The Role of Clay Mineral Composition on Silicified Coal Genesa Based on X-Ray Diffraction (XRD)

The interesting concern in this study is that in the same outcrop it was found that the first coal seam (coal seam-D) contains silicified coal and the second coal seam (coal seam-E) does not contain silicified coal and how to find out the cause. In general, the characteristics of silicified coal seam-D are in the form of lenses; the distribution extends from a few meters to several tens of meters with a thickness of three to five centimeters and some are in nodules. Environmental conditions are the main factor in the formation of silicified coal where the acidity level of coal bearing sediment seam-D which is between 3.4 (roof sediment) to 4.2 (floor sediment) is higher than coal bearing sediment seam-E which has a pH of 5.4. This condition indicates that the coal bearing sediment seam-D environment is more acidic than coal seam-E. The results of XRD analysis show that the seam-D containing silicified coal has a composition of 47.47% kaolinite, 21.27% illite, 23.73% smectite and 17.54% mixed layer, while the seam-E which does not contain silicified coal has a composition of kaolinite 35.78%, illite 17.88%, smectite 21.59% and mixed layer 24.76%. Acidic environmental conditions (pH) and silica enrichment in clay minerals (kaolinite) in the roof sediments during sedimentation, accelerated the process of deposition of colloidal silica in the wood structure of coal seams into silicified coal.