Simulations of the neutralizing capacity of silicate rocks in acid mine drainage environments

Abstract

Pyritic rocks with little or no carbonate mineral content generally produce acid mine drainage when exposed to the atmosphere and moisture. In the absence of carbonate minerals, it is often suggested that silicate minerals can provide some level of acid buffering. At the current time, databases of reaction kinetics are sufficiently detailed to allow calculations of the rates of silicate mineral dissolution reactions relative to the rate of pyrite oxidation. These types of calculations have been conducted for conditions of abiotic oxidation of pyrite by O2 to estimate the acid neutralizing capacity of silicate rock types in terms of specific mineral contents. Using the criteria of maintaining a pH > 5.0 for 10-years, these calculations yield the following results for some generalized igneous rock types. (Table presented) The results show that it is feasible to use geochemical model calculations to estimate neutralization potentials through reaction rate calculations, although these calculations tend to overestimate neutralization potentials of mafic rock types compared to experimental data. The rate calculations also point out the importance of the mafic mineral content for neutralizing acidity in that minerals, such as calcic plagioclase, forsterite, augite, and hornblende, may react rapidly enough to keep up with the rate of acid generation by pyrite oxidation under the right circumstances, whereas the felsic minerals, such as K-feldspar and albite, react too slowly to mitigate acid generation. The calculations also show the importance of secondary mineral formation in estimating neutralization potential in that equilibrium with goethite and gibbsite yields the low end of the range listed in the above table relative to equilibrium with amorphous Fe and Al hydroxides. Overall, the kinetics simulation procedures provide an efficient and inexpensive means to conduct scoping studies of likely pH conditions for carbonate-poor rocks that can be used to optimize sample selection for more detailed experimental testing or prediction of potential impacts from acid mine drainage based on petrological and mineralogical data in the absence of detailed experimental data.