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• occur between bedding planes and are prevalent throughout the study area in the 20 foot thick <br />sandstone strata which lies about 40 to 60 feet above the base of the lower Dakota coal seam <br />(Figure 2.04.6-1 and Attachment 2.04.6-1). The gypsiferous sandstone is situated within a <br />transitional zone between oxidized sulfate-bearing horizons and unoxidized sulfide-bearing, <br />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 <br />70 feet above the base of the lower Dakota coal seam (Figure 2.04.6-1 and Attachment 2.04.6-1 ). <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 />Mineralogy. Iron disulfides (pyrite [FeSz] and marcasite [FeS~]) 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 <br />are the most reactive form of pyrites, while massive pyrite or nodules react very slowly. Pyrite <br />nodules 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 (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 <br />in spoil materials. <br />• REVISED Auguat 2006 2.04.6-1 <br />