Iron pyrites metastable

Metastable Iron Sulfides Organic Sulfur Elemental Sulfur MECHANISM OF PYRITE FORMATION. 4.1 Evidence from Experimental Studies. 4.2 Isotope Effects during Experimental Pyrite Formation. 4.3 Origin of Morphological Variations in Pyrite SULFUR DIAGENETIC PROCESSES IN MARINE SEDIMENTS. 5.1 Depth Distribution of Diagenetic Sulfur Products. 5.2 Rates of Sulfate Reduction... 

In addition to pyrite, there are two additional iron-sulfide minerals commonly found in recent sediments. These phases have been termed metastable iron sulfides, because they are thermodynamically unstable with respect to transformation to pyrite, or to a mixture of pyrite plus pyrrhotite (Berner, 1967). They are also known as acid-volatile iron sulfides, because in contrast to pyrite they are soluble in nonoxidizing mineral acids such as HCl. The dominant acid-volatile sulfides found in both natural and experimental systems are mackinawite and greigite. 

Bioturbation and other physical processes associated with the upper portions of marine sediments may lead to rapid exchange between pore-water and overlying depositional water. Depending on the intensity of bioturbation, sulfate in depth zones 1 and 11 and the uppermost part of zone 111 (Figure 4) may be effectively in contact with an infinite reservoir of seawater sulfate. When this is the case, pore-water SO will have a nearly constant 8 value with depth regardless of the withdrawal of isotopically light sulfur to form H2S. The initial isotopic composition of H2S produced by SRB in zones 1 and 11 will be equal to the instantaneous isotopic separation between seawater sulfate and bacterial sulfide (i.e., up to about Aso -HjS = 45%o). Metastable iron sulfides and pyrite formed from this H2S will have an isotopic composition very close to this initial H2S because of the small fractionation observed during sulfidization of iron minerals. 

There is a metastable phase of FeS2, marcasite, which is the orthorhombic dimorph of pyrite and appears also in several coals. Marcasite has slightly different IS and QS (Table I). When the amount of marcasite in coal is more than 20% of the total iron disulfide content, its detection using Mossbauer spectroscopy is possible. In general, petrographical techniques... 

At the surface of some marine sediments, organic sulphur can comprise as much as 50% of the total sulphur present (Francois, 1987) due to biosynthesis which incorporates sulphur of all oxidation states, but also, because of the reactivity of sulphides and polysulphides, by chemical addition. There is usually an increasing S/C ratio with depth in sediments, partly associated with humic substances, and most of this increase occurs in the oxic and suboxic zones. This organic repository may be the source of the sulphur required to convert metastable iron sulphides, formed in the lower part of the sulphur reduction zone, to framboidal pyrite, which is often found closely associated with organic matter. 

A number of the observed minerals (formulae given in Table 16.4) do not form in the simulation. Wurtzite is metastable with respect to sphalerite, so it cannot be expected to appear in the calculation results. Similarly, the formation of pyrite in the simulation probably precludes the possibility of pyrrhotite precipitating. In the laboratory, and presumably in nature, pyrite forms slowly, allowing less stable iron sulfides to precipitate. Elemental sulfur at the site probably results from incomplete oxidation of H2S(aq), a process not accounted... 

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