Clay Minerals in Northern Plains Coal Overburden as Measured by X‐ray Diffraction

Abstract

Abstract Mathematical models were tested for changing x‐ray diffraction data to percentages of clay minerals in coal overburden. Various factors for adjusting peak areas were tested on 50 eastern Montana samples that contained smectite, illite, and kaolinite, with lesser amounts of other minerals. Cation exchange capacities (CEC) of the clays were estimated from the calculated mineral percentages and correlated against measured CEC. The best model gave an r 2 of 0.89. It was used for estimating clay mineralogy at six mine sites in the Northern Great Plains. Average mineral contents in the surface 8 to 38 m of five of seven drill holes in the Montana‐Wyoming border area were 40% smectite, with 20% each of illite and kaolinite. Clays from greater depths in the same area had no smectite and an average of 50% each of illite and kaolinite. All samples from a mine in central North Dakota were high in swelling clay, with an average of 60% smectite and 10% vermiculite. Samples from four holes at a mine in eastern Wyoming were all high in kaolinite, having an average of 50% with 30% illite and 10% interstratified smectite‐vermiculite.