Naturally Weathered Plagioclase Grains from the Idaho Batholith: Observations Using Scanning Electron Microscopy

Rates of primary silicate mineral weathering are important to understanding proton buffering and biogeochemical cycling of Si, Al, alkali metals, and alkaline earth metals in natural watersheds. The objectives of this study were to characterize the surface morphology and secondary coatings of naturally weathered oligoclase grains sampled from the Silver Creek Experimental Watersheds in the southwestern Idaho batholith. Oligoclase grains (0.5–1.0 mm) were sampled from soil, sediment, and rocks of various weathering classes within the watershed, and were characterized using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDXRA). Oligoclase grains from unweathered bedrock (Class 1 rock) and very weakly weathered rock (Class 2) showed little evidence of weathering features such as etch pits and secondary coatings on the mineral surface. As the weathering intensity increased to Class 5, Class 6, R horizon, and C horizon oligoclase samples, the presence of etch pits and secondary coatings increased. Grains from Class 6 rock and the R horizon were characterized by numerous etch pits and secondary coatings, which covered the majority of the grain surface. The EDXRA analyses showed that the secondary coatings had Al/Si ratios approximately two times and Ca/Si ratios approximately 0.2 times that of Class 1 and Class 2 rock, indicating significant Al enrichment and Ca depletion in the weathering products. X-ray diffraction was used to positively identify a kaolin layer silicate (presumably kaolinite) as a major component of the secondary phases found on the feldsdpar surface. Observations of naturally weathered feldspar grains suggest significant differences in weathering environment or intensity between many laboratory studies and actual watersheds. Environmental conditions that may be responsible for differences in weathering intensities between natural watersheds and laboratory studies include physical factors such as solid/solution ratios, water residence times, wetting and drying cycles, and chemical factors such as the presence of secondary coatings on the mineral surface and the reactive surface area of weathered grains. Contribution no. J-2823 from the Montana Agric. Exp. Stn.