Masters Theses

Date of Award

8-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Otto C. Kopp

Committee Members

Richard W. Arnseth, Don W. Byerly

Abstract

Recent research has shown that shales and the clay-rich rocks have the best potential as geologic hosts for the disposal of high-level radioactive waste. Beneficial properties include low hydraulic conductivity, high elastic limit, good ion exchange (sorptive) capabilities, and low solubility. These properties are largely dependent upon the mineralogical composition, the textural and diagenetic character, and the effects of deformation of a particular formation. X-ray diffraction was investigated as a method for determining mineral compositions, and scanning electron microscopy was used to observe textural and diagenetic features such as recrystallization, cementation and grain orientation of several end-member shale formations.

A pressed-pellet technique of powder sample preparation for X-ray diffraction was tested for precision using two clay mineral standards. Analyte diffraction intensities (in ops) were reproduced from any of six positions on a sample with a precision of between 1.6% and 5.8% relative error. Analyte diffraction intensities varied between 0.5% and 3.0% for samples composed of the same material. A linear relationship is established between diffraction line intensities (corrected for the variation of an Internal standard) and the weight percent quartz in samples of illite, kaolinite, and smectite.

Two samples were taken from cores of the Conasauga Group, the Rhinestreet Shale, the Pierre Shale, and the Green River Formation and examined for bulk mineral composition using Chung's (1974 a and b) "matrix-flushing" and relative intensity method (RIM). Selected minerals were chosen for spiking in each of the samples so that linear regressions could be performed and mineral concentrations determined by a second method. Differences in concentrations calculated by the two methods are noted and are most likely due to variations in crystallinity and chemical composition of the standards. Fractured blocks were examined with the scanning electron microscope for textural and diagetic features. Semi-quantitative chemical analyses were performed for specific grains.

The Conasauga samples show variability in both mineralogy and texture. Authigenic quartz and calcite is abundant and porosity is attenuated by cementation, compaction, and alignment of clay minerals. The Rhinestreet samples are more homogeneous. They are low porosity, illite-rich rocks with a high degree of preferred orientation. Detrital quartz and albite and pore filling oxides are present. The Pierre Shale samples are much less "mature" than the Conasauga or Rhinestreet samples. They are rich in mixed-layer clays in a random orientation with calcareous and phosphatic fossil grains. Porosity is evident and euhedral growths of pyrite and fibrous calcite are noted. The Green River Formation is an organic-rich, clay-deficient marlstone showing only minor amounts of illite. It is rich in dolomite and calcite probably of an authigenic nature and exhibits considerable porosity.

The pressed pellet-technique and X-ray diffraction are evaluted as quantitative tools for determining mineral concentrations in shales and argillaceous rocks. The mineralogical and textural information are used to speculate on the properties of each of the formations and their potential as geologic hosts for high-level radioactive waste storage.

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