Laserfiche WebLink
early in the history of coal diagenesis by bacterial reduction of sulfate and ferrous iron <br /> in order to metabolize the organic compounds presents. (In this discussion, the name <br /> "pyrite" will be used to indicate both pyrite and marcasite). It is apparently in this <br /> setting that framboidal pyrite forms (Caruccio and others, 1977). Framboidal pyrite, is <br /> comprised of submicron-sized crystals of pyrite. Studies by Caruccio (Caruccio and <br /> Parizek, 1967; Caruccio, 1968; Caruccio, 1970; Caruccio and Geidel, 1982) have shown that <br /> framboidal pyrite is responsible for the production of acid, while other forms (massive, <br /> plant replacement, primary euhedral and secondary cleat filling) do not react rapidly <br /> enough to produce substantial acid. According to Caruccio and others (1977), the <br /> occurrence of framboidal pyrite is related to depositional environment. It is found more <br /> commonly in near marine (back bay and lower delta plain) than in upper delta plain <br /> environments. The second important factor in the production of acid is the presence or <br /> absence of carbonate minerals in rocks associated with the pyrite. <br /> Different laboratory techniques have been used for predicting the potential for acid <br /> generation, and result in a value termed the "Acid Base Potential" or ABP. in order to <br /> measure or calculate the potential acidity either (1) the pyrite (plus organic sulfur, in <br /> some techniques) content is measured, and potential acidity is calculated, or (2) a sample <br /> is oxidized with hydrogen peroxide (after treating with acid to remove carbonates and <br /> gypsum, and rinsing) and the amount of acid produced is measured by titration. The <br /> neutralization potential is measured by titration, and is indicative of the content of <br /> alkaline carbonates, exchangeable bases, and readily weatherable silicates. These two <br /> measurements are expressed in terms of equivalent CaCO3. The ABP is calculated by <br /> subtracting the potential acidity from the neutralization potential. If the ABP is <br /> positive, reactions of the overburden with water are likely to yield pH's greater than 7. <br /> A negative ABP indicates that acidic conditions may develop. <br /> An important assumption in this approach is that all the pyrite present is reactive. When <br /> the acid potential is determined by oxidation with hydrogen peroxide, the use of elevated <br /> temperatures encourages oxidation of the non-framboidal pyrite. Smith and others (1974, <br /> p. 48) discuss the excellent agreement between potential acidity determined by oxidation <br /> and that calculated from total pyrite content, which indicates that pyrite oxidation <br /> during the laboratory procedure is complete, or nearly so. Hence, the potential acidity <br /> parameter overestimates the amount of acid that might be produced because non-framboidal <br /> forms of pyrite are oxidized in addition to the other forms. <br /> 17-39 Revised 04/11/88 <br />