite-derived soils from each site tested positive for iron-oxidizing bacteria, and mineral dissolution in weathered samples appeared to follow the order (from first to dissolve to last): pyroxene > iron-rich serpentine > magnesium-rich serpentine > aluminum-rich serpentine. The third chapter of this thesis examines the natural dissolution of olivine in an arid environment at Black Rock Flow, Nevada. Etch pits, an apparent dissolution feature, were observed and used to measure a surface-area normalized rate of olivine dissolution (3.48x10-15 mol forsterite/m2s). These findings help quantify the incipient processes affecting serpentine soil formation and provide the first known dissolution rate for naturally-weathered olivine in an arid environment.