Sulphide Minerals

Crystalline substances that contain sulphur combined with a metal (e.g., iron) or semi-metal (e.g., arsenic) but no oxygen are called sulphide minerals (Table 6.1). If a metal or semi-metal are both present in a mineral (e.g., arsenopyrite, FeAsS), the semi-metal substitutes for sulphur in the crystal structure. These minerals form in strongly anoxic (i.e., chemically reducing) environments, as indicated by sulphur, which is present in its lowest natural oxidation state. In oxygenated environments, sulphur exists in higher oxidation states, such as S,0,^2-, SO,^2-, and SO,^2- (sulphate) and forms minerals with oxygen (e.g., gypsum, CaS0₄-2H,0).

Under certain geological conditions, most notably near-surface, low-temperature deposits (bogs and swamps, etc.) and/or rapid deposition (e.g., mid-oceanic ridge sulphide deposits), sulphide may be precipitated in amorphous (noncrystalline) or poorly crystalline forms. For iron-sulphide minerals, amorphous FeS or greigite may form initially and then alter to pyrite via sulphurization. This process may lead to the formation of raspberry-like balls or “framboids” of fine-grained pyrite crystals. This framboidal pyrite has a significantly higher rate of acid generation when exposed to an oxidizing environment than coarsely grained, euhedral pyrite.

Marcasite (Table 6.1) is a low-temperature iron-sulphide mineral that may form instead of pyrite and which reportedly has a higher rate of acid generation under oxidizing conditions than crystalline pyrite. Marcasite may also be found in higher temperature paleoenvironments where it is metastable with respect to pyrite at temperatures >157°C.

At elevated temperatures, sulphide may be mobile, leading to recrystallization as a massive sulphide sometimes found at metal mines. The rate of acid generation from massive sulphide may be

TABLE 6.1

Summary of Common Sulphide Minerals and Their Oxidation Products

^a Intermediate species such as ferrous iron (Fe²) and S₂0₃²~ may be important.

^b Depending on overall water chemistry, other minerals may form with, or instead of, the minerals listed here.

Source: From Draft Acid Rock Drainage Technical Guide, Vol. 1 (Vancouver, Canada: BiTech Publishers, 1989). With permission.

relatively slow, but the rate may be accelerated during the mining process through blasting and grinding. In general, the relative rates of oxidation for sulphide minerals under typical environmental conditions are unclear and detailed experimentation is recommended.

Oxidation of these minerals may lead to the formation of secondary minerals after some degree of pH neutralization or when pH is maintained near neutral during oxidation. Some of these minerals are listed in Table 6.1; other minerals may form in addition to, or instead of, these minerals depending on water chemistry, the extent of oxidation, and the presence of other compounds such as aluminosilicates. These secondary minerals may encapsulate the sulphide mineral and/or any neutralizing mineral, slowing the reaction rate.