Date of Award

1987

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

F.R. Karner

Abstract

Study of the Harmon lignite bed at the Gascoyne Mine in Bowman County, North Dakota showed that most of the minerals in the coal lithobodies were detrital in origin and that variable ash deposition during combustion may be caused by variations in types and quantities of mineral phases.

The Harmon bed is part of the Bullion Creek Formation (Paleocene). Objectives of this study included: to develop scanning electron microscope/microprobe techniques for the study of minerals in coal; to determine the origins of the mineral phases; to postulate a depositional environment for the Harmon lignite; and to ascertain whether variable ash deposition behavior of the Gascoyne lignite is related to mineral content.

Scanning electron microscopy and electron microprobe analysis was used to identify and determine the abundance of minerals in the lignite. The average mineral content in weight percent was 44% quartz, 31% illite, 13% kaolinite, 5% montmorillonite, 5% pyrite, and 2% gypsum. The amount of mineral matter as discrete phases, not organically bound inorganic constituents, varied directly with total ash.

The Blue Pit has a higher inorganic content than the White Pit because of a greater amount of quartz and clays in the B seam. This result demonstrates the lateral variability in inorganic content in the Gascoyne lignite. Minerals also varied in vertical distribution. Quartz and clay minerals were more abundant in lithologic layers that were adjacent to clay silt partings, overburden, and underclay.

Quartz, illite, and kaolinite are primarily detrital in origin. Framboidal pyrite and possibly some kaolinite and phosphate minerals formed authigenically during early peat stages. Massive pyrite, gypsum, barite, celestite, siderite, and jarosite were probably epigenetic products which formed after seam compaction and coalification. The present mineral content of the Harmon lignite is probably the result of the characteristics of the depositional environment. Authigenic processes during early peat stages or after compaction and coalification had a minor influence on the current mineral content.

The Harmon lignite was probably deposited as part of a lacustrine depositional environment. Periodic transgression and regression of the freshwater body would explain best the types of minerals observed and their distributions. Variations in ash deposition behavior within the Gascoyne Mine may be caused by the variation in types and quantities of discrete mineral phases.

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