MINERALOGY OF GRANITE AND RHYOLITE UNITS IN THE ST. FRANCOIS MOUNTAINS

The purpose of this study is to examine several plutonic and volcanic felsic rock units exposed in the St. Francois Mountains to determine similarities in mineral content. Rock units sampled include the Grassy Mountain Ignimbrite, Knob Lick Granite, Lake Kilarney Rhyolite, Butler Hill Granite, as well as multiple samples within the Johnson Shut-Ins ash flows. The mineralogy of each rock unit is being examined to test between two hypotheses. The first hypothesis suggests that the mineralogy between the fine-grained and coarse-grained units is the similar and only the grain size differs. The second hypothesis is that although the mineralogy is similar, the volcanic rock units contain high temperature polymorphs of both quartz and the potassium feldspar. Mineral composition analysis is being completed using a Miniflex x-ray diffractometer (XRD) with a Cu radiation source. Prior to analysis, samples were crushed and sieved to a particle size <63 µm. After the samples were analyzed, the Ka₂ and background noise were eliminated from the diffraction patterns using Jade software. The prominent peaks were identified for comparison with other samples and known minerals. Preliminary evaluations of the diffraction patterns indicate that the two primary minerals are quartz and orthoclase. Qualitative comparisons were completed for location and intensity of peaks to determine the similarity in mineralogy. Evaluation of the XRD patterns from the Grassy Mountain Ignimbrite and the Knob Lick Granite showed that the locations of all of the major peaks matched and most of the minor peaks as well. However, the peak intensities between the XRD patterns these two rock units vary slightly suggesting different modal percentages of the minerals present. Similar comparisons were completed between the other units. Based on these preliminary results, our data suggests that the minerals present in these igneous rocks are essentially the same with some variation in the modal percentages.