Pyrite, ambient

In this case study, the selected phases are pyrite, amorphous FeS, calcite (present in limestones in the roof strata Fig. 5), dolomite (possibly also present in the limestones), siderite (which occurs as nodules in roof-strata mudstones), ankerite (present on coal cleats in the Shilbottle Seam), melanterite and potassium-jarosite (representing the hydroxysulphate minerals see Table 3), amorphous ferric hydroxide (i.e., the ochre commonly observed in these workings, forming by precipitation from ferruginous mine waters), and gypsum (a mineral known to precipitate subaqueously from mine waters with SO4 contents in excess of about 2500 mg/L at ambient groundwater temperatures in this region, and with which most of the mine waters in the district are known to be in equilibrium). In addition, sorption reactions were included in some of the simulations, to contribute to the mole transfer balances for Ca, Na, and Fe. 

One of the most commonly applied SR-based scattering methods is powder X-ray diffraction, usually accompanied by Rietveld profile fitting of the diffraction pattern (e.g., Hazemann et al. 1991 Parise 1999 Lee et al. 2001). Special in situ reaction cells have been designed to study crystal growth and phase transformations in aqueous solutions at ambient to moderate temperatures ( 200°C) using X-ray diffraction (Cahill et al. 1998). Such cells have been used to study pyrite growth from aqueous solutions (Cahill et al. 

Six iron-sulfur minerals are stable enough to exist in nature all contain the ferrous (Fe +) form of iron. Those minerals with iron-to-sulfur ratios of 1 1 (FeS) are mackinawite and pyrrhotite. Those with iron-to-sulfur ratios of 3 4 0 0384) are greigite and smythite. All the iron sulfides with ratios of 1 1 and 3 4 are soluble in mild acids with formation of H2S. Those minerals with iron-to-sulfur ratios of 1 2 (FeSj) are pyrite and marcasite. Pyrite and marcasite are distinguished from the other four iron sulfides by their insolubility in concentrated HCl. Pyrite is highly pH- and temperature-stable. Due to this, it is found often in nature. This inertness also makes pyrite a desirable reaction product for sulfide removal using an iron-based scavenger. Various iron sulfides can be formed chemically from iron compounds reacting with soluble sulfides at ambient conditions in aqueous systems. The specific reaction conditions control both the products formed and the rate of reaction. 

The related sulfate minerals (Table 7.3) are not as common as the sulfides and are not usually present in unweathered fresh coals. Obviously, the anaerobic maturation of (the majority of) coals are not conducive to the formation of sulfates. For example, pyrite is markedly susceptible to oxidation and will decompose to iron sulfate minerals even under ambient conditions. 

Figure 7.44. ATR spectra of surface compounds on pyrite electrode in 10" M potassium n-butyl xanthate solution in 0.01 M borate buffer (pH 9.18) at ambient atmosphere at potentials starting at -0.6 V. Reference taken at -0.6 V. Spectra obtained with Perkin-Elmer 1760X FTIR spectrometer with MCT detector. Each spectmm is average of 200 scans with 8 cm" resolution and multiplied by factor of (a) 300 (b) 50 (c) 10. Reprinted, by permission, from I. V. Chernyshova and V. P. Tolstoy, Appl. Spectrosc. 49, 665 (1995), p. 668, Fig. 4. Copyright 1995 Society for Applied Specfroscopy.

Another mysterious phenomenon is ambient pyrite. Grains a few microns in diameter have moved through solid chert (an extremely fine-grained quartz), leaving a trail of coarse-grained quartz. This was described by Tyler, Knoll, and Barghoorn (172) (Figure 1.9). 

Fong R, Dahn JR, Jones CHW (1989) Electrochemistry of pyrite-based cathodes for ambient temperature lithium batteries. J Electrochem Soc 136 3206-3210... 

Shao-Hom Y, Osmialowski S, Horn QC (2002) Reinvestigation of lithium reaction mechanisms in FeS2 pyrite at ambient temperature. J Electrochem Soc 149 A1547-A1555... 

Because the activation energy for the phase transformation of mackinawite to the thermodynamically more stable FeS phase hexagonal pyrrhotite is quite high (493 kJ/mol (31)), mackinawite is more susceptible to oxidation to other mineral phases such as greigite and pyrite than transformation to pyrrhotite under ambient conditions (31). 

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