Pyrite forms

The pyrite formed in the above reaction is well known as fool s gold , and was certainly present on the young Earth. 

Established high PSC of iron on beryllium deposits are connected with pyrite form of iron in fluorite-beryllium ores containing to 50-70 % of fluorite. On this beryllium... 

The mineral marcasite constitutes yellow rhombic crystals density 4.87g/cm3 transforms to more stable pyrite form when heated at 480°C insoluble in dilute acids. Both forms dissolve in concentrated nitric acid and are insoluble in water (4.9 mg/L at 20°C). 

MINEEALS OF TIN.—Tin is found almost invariably in the form of biDoxido of tin or tin-stone, and in some instances associated with iron and copper pyrites, forming what is called tin pyrites or bell-metal ore, but the quantity so existing is very small. 

Organic sulfur is the dominant form in peats described in these studies. Pyrite, however, is abundant in brackish and marine peats, occurring in void spaces in or between plant debris (3). In a study of pyrite formation in freshwater peats, Altschuler et al. (5) determined parallel decline in ester sulfate with increases in pyrite as depth increased and concluded that pyrite formed at the expense of organic sulfur. In general, framboidal morphology is present at all salinities. Altschuler et al. (5) and Lowe and Bustin (10) found monosulfides to be minor in peats. 

Pyrite Morphology. Pyrite formed during early diagenesis in marsh sediments nas essentially two modes of occurrence, as small single crystals and as framboids (18). The processes that lead to one form or the other remain unclear. Euhedra may form by direct... 

Previous studies (4-6) have illustrated the difficulties in relating the isotopic composition of the organic and pyritic forms of sulfur in coals. This is particularly so where large additions of secondary pyrite further distort the frail relationship, if... 

Starting Materials. Pyrite isolated from the two kerogens constitutes approximately 2 wt % and has S S vaiues Qf +8.7 per mil. These values are enriched by 20-30 per mil compared to pyrite formed from near-surface biogenically reduced sulfate in porewaters of "normal" near-surface marine sediments (5). Bitumens extracted from the two samples contain 8.0 and 8.7 wt % organic sulfur with 5 4 vaiues 0f + 17.3 and +18.5 per mil respectively. Both pyrite free kerogens contained 10.3 wt % organic sulfur with isotope values of +15.9 per mil and +17.0 per mil respectively. 

The existence of low-spin Fe2+ in pyrite formed the basis of an early estimate that spin-pairing of Fe2+ would occur in olivine at a depth in the Mantle of... 

Ainsworth, C. C. 1979. Pyrite forms and oxidation rates in Missouri shales. M.S. Thesis, University of Missouri,... 

Fig. 6.21. Calculated dispersion of energy bands along some principal symmetry directions in (a) marcasite and (b) pyrite forms of FeS, (from the band-structure calculations of Bullett, 1982 reproduced with the publisher s permission).

Three variables are of potential importance in controlling the total amount of pyrite formed. These are the availability of sulfate, iron, and organic matter. The effect of the hrst is relatively minor in marine sediments, whereas last two are more critical. 

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. 

Habicht and Canheld (2001) provide evidence that the pyrite forming in marine sediments is inhuenced by such a pathway by simultaneously determining the isotopic composition of H2S produced by SRB and that of iron-sulhde minerals coexisting in the same sediments. Their measured bacterial fractionations could only explain 41-85% of the observed depletion in iron sulhdes from a range of marine environments. They calculated a closed-system model showing that microbial-disproportionation fractionations involving partially oxidized sulfur species, as documented in the laboratory, could explain their data. Their calculations assumed that all H2S initially produced by SRB was... 

Conventional mining is both of historical interest and is still used to produce nearly a quarter of the world s sulfur (Table 9.2). Mining brings to the surface lumps of either a volcanic or one of the many pyritic forms of sulfur. Some of the pyritic forms are pyrite (FeSi) itself, chalcocite (CuiS), and chalcopyrite (CuFeSi). The sulfur content of the raw mineral is usually 25-35%, but may run as high as 50%. To obtain the sulfur in a separated form, the original procedure was to pile the lumps of ore outside and seal these with clay or earth. Burning a part of the contained sulfur sealed into these mounds, with careful control of the air generated sufficient heat to melt any elemental sulfur present, and thermally decompose the pyrite (Eqs. 9.1-9.3) [10]. 

