Date of Award

1992

Document Type

Thesis

Degree Name

Bachelor of Science (BS)

Department

Geology

Abstract

Longer sample grinding time results in reduced variation in peak intensities, allowing for precise semi-quantitative X-ray diffraction (XRD) analysis of abundance of crystalline phases; but also creates difficulties by reducing peak intensities and broadening peaks. Grinding samples for an extended period is known to reduce preferred orientation and the particle size in sample mounts. Grinding also increases the number of particles in a sample, which increases the probability of equal representation for all crystal orientations during XRD analyses and reduces the variation of integrated intensities.

Three separate samples of a dolomite-rich dolostone, a quartzose sandstone, and plagioclase-rich gabbro were ground with a tungsten carbide mortar and pestle until each passed through a 230 mesh (63 micron) sieve. The samples were ground in a Spex Mixer Mill from 5 to 60 minutes, packed into side drift plates, and analyzed using a Philips x-ray diffractometer. The computer software programs, Jade and Micro-ID, were used to analyze the XRD data.

The highest intensity peak of dolomite was examined in ten samples with grinding times of 5, 15, 30, and 60 minutes. The coefficient of variation for the mean integrated intensities decreased from 14% at 5 minutes to 5% at 60 minutes; the mean integrated intensity decreased from 1.47 x 105 to 1.03 x 105 counts; and the peak width at half-maximum increased from 0.179 degrees 2-theta to 0.210 degrees 2-theta.

The highest intensity peak of calcic plagioclase was studied in three samples at 0, 15, and 30 minutes of grinding time. The mean integrated intensity decreased from 4.67 x 104 to 3.33 x 104 counts and the peak width at half maximum increased from 0.127 to 0.136 2-theta.

The highest intensity peak of the quartz in the sandstone was examined in 3 samples at 0, 15, and 30 minutes of grinding time. The mean integrated intensity of the quartz samples decreased from 3.48 x 104 to 2.98 x 104 counts and the peak width at half maximum increased from 0.180 to 0.199 2-theta.

Increased grinding time in a Spex Mixer Mill reduced the observed integrated intensities (peak area) and the coefficient of variation of these intensities for all three materials. Increased grinding time also resulted in an increase in peak width. Higher integrated intensities and narrower peaks of more coarsely ground samples aid in qualitative identification of phases present in a sample, but the accompanying highly variable intensities decrease precision in semi-quantitative analysis. Variation in integrated intensities can be minimized by longer grinding. For consistent semi-quantitative results each sample should be prepared in a similar manner to ensure similar particle sizes. For mixtures of minerals with different resistances to grinding, sample preparation must be designed based on project objectives.

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