NATURALLY WEATHERED PLAGIOCLASE GRAINS FROM THE IDAHO BATHOLITH - OBSERVATIONS USING SCANNING ELECTRON-MICROSCOPY

Rates of primary silicate mineral weathering are important to understa nding proton buffering and biogeochemical cycling of Si, Al, alkali me tals, and alkaline earth metals in natural watersheds. The objectives of this study were to characterize the surface morphology and secondar y coatings of naturally weathered oligoclase grains sampled from the S ilver Creek Experimental Watersheds in the southwestern Idaho batholit h. Oligoclase grains (0.5-1.0 mm) were sampled from soil, sediment, an d rocks of various weathering classes within the watershed, and were c haracterized using scanning electron microscopy (SEM) and energy dispe rsive x-ray analysis (EDXRA). Oligoclase grains from unweathered bedro ck (Class 1 rock) and very weakly weathered rock (Class 2) showed litt le evidence of weathering features such as etch pits and secondary coa tings on the mineral surface. As the weathering intensity increased to Class 5, Class 6, R horizon, and C horizon oligoclase samples, the pr esence of etch pits and secondary coatings increased. Grains from Clas s 6 rock and the R horizon were characterized by numerous etch pits an d secondary coatings, which covered the majority of the grain surface. The EDXRA analyses showed that the secondary coatings had Al/Si ratio s approximately two times and Ca/Si ratios approximately 0.2 times tha t of Class 1 and Class 2 rock, indicating significant Al enrichment an d 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 su rface. Observations of naturally weathered feldspar grains suggest sig nificant differences in weathering environment or intensity between ma ny laboratory studies and actual watersheds. Environmental conditions that may be responsible for differences in weathering intensities betw een natural watersheds and laboratory studies include physical factors such as solid/solution ratios, water residence times, wetting and dry ing cycles, and chemical factors such as the presence of secondary coa tings on the mineral surface and the reactive surface area of weathere d grains.