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Symposium on the Geology of Rocky Mountain Coal, October 2-4,
OCCURRENCE OF DETRITAL, AUTHIGENIC AND ADSORBED INORGANIC CONSTITUENTS IN LIGNITE FROM THE BEULAH MINE, NORTH DAKOTA
Inorganic constituents in the lignite of the Beulah- Zap Bed consist primarily of: 1) detrital and authigenic quartz, clay minerals and pyrite with lesser silicates, carbonates, and accessory minerals; and 2) organically adsorbed and bound S, Ca, Na, Al, Mg and other elements.
Samples from the Beulah Mine were studied by scanning electron microscope (SEM) and microprobe (EMPA) techniques. Mineral constituents were investigated in polished sections by SEM-EPMA study of samples from nine seam locations. Abundant quartz was observed as 20-30 micron diameter subangular grains. Clay minerals occurred as fine-grained masses in specific mineralized areas 30-400 microns in diameter. Members of the kaolinite, smectite and mica/illite groups were observed. Pyrite occurred as rounded blebs or framboids 20-30 microns across. Samples at the base of the seam contained massive pyritic replacements of wood. Carbonates included small amounts of calcite, magnesite, and dolomite predominantly in fractures. Minor minerals included hematite, feldspars, rutile, zircon, barite and gypsum.
Adsorbed constituents were investigated by SEM-EPMA techniques in the study of fractured surfaces. Some correlation of element distribution and SEM appearance was noted. For example, some obviously woody fragments had high Na values with relatively lower Ca and S when compared to other morphological forms of organic material. Many elements showed distinctly polymodal distributions when results from all morphological types were grouped.
Results of the studies reported here show that major types of inorganic constituents are highly variable in the coal in both mode of occurrence and distribution within the seam. This suggests a complex depositional, diagenetic and post-diagenetic geologic history for the Beulah-Zap Bed.
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