The conditions most conducive to acid formation are high pyrite contents with little base material present. Conversely, an excess of base relative to pyrite is most likely to preclude acid formation. Sites containing low quantities of pyrite and bases produce the most variable drainage quality and are the most difficult to assess with premining predictive techniques. Lithologic Controls Lithology or rock type also influences spoil and drainage quality. Physical characteristics of the rock, such as porosity, and accessory minerals can exert various constraints or enhancements to the overall chemical weathering process.
Argillaceous rocks tend to release their acid load over a longer period of time.
The mineral pyrite occurs in several different morphological forms and a range of grain sizes. Pyrite can occur in grain sizes ranging from invisible to the eye up to several inches. Framboids and other fine grained pyrites with a large surface area are much more chemically reactive than the coarser forms. The reactivity of fine grained pyrites reflects the fact that acid generating reactions occur at the mineral surface. The products of pyrite oxidation are free acid and soluble acid salts.