STUDY OF CLAY MINERALS FROM THE MORTON GNEISS SAPROLITE

Various outcrops of the Morton Gneiss, located in the Minnesota River Valley, are now weathered to saprolite. Early research placed the main weathering event during the late Cretaceous, sometime between 99 and 65 Ma. Recent research and ⁴⁰Ar/³⁹Ar dating on a white mica from Redwood Falls, MN, however, dated the weathering process at about 500 Ma. The grains tested were extremely small, so it is possible that this date may have been confounded by Argon loss. Petrographic analysis shows that the Redwood Falls saprolite is mainly comprised of quartz, clay minerals, oxides, mica, and a few heavy minerals including zircons. Here we provide additional chemical analysis and study of the clay minerals to further understand the processes of weathering that took place. After isolating the clay particles, we used x-ray diffractometry (XRD), scanning electron microscopy (SEM), and petrographic analysis. Our preliminary XRD findings suggest that the main clay mineral, in addition to the white mica, is kaolinite.