Gallium and Germanium in Pennsylvanian Coals, Shales, and Paleosols in Indiana

Gallium (Ga) and germanium (Ge) are two elements of high industrial interest, included in the current United States and European Union lists of critical raw materials. This paper compares Ga and Ge concentrations between coals and associated paleosols and shales in Indiana. In addition, we include Ga and Ge data on the coal waste from coal preparation plants (coal tailings) to evaluate the coal tailings as potential sources of these elements. Paleosols have an average Ga content of 26.70 ppm, more than shales with an 18.77 ppm average, and coals with a 4.61 ppm average. The Springfield coal tailings average 15.60 ppm of Ga and the Brazil and Staunton Formation coal tailings average 31.90 ppm. For Ge, the coals have the highest average concentration (6.46 ppm), then shales (1.36 ppm), and paleosols (0.96 ppm). In coal tailings, the average Ge concentration is 13.90 ppm for the Springfield and 47.70 ppm for the Brazil and Staunton Formation coal waste. For Ga, a very strong positive correlation was obtained with aluminum (Al) in paleosols (R2=0.89) and coal tailings. A strong correlation was also obtained for shales (R2=0.86), together indicating that Al content is a great predictor of Ga concentrations in these materials. For Ge, there seems to be a tendency of decreasing Ge concentration with an increase in Al content in coals, suggesting an organic and not mineral association for this element. The organic association is also supported by a strong negative correlation between Ge and ash yield in coal tailings.