The sulphite (SO2-) is subsequently oxidized to SOI-. Sedimentary pyrite, formed as a byproduct of sulphate reduction in marine sediments, is a major sink for seawater SO -. The presence of pyrite in ancient marine sediments shows that SO4- reduction has occurred for hundreds of millions of years. On a geological timescale, removal of SO4- from seawater by sedimentary pyrite formation is thought to be about equal to that removed by evaporite deposition (Section 6.4.2). Compilations of pyrite abundance and accumulation rates are used to calculate modern SO - removal by this mechanism and to derive the estimate in Table 6.2. 

The discussion thus far has assumed a syngenetic origin for the pyrite. Observations of sulfide morphology suggest that at least some of the sulfide may be epigenetic. Edwards and Baker (32) showed that pyrite forms in marine environments whereas marcasite forms under more acidic conditions. Recent experimental work has shown that pyrite forms at a pH of 5.0, whereas marcasite forms at a pH of... 

The distributions of minerals within the Lower Kittanning seam can be related to depositional environment. Pyrite content is highest in areas which may have experienced brackish conditions. This distribution reflects the availability of iron and sulfur, and pH conditions within the swamp. Whereas much of the pyrite formed syngenetically, observations of sulfide modes of occurrence suggest that marcasite formed epigenetically. 

Os(Te2) has been prepared by heating the elements to ca. 923 K. Up to 713 K the reaction is slow but above that temperature a marcasite phase is formed. The phase change to the pyrite form above 753 K is thought to occur via formation of gaseous Os and Te2. 

Pyrite occurs both as early and late cement in Hibernia Field. Early pyrite is present in early calcite cement, fossil fragments and siderite nodules. Late pyrite is found in intergranular pores and fractures. The association of pyrite with a porous zone in Hibernia Sandstone and its absence in adjacent ferroan-calcite cemented sandstone suggests that this pyrite formed after the dissolution of the ferroan calcite. 

The discrepancy between the relative rates of sulfate reduction, FeS inventories, and the resulting quantity of pyrite formed at each station may be due to several processes acting alone or in concert (1) the rate of conversion of FeS to FeSj may differ, although this was largely discounted previously (2) FeS may be reoxidized prior to pyrite formation or (3) FeSz may be oxidized after formation. 

Iron sulfides represent the most important minerals that form in association with both organoclastic and methanotrophic snlfate reduction, or - more precisely - as a resnlt of the hydrogen snlfide prodnced by these processes. The different pathways of pyrite formation via intermediate iron snlfides will be described in more detail in Section 8.4.2. The first step in all pyrite forming seqnences involves a reaction of hydrogen snlfide with either dissolved Fe " or solid-state iron (oxyhydr)oxides. The reactivity of oxidized iron minerals towards snlfide varies... 

All pyrite forming pathways identified so far involve several reaction steps. First, hydrogen sulfide, produced during sulfate reduction (Eq. 

As reviewed in detail by Schoonen (2004) these different conversion mechanisms - and in particular the H,S pathway - have received controversial discussion. However, field studies have shown that hydrogen sulfide can indeed sulfidize amorphous FeS and form pyrite. Rickard (1997) found that the H,S process is by far the most rapid of the pyrite-forming reactions hitherto identified and suggested that it represents the dominant pyrite forming pathway in strictly anoxic systems. In addition, Morse (2002) discussed that the oxidation of FeS by hydrogen sulfide is the faster process compared with the oxidation by elemental sulfur. Berner (1970) suggested that, in the presence of zero-valent sulfur, a complete transformation of FeS to pyrite should be possible on a time scale of years. An incomplete conversion of FeS to pyrite, as often observed, e g. in... 

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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