Mineralogical characteristics and weathering environments in Texas lignite overburdens

Abstract

Seven cores and five soils were investigated to characterize the mineralogy of their coarse fractions (>2 μm). The cores and associated soils were sampled from the Wilcox Group sediments overlying lignite at several geographical locations. Heavy-light mineral separation, XRD, IR, SEM, thin section examination, and total chemical analysis were used. Coarse fractions were dominated by quartz and contained variable amounts of weatherable minerals, including mica, K -feldspars, Na-Ca feldspars, chlorite, and kaolinite. Mineral abundance of mica, chlorite, and kaolinte increased with the decrease in particle size from sand to fine silt. Potassium feldspars and Na-Ca feldspars were more abundant in the coarse and medium silt fraction than either the sand, and in most cases, the fine silt. Iron-bearing minerals identified in the coarse fractions included goethite and hematite in the oxidized zones and siderite, chlorite, and pyrite in the reduced zones. Mineral distribution and abundance was related to lithology, soil pedogenic processes, and the status of oxidation of the sediment. Potassium feldspars and particularly, plagioclase feldspars were the lowest in the soil solum, suggesting alteration by the processes of soil formation. Sediment oxidation had caused a moderate reduction in the amounts of weatherable minerals. The magnitude of this reduction was related to the texture of the sediment. Oxidation and associated processes had caused relatively higher mineral weathering in the coarse-textured sediments than the fine-textured ones. The abundance of iron-bearing minerals was mainly related to the status of sediment oxidation. The abundance of weatherable minerals within the Wilcox Group sediments were related to lithology. The Carrizo Formation was composed mainly of quartz, whereas the Simsboro and the Calvert Bluff Formations contained substantial amounts of weatherable minerals in addition to quartz. The structural states of K -feldspars of the Wilcox Group sediments indicated a similar sediment source which suggested that variations in the sedimentation environment have greatly influenced the observed mineralogical differences...