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calcareous strata. The "upper" sandstone unit of the Dakota (Young, 1973) is absent within the <br />study area. Two thin, white clay (bentonitic) beds are widely traceable in the western half of the <br />study area. These clays average 0.5 to 2.5 feet in thickness and are found approximately 60 to 70 <br />feet above the base of the lower Dakota coal seam (Figure 2.04.6-1 and Peabody Appendix 6-1 ). <br />Conglomerates are absent in overburden strata. However, a 2 to 4 foot conglomerate marks the <br />base of the Dakota Formation. This conglomerate is the "lower" lithologic unit of the Dakota <br />described by Young (1973). <br />Mineralocty. Iron disulfides (pyrite [FeSZ] and marcasite [FeS2]) are present in the lower ten feet <br />of overburden and the interburden strata at the New Horizon 2 study area. These minerals are <br />associated with coal-bearing rocks in which reducing conditions are prevalent at the time of <br />deposition. When exposed to an oxidizing environment in the presence of percolating water, iron <br />disulfides often react to form hydrous iron sulfates. These compounds commonly appear as white <br />and yellow salt crusts on weathered rock surfaces. According to Caruccio et al. (1977), the rate <br />at which these acid leachates are produced is dependent on the quantity and grain-size distribution <br />of pyritic material, availability of oxygen, presence of iron bacteria, amount of calcareous material, <br />and relationship to the ground water aquifer. According to Nordstrom (1982), framboidal pyrites are <br />the most reactive form of pyrites, while massive pyrite or nodules react very slowly. Pyrite nodules <br />. are common within the New Horizon 2 mine study area. <br />Another mineral commonly encountered in overburden strata at the New Horizon 2 mine area is <br />gypsum (CaSO4 2H2O). Gypsum is soluble in water and, according to Hounslow et al. (1978), <br />"precipitates readily from solutions produced by the oxidation of pyrite coupled with the dissolution <br />of limestone". As gypsum occurs both as a primary and secondary diagenic mineral in strata within <br />the New Horizon 2 mine study area, a similar cycle of solution and precipitation can be expected <br />in spoil materials. <br />Quartz, feldspar, and clay minerals occur as primary detrital minerals in sandstones and shales. <br />Clays, shales, and organic residues commonly have higher quantities of trace metals occurring as <br />adsorbed ions than do coarser grained strata. These ions may be released into solution as a result <br />of oxidation and low pH. Other less commonly occurring minerals found in overburden strata <br />include calcite (CaCO,), limonite (Fe2O,) and possibly siderite (FeCO3). Small deposits of copiapite <br />(FeO,) and halotrichite (Fe or Mg SO,) may occur as crusts on the faces of coal seams and <br />associated strata along outcrops near the New Horizon 2 mine area, (Caruccio et al., 1977). <br />• Revised 27 Aug 2002 6 <br />