GEOCHEMISRY OF SERICITE FROM A SAPROLITE OF THE PRECAMBRIAN MORTON GNEISS IN THE MINNESOTA RIVER VALLEY OF SOUTHWESTERN MINNESOTA

The saprolite developed on the Precambrian Morton Gneiss in the Minnesota River valley of southwestern Minnesota is ideal for studying the rates of chemical and physical weathering. In addition to the few remaining original minerals of the parent rock, the saprolite contains secondary minerals such as kaolinite and sericite (white mica). Sericite is a by-product of weathering of K-feldspar, and makes up ~ 1-2 % of the saprolite. The sericite grains range in size from 0.3 to 0.5 mm in diameter. For this study, sericites were separated from the bulk sample and analyzed for their chemistry using X-ray diffractometer (XRD), scanning electron microscopy (SEM), scanning electron microprobe (SEMP), and Fourier-transform infrared spectroscopy (FTIR). Bulk sample was analyzed for major, minor and trace elements using X-ray fluorescence. In the field, in the hand sample, under the SEM, and under the petrographic microscope, the sericite appears to be true white mica. However, analysis of the sericite using SEM, SEMP and XRF indicate that the sericite within the saprolite are highly potassium (K) depleted, with range of 0.04 - 0.11 wt. % K compared to the published range of ~ 8-11 wt. %. We propose that the K-depletion of the sericite resulted from an additional weathering event that took place since the formation of the sericite and the sericite appears to be transitioning chemically towards kaolinite.