Pyrite stability

KnUemd, G. and Yoder, H.S. 1959. Pyrite stability relations in the Fe-S system. Economic Geology, 54 533-572. 

With respect to non-noble and non-Ru catalysts, transition metal chalcogenides with spinel and pyrite structures have been investigated and shown that these can also be active to oxygen reduction processes. The motivation in the present case is that chalcogen addition might enhance the stability and activity toward the ORR... 

Figure 1.96. Log /oj-pH diagram constructed for temperature = 200°C, ionic strength = 1, ES = 10 m, and EC = 10 m. Solid line represents aqueous sulfur and carbon species boundaries which are loci of equal molalities. Dashed lines represent the stability boundaries for some minerals. Ad adularia. Bn bomite, Cp chalcopyrite, Ht hematite, Ka kaolinite, Mt magnetite, Po pyrrhotite, Py pyrite, Se sericite. Heavy dashed lines (1), (2), and (3) are iso-activity lines for ZnCOs component in carbonate in equilibrium with sphalerite (1) 4 co3=0-1- (2) 4 ,co3=0-01- (3) 4 co3 =0-001 (Shikazono, 1977b).

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

This equation represents the overall reaction which must occur in several steps. The mineral must produce some ferrous ion in solution which reacts with CFT to form Fe(CN)5 . Using measured oxidation potential and pyrite solubility values, Eligillani and Fuerstenau (1968)delineated the stability domains of the compound... 

It can be seen from Fig. 8.5 that pyrite still exhibits the cathodic characteristic when sphalerite is used as the opposite electrode at static state. The corrosion potential of the pyrite electrode decreases at the beginning and is finally stabilized at about 140 mV. The pyrite electrode has not exhibited obvious cathode current. When sphalerite is used as the grinding media as seen from Fig. 8.6, the potential of pyrite electrode decreases with the increase of the mechanical pressure exerted on it and the grinding time. Pyrite exhibits cathodic characteristic, but the degree of cathode polarization is less than that in Fe grinding media. Corrosion potential of the pyrite electrode reaches to the lowest value about 145 mV at pressure of 800 g and 8 min. 

The most important FeSa surface is the (100) surface, which is the most common growth surface and is also the perfect cleavage surface. Research from Nesbitt et al. (1998) suggest that the (100) surface of pyrite exhibits good stability and only minimal relaxation fi om the truncated solid. Therefore, our adsorption calculation is based on FeSa (100) surface and the relaxation of surface is ignored. 

Adding sulfur to the system (molality of solutes = 10 figure 8.22C), a wide stability field opens for pyrite FeS2 in reducing conditions and, almost at the lower stability limit of water, a limited field of pyrrhotite FeS is observed. 

Stability of iron sulfides in lake sediments has not been thoroughly examined. Pyrite undergoes dynamic seasonal oxidation in salt marshes and coastal marine sediments (142, 184, 191-195). Pyrite oxidation is mediated... 

Fe(III) can oxidize arsenopyrite about 10 times faster than pyrite and the rates are even more rapid if Acidithiobacillus ferrooxidans is present (Welch et al., 2000, 597 Gleisner and Herbert, 2002, 140 Evangelou, Seta and Holt, 1998, 2084). Scorodite, a product of Reaction 3.52, may form colloids in water and natural organic matter (NOM) could assist in stabilizing the colloids (Buschmann et al., 2006, 6019). 

It has been shown by mineralogical, chemical and X-ray-diffraction analyses that the major part of reduced sulfur occurs in the form of pyrite in ancient sediments (Lein, 1978)81). It has been also established that pyrite may form rapidly in muds of recent sediments. In anoxic bottom waters, pyrite formation can take place before and after burial even during sedimentation (Berner, 1984)89). Also the geological occurrence and chemical stability relations indicate that authigenic pyrites can be synsedimentary or diagenetic (Kalliokosky, 1966)90). 

Nickel, E. H. (1968a) Structural stability of of minerals with the pyrite, marcasite, arsenopyrite and lollingite structures. Canad. Mineral., 9, 311—21. 

Chalcocite forms a limited binary solid solution series (Cu2 S) extending into the ternary diagram beyond a composition of bornite. The central portion of the system is dominated by the biconvex chalcopyrite stability field. Pyrrhotite displays binary as well as ternary solid solubility, in both cases >4 wt.%, whereas pyrite has... 

Foley and Ayuso (2008) suggest that typical processes that could explain the release of arsenic from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulfides, and these in general rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of arsenic-bearing iron—cobalt— nickel-sulfide minerals, dissolution (by reduction) of arsenic-bearing secondary arsenic and iron hydroxide and sulfate minerals, carbonation and/or oxidation of As-sulfide minerals, and desorption of arsenic from Fe-hydroxide mineral surfaces are all thought to be implicated. All of these processes contribute to the occurrence of arsenic in groundwaters in coastal Maine, as a result of the variability in composition and overlap in stability of the arsenic source minerals. Also, Lipfert et al. (2007) concluded that as sea level rose, environmental conditions favored reduction of bedrock minerals, and that under the current anaerobic conditions in the bedrock, bacteria reduction of the Fe-and Mn-oxyhydroxides are implicated with arsenic releases. 

This is, even to a greater extent, true of oxide glasses with the coordination number Z > 6. It is obvious that with another increase in pressure to P 1 Mbar (the stability region of an a-PbC>2 structure type), the coordination number in glassy silica should slightly rise [98] and approach eight at P 2-3 Mbar (the stability region of a pyrite-like structure of crystalline silica). This state of the... 

Pyrite-Pyrrhotite Stability Field. The first graphical representation (Figure l) is of a pyrite-pyrrhotite stability field, in the form log fugacity of sulfur (log fc ) vs. temperature, T. This diagram derives... 

Phase Diagrams. The phase diagrams shown in Figures 2A-2E display the stability regions of iron, the iron oxides, pyrite and the pyrrhotites, and will assist in interpreting the significance of the form of pyrrhotite observed in coal liquefaction experiments. 

Figure 1. Pyrite-pyrrhotite stability field Py = pyrite and Po = pyrrhotite.

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