Marcasite: oxidation

What are the two key pyrite-marcasite oxidation reactions that cause the weathering of FeS2 Compare and discuss the meaning of the acidity produced by the overall oxidation reactions to that of the corresponding oxidation half-cell reactions. 

The acidification of dumps and waters is a common problem in many mining areas. The according acidic mine drainage (AMD) has been investigated (e.g. Alpers and Blowes 1994 Fischer et al. 1987 U.S. Dept, of the Interior 1994). Oxidation of marcasite in dumps results in a first step in acid-reaction products which are then washed out. The most significant reactions of pyrite and marcasite oxidation are listed in Table 8.2 (after Stumm and Morgan 1981 MattheB 1990). 

Iron disulfide is obtained from its naturally occurring minerals, pyrite and marcasite. In the laboratory it may be prepared along with iron(II) sulfide by passing dry hydrogen sulfide through a suspension of hydrated iron (III) oxide or iron(III) hydroxide in alkaline medium. The unstable product formed decomposes to FeS2 and FeS. 

Pyrite is the most common sulfide mineral. It is a major contributor to the formation of mine drainage and sulfate-rich natural runoff. The oxidation of pyrite and other Fe(II) sulfides (e.g. marcasite and pyrrhotite) involves both iron and sulfur, as well as any arsenic impurities. Activation energies suggest that surface reactions dominate the oxidation of pyrite (Lengke and Tempel, 2005). Furthermore, evidence from pyrites in coal and ore deposits suggests that arsenian pyrite is more susceptible to oxidation from weathering than low-arsenic pyrite (Savage et al., 2000, 1239). 

The chief commercial use of pyrites (including marcasite) is in the manufacture of sulphuric acid.5 The pyrites, on being roasted in air, yields sulphur dioxide and a residue of ferric oxide. Thus —... 

The principal sulfides that have been observed in oxidized mine wastes are marcasite [FeS2] and... 

In contrast to the spatial restriction of marcasite to replacement of pyrrhotite, most covellite in mine wastes results from redeposition of solubilized copper that is typically derived from primary chalcopyrite. Sorption of copper on iron oxyhydr-oxides is common, but redeposition as a sulfide occurs where reductive conditions are present. Such conditions seem to be available locally on a micro scale in proximity to altering pyrrhotite, but on a broader scale the formation of covellite is predominant at the interface between the oxidized and reduced zones of a waste body, thus emulating the supergene enrichment process that takes place in sulfide deposits, especially those of porphyry copper. In mine wastes, other copper sulfides that resemble covellite in reflected light may also be present, but small grain size has impeded specific identification. 

This principal environmental problem posed by coal-cleaning waste is that the pyrite and marcasite in the waste are oxidized to sulfuric acid in the presenee of air, water, Ferrobacillus ferrooxidans, and Thiobacillus ferrooxidans. The sulfuric acid is usually sufficiently concentrated to dissolve numerous metallic constituents and large quantities of iron from the pyrite in the leachate. Any ECT intended to prevent sulfuric acid formation must eliminate either the air, the water, or the oxidizable sulfur compounds in the waste, or inactive Ferrobacillus ferrooxidans and Thiobacillus ferrooxidans by maintaining alkaline conditions. Post-treatment of pile drainage comprises ECTs designed to neutralize the acid in the effluent and remove the metal ions by some sort of precipitation, adsorption, flocculation, or ion-exchange phenomenon. 

Based on highly accurate adiabatic-shield calorimetry measurements, Gronvold and Westrura ( ) reported that the enthalpy of transformation of marcasite to pyrite is -1.05 0.05 kcal raol" at 700 K. The adopted value of AjH (298.15 K) is selected to reproduce this enthalpy of reaction within the reported uncertainty. Lipin et al. (2), based on combustion calorimetry, reported a value of -5.6 kcal mol" for the marcasite-pyrite transformation at 298.15 K. Due to the state of the art in combustion calorimetry at the time of this measurement and uncertainty in the products (oxides of sulfur), this value must have a high uncertainty and is given no weight in our selection process. 

A number of important structure types are found in transition-metal sulphides which have no counterparts among oxide structures, notably the various layer structures and the pyrites, marcasite, and NiAs structures. Further, many sulphides, particularly of the transition metals, behave like alloys, the resemblance being shown by their formulae (in which the elements do not exhibit their normal chemical valences, as in 0983, Pd4S, TiSa), their variable composition, and their physical properties-metallic lustre, reflectivity, and conductivity. The crystal structures of many transition-metal sulphides show that in addition to M-S bonds there are metal-metal bonds as, for example, in monosulphides with the NiAs structure (see later), in chromium sulphides, and in many sub-sulphides such as Hf2S,... 

Integration yields n = /j exp(-/ +/), where n and are the total number of atoms present at t and r = 0. The half-time of radioactive decay is defined by r 2 = 0.693/A+, where n = 0.5, and n =. Other examples of first-order reactions (see Section 2.7) are the oxidation of organic matter and sulfate reduction, gypsum (CaS04 2H2O) dissolution, and the oxidation of pyrite and marcasite (FeS2). 

A few minerals produce acid when they contact water. These minerals can be described as salts of weak bases and strong acids. They chiefly result from weathering and oxidation of the pyrite or marcasite (FeS2) exposed in the mining of mineral deposits and coal. Such acid minerals, which are dominantly Fe sulfates and to a minor extent AP sulfates, typically form from the evaporation of pooled acid-mine waters or of the moisture in unsaturated mine wastes or spoils that contain the sulfides. Acidity is produced when they are dissolved by fresh runoff or recharge. For example... 

Pyrite (FeS2> is by far the most abundant sulfide mineral, occurring in most types of geologic formations. Its less common polymorph, marcasite, usually forms in near-surface, low-temperature environments. At 25°C pyrite is more stable than marcasite by about -0.4 kcal/mol. The oxidative breakdown of these minerals as the result of exposure to aerobic conditions due to mining is the chief cause of acid mine waters. 

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