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

Minerals

Composition

Aqueous End Products of Complete Oxidation

Possible Secondary Minerals Formed at Neutral pH after Complete Oxidation and Neutralization11

Pyrile

FeS,

Fe5. S042-. H'

Ferric hydroxides and sulphates; gypsum

Marcasite

FeS,

Fe5. S042-. H'

Ferric hydroxides and sulphates; gypsum

Pyrrhotite

Fe,.,S

Fe5. S042-. H

Ferric hydroxides and sulphates; gypsum

Smythite, greigite

Fc ;S 4

Fe5. S042-. H

Ferric hydroxides and sulphates; gypsum

Mackinawite

FeS

Fe5. S042-. H

Ferric hydroxides and sulphates; gypsum

Amorphous

FeS

Fe5. S042-. H

Ferric hydroxides and sulphates; gypsum

Chalcopyrite

CuFcS,

Cu2. Fe5. S042-. H

Ferric hydroxides and sulphates; copper hydroxides and carbonates; gypsum

Chalcocite

Cu,S

Cu2. S042-. H

Copper hydroxides and carbonates; gypsum

Bornite

Cu5FeS4

Cu2. Fe5. S042-. H

Ferric hydroxides and sulphates; copper hydroxides and carbonates; gypsum

Arsenopyrite

FeAsS

Fe5. As045-, S042-. H

Ferric hydroxides and sulphates; ferric and calcium arsenates; gypsum

Realgar

AsS

As045-, so42-. h

Ferric and calcium arsenates; gypsum

Orpiment

As,S,

As045-, so42-. h

Ferric and calcium arsenates; gypsum

Tetrahed rite and tennenite

CU|2(Sb,As)4S|3

Cu2. Sb05-.As045-, so42-, H

Copper hydroxides and carbonates; calcium and ferric arsenates; antimony materials; gypsum

Molybdenite

MoS,

Mo042-, so42-. H

Ferric hydroxides; sulphates; molybdates; molybdenum oxides; gypsum

Sphalerite

ZnS

Zn2. S042-. H

Zinc hydroxides and carbonates; gypsum

Galena

PbS

Pb2. so42-. H

Lead hydroxides, carbonates, and sulphates; gypsum

Cinnabar

HgS

Hg2. so42-. H

Mercuric hydroxide; gypsum

Cobaltite

CoAsS

Co2. As042-, S042-, H

Cobalt hydroxides and carbonates; ferric and calcium arsenates; gypsum

Niccolite

NiAs

Ni2. As045-, S042-. H

Nickel hydroxides and carbonates; ferric, nickel, and calcium arsenates; gypsum

Pentlandite

(Fe, Ni),Ss

Fe5, Ni2, S042-. H

Ferric and nickel hydroxides; gypsum

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.

 
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