HEAVY MINERAL ANALYSIS OF SANDSTONES BY RIETVELD ANALYSIS

Abstract

Rietveld analysis of heavy minerals has been tested using an Eocene sandstone from the Medicine Pole Hills south of Rhame, ND. Heavy minerals were separated from three size fractions covering the range of 0.18 to 0.30 mm. Polished grain mounts prepared from each size fraction were used to identify and measure the size of 1,517 grains by optical microscopy. Mineral compositions were determined by microanalysis of 101 grains using a scanning electron microscope (SEM) with an energy dispersive X-ray analysis system. Powdered sample mixed with a rutile internal standard (10%) was used to collect an X-ray diffraction pattern (20-80° 2 θ). Rietveld refinements were carried out using GSAS and an averaging technique. Based on optical/SEM work, heavy minerals in the sandstone consist of diopside (56.1%), edenite (19.9%), epidote (6.1%), augite (5.3%), paragasite (4.2%), almandine-rich garnet (3.6%), and ten minor phases (4.8%). Results of Rietveld refinements completed thus far compare reasonably well with the optical/SEM results for phases of at least modest abundance. One notable difference from optical/SEM results was the abundance of diopside (46.0%). Other differences included higher abundances for two minor phases (4.2% plagioclase and 2.9% talc) and inclusion of goethite (1.1%) and quartz (0.7%), which were not identified as grains in the optical/SEM work (although quartz was seen as inclusions within epidote). The greatest difficulty with the refinements was severe preferred orientation of several phases. The spherical harmonics corrections applied seem reasonable, but additional work on this problem is needed.