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• bedding planes and are prevalent throughout the study area in the 20 foot thick sandstone strata <br />which lies about 40 to 60 feel above the base of the lower Dakota coal seam (Figure 2.04.6-1 and <br />Peabody Appendix 6-1). The gypsiferous sandstone is situated within a transitional zone between <br />oxidized sulfate-bearing horizons and unoxidized sulfide-bearing, calcareous strata. The "upper" <br />sandstone unit of the Dakota (Young, 1973) is absent within the study area. Two thin, white clay <br />(bentonitic) beds are widely traceable in the western half of the study area. These clays average 0.5 <br />to 2.5 feet in thickness and are found approximately 60 to 70 feet above the base of the lower Dakota <br />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 base <br />of the Dakota Formation. This conglomerate is the "lower' lithologic unit of the Dakota described by <br />Young (1973). <br />Mineraloav. Iron disulfides (pyrite [FeS~] and marcasite [FeS~) are present in the lower ten feet of <br />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 at <br />which these acid leachates are produced is dependent on the quantity and grain-size distribution of <br />pyritic material, availability of oxygen, presence of iron bacteria, amount of calcareous material, and <br />relationship to the ground water aquifer. According to Nordstrom (1982), framboidal pyrites are the <br />most reactive form of pyrites, while massive pyrite or nodules react very slowly. Pyrite nodules are <br />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 (CaSO; 2Hz0). 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 in <br />spoil materials. <br />Revised 9/99 2.04.6-5 <br />