CHARACTERISTICS AND CONTROLLING FACTORS OF FELDSPAR RECYCLING FROM SEDIMENTARY ROCKS

Compared to quartz feldspars have been generally believed to be easily weathered under Earth's surface conditions because they are more susceptible to weathering. Even though there have been many studies dealing with weathering of feldspars, few results have been reported regarding the weathering phenomenon of feldspars in feldspathic sandstones, especially under natural conditions. In this study, we have examined bedrock sandstones and their derivative stream sediments collected from three small watersheds that are composed of Cretaceous and Triassic feldspathic sandstones in southern and eastern Korea, respectively. Sandstones consist mainly of monocrystalline quartz, plagioclase, alkali feldspar, with minor amounts of rock fragments, micas and opaque minerals. Each watershed has single lithology with limited compositions so that the compositional variations found in stream sediments would reflect weathering processes instead of changes in source rock lithology. Most of plagioclases are albitized in source sandstones of two watersheds, and their composition ranges are An 0~14. The ratios of plagioclase to alkali feldspar are higher in sands than in sandstones in both areas although they show some fluctuations. Although plagioclase is generally thought to be more labile than K-feldspar during weathering, the result indicates that weathering rate of K-feldspar is higher than that of albitized plagioclase, which is opposite to common thought. In addition, the two watersheds show slightly different rates of weathering, which is probably caused differences in temperature, precipitation and slope gradient. In the third watershed where plagioclases of sandstones are of volcanic origin with composition between An 0 and 64, alkali feldspar and plagioclase show normal weathering susceptibility as generally thought. The result of this study suggests that weathering of recycled feldspars is mainly controlled by the chemical composition of feldspar, diagenesis and environmental condition. Albitization during diagenesis in sedimentary rocks affects both chemical composition and surface texture of feldspars, which could be one of the important controlling factors of feldspar weathering in sedimentary rocks.