A STUDY ON THE CHEMICAL WEATHERING OF AMPHIBOLITE GNEISS FROM THE PIEDMONT PROVINCE OF GEORGIA

Silicate mineral weathering has been the focus of many studies on soil production as well as soil water and groundwater chemistry, mainly because 83% of the continental rocks exposed at the surface are silicate rock types, with the remainder represented by carbonates (16%) and evaporates (~1%) (Meybeck, 1987). The importance of silicate weathering on the reduction of atmospheric carbon dioxide over long periods of geologic time is well recognized (Walker et al., 1981; Berner, 1995), in particular the Ca-Mg silicates (Gislason and Oelkers, 2011). Based on global silicate weathering rates (Dessert et al. 2003) calculated that 30-35% of the carbon dioxide removed by silicate weathering is due to the weathering of basalt, despite the small percentage of basalt exposed at the surface on the continents and volcanic islands. Weathering of Ca-Mg silicates provides the Ca and Mg divalent cations necessary for the formation of carbonate as a secondary mineral, resulting in the sequestering of atmospheric carbon dioxide.

Primary silicate minerals that are most susceptible to chemical changes during weathering are the ferromagnesian silicates, of which the chain silicates are the dominant ferromagnesian minerals, comprising 16% of the earth’s crust (Klein, 2002). Although many weathering studies focus on granitic material, fewer studies involve mafic weathering, in particular metamorphosed mafic rocks, despite their higher amounts of ferromagnesian minerals and susceptibility to weathering. Our focus on ferromagnesian silicate weathering involves the study of amphibolite gneisses exposed in several locations around Carroll County in the Piedmont Province of Georgia. Variably weathered samples were collected and bulk chemistries determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Qualitative mineralogy was determined by x-ray powder diffraction techniques (XRD). We use the Weathering Index of Parker (WIP) to characterize the amount of geochemical weathering, and relate this to the mineralogical changes seen among the samples.