GEOCHEMICAL AND MINERALOGICAL CHARACTERIZATION OF OVERBURDEN MATERIALS FOR THE ADTI-WP2 PROCEDURE FOR COAL MINE DRAINAGE PREDICTION

Rocks typical of coal overburden in the eastern United States (shales, sandstones) were sampled for testing of the proposed ADTI-WP2 (leaching column) weathering protocol. The rocks vary in geochemical, mineralogical, textural, and acid-base accounting characteristics. The abundance and composition of minerals are important for the interpretation of leaching column data because minerals have different acid-generating (AP) or acid-neutralizing potentials (NP), trace-metal contents, and vary in weathering characteristics. Unweathered rocks were characterized by optical and scanning-electron microscopy, quantitative powder x-ray diffraction, electron probe microanalysis, bulk geochemical analysis, and low-temperature ash (LTA) determinations. Shales had variable total sulfur (0.2 to 4.3 weight percent), NP (13 to 48 ppt CaCO₃), arsenic (8 to 56 ppm), selenium (<3 to 82 ppm), pyrite (1 to 17 weight percent), carbonate minerals (2 to 6 weight percent), and LTA (79 to 99 weight percent). Quartz was the dominant mineral, with micas, chlorite, and clay. Pyrite framboids (<1 to 5 micrometers) were associated with Mg-calcite, ankerite, siderite, or organic material. Ankerites were zoned (Ca-rich cores, Fe-rich rims); both calcite and ankerite contained up to 3 weight percent MnO. Gypsum or barite contributed to the total sulfur endowment of some samples. Sandstones contained <0.1 weight percent total sulfur. Pyrites sequester trace elements (up to 0.4 weight percent selenium, 0.2 weight percent arsenic). Despite an acid-base account that predicts potential for acid effluent for two of the four shale samples, preliminary data indicate that effluents from shale columns (some initially acidic) were all net alkaline with near-neutral pH from four to 15 weeks. Rapid, continued dissolution of Ca-bearing carbonates in the CO₂-enriched atmosphere of the leach protocol could maintain near-neutral pH, slowing the rate of pyrite dissolution. Dissolution of fine-grained Fe-bearing carbonates, as well as pyrite, contributes iron to effluent in the weathering experiments. Mineralogical characterization of these samples facilitates interpretation of effluent data.