X-RAY POWDER DIFFRACTION (XRPD) RIETVELD QUANTITATIVE PHASE ANALYSIS OF THE MOUNT SCOTT AND MEDICINE PARK GRANITES FROM THE WICHITA MOUNTAINS, OKLAHOMA

The members of the Wichita Granite Group exhibit a small but significant diversity in their mineral mode. The differences are useful in discriminating events and processes that formed the shallow plutons. Traditionally, modes have been assessed by point counting thin sections. Relatively new improvements in X-ray powder diffraction (XRPD) software coupled with affordable but robust hardware permit the possibility of faster modal assessment using XRPD.

In this study, we used rapid quantitative phase analysis to characterize two Wichita Granite Group samples: the Medicine Park Granite and the Mount Scott Granite. Both samples were previously characterized for mode and geochemical composition, including the geochemistry of the feldspar populations. Both have not been previously characterized by powder diffraction. Samples were powdered to ~60 microns using a ring-and-puck crusher. We analyzed the samples using a Rigaku MiniFlex600 XRD (Cu radiation with monochronometer and scintillation detector) and its accompanying PDXL2 software with the ICPDD PDF-2 catalog. We collected long scans from 2-theta of 5° to 75° at 3 seconds in 0.01° steps. We optimized the resultant patterns for all peaks and background, and matched lines from robust ICPDD cards that produced aligned peak position and intensity matches. Analyses show the presence of both microcline and orthoclase polymorphs in both granites, consistent with prior arguments for multiple stage crystallization and exsolution. PDXL permits two techniques of mode assessment: the Relative Intensity Ratio (RIR) method and Rietveld refinement technique. The RIR gives unsatisfactory results for the Medicine Park Granite, but Rietveld refinement of the structural parameters yields modes within 2% of those determined by point counting. The RIR analysis of the Mount Scott Granite sample gives expected compositions for quartz and perthitic microcline, but refinement of the calculated spectra skews the quartz in favor of albite. Further refinements that consider the preferred orientation of biotite and exsolved albite crystallites yields results close to the point-counted modes. Results suggest that XRPD could be used for rapid assessment of these granites and as a technique to amplify differences in the mineral populations.