CHARACTERIZATION AND DETERMINATION OF CRETACEOUS ARKANSAS ALKALI MAGMATITES, WITH EMPHASIS ON XRPD-RIETVELD METHODOLOGY

Methods of extraction of important data characterizing geological solids are numerous. If combined they lead to a meaningful synthesis. Essentially, they include local/ regional geology, mineralogy, petrography, fabric analysis, and geochemistry. Quantification should be applied whenever possible and necessary. Common problems encountered are mineral grain size, hydrothermal alteration, glass content, magmatic xenocrystals, and changes by metamorphic processes. X-ray powder diffractometry with Rietveld analysis yields detailed constituent, mostly mineral, information and quantification [weight or volume percentages] based on modern instrumentation and software (TOPAS, DIFFRAC EVA). However, prior sample handling and preparation techniques have to be carefully observed including the use of a micronizer to assure a unimodal particle size of about 15 microns. Incorrect preparation can result in questionable or unusable diffraction patterns. A uniform sample powder surface for diffraction is ensured by applying a proper loading technique using backfilling, zero background plate or front filling. Rietveld analysis diffraction patterns are constrained by a goodness of fit less than 1.5, preferred orientation, and matching diffraction peak patterns. The geologic characterization criterion must be considered to validate results. Cross matching of results is absolutely necessary. Minerals determined by XRDP-Rietveld must be checked against light optical data obtained from polarization microscopy / transmitted and reflected light, and SEM/EDS observations and data if available. Recalculated data plots on QAPF diagrams for magmatites give rock names that must match the geochemically derived names read from the TAS diagrams for aphanites and phanerites.

Application of these methods to selected rocks from the Cretaceous Arkansas alkali magmatic province, mostly the subvulcanic Magnet Cove Igneous Complex (MCIC), show that all plot on the APF triangle. Geochemical plots on the extended TAS diagrams fall in the alkali field ranging from jacupirangite and lamprophyres to nepheline syenites / phonolites. Moreover, refinement of names, mineralogical and geochemical compositions, and other new data provide a much improved base for further petrologic and magmatogenic investigations.