Relationship with iron sulfides

The Gibbs phase rule is the basis for organizing the models. In general, the number of independent variables (degrees of freedom) is equal to the number of variables minus the number of independent relationships. For each unique phase equilibria, we may write one independent relationship. In addition to this (with no other special stipulations), we may write one additional independent relationship to maintain electroneutrality. Table I summarizes the chemical constituents considered as variables in this study and by means of chemical reactions depicts independent relationships. (Throughout the paper, activity coefficients are calculated by the Debye-Hiickel relationship). Since there are no data available on pressure dependence, pressure is considered a constant at 1 atm. Sulfate and chloride are not considered variables because little specific data concerning their equilibria are available. Sulfate may be involved in a redox reaction with iron sulfides (e.g., hydrotroilite), and/or it may be in equilibrium with barite (BaS04) or some solid solution combinations. Chloride may reach no simple chemical equilibrium with respect to a phase. Therefore, these two ions are considered only to the... 

Stannite is the most common tin sulfide mineral in the ore deposits associated with tin mineralization. This mineral sometimes contains appreciable amounts of zinc, together with iron. Several workers have suggested that the zinc and iron contents of stannite are related to temperature. With respect to the study of the phase relationships in the pseudobinary stannite-kesterite system. Springer (1972) proposed zincic stannite as a possible geothermometer mainly based on the chemical compositions of the two exsolved phases (stannite and kesterite). Nekrasov et al. (1979) and Nakamura and Shima (1982) experimentally determined the temperature dependency of iron and zinc partitioning between stannite and sphalerite. 

If iron limits retention of S in sediments (cf. 50, 30) it would be expected that the fraction of S present as iron sulfides would increase with increasing Fe content of sediments. Although this relationship is observed in deep sediments (Figure 7), fractionation of S between organic and inorganic forms is not determined by iron content in surface sediments. Nor is there any relationship between Fe content and total S content in surface sediments for all lakes reported in the literature (Figure lc). In deep sediments where C S ratios indicate that seston was the major source of sedimentary sulfur... 

Iron monosulfides show an antithetic relationship with pyritic sulfur (Table I). The highest amounts of acid volatile sulfides (which remained small compared to S in pyrite) occur directly beneath the marsh sediments within the upper portions of the tidal flat deposits. The high value for iron monosulfides in the upper part of the tidal creek sediments may be related to high rates of sulfate reduction at these depths. 

The relationships between the silicate and sulfide sediments are also determined mainly by the content of sulfur in the water, and pH and Eh. For given sulfur activities the sulfide field is curtailed due to formation of silicates only in highly alkaline environments (pH >11). Deposition of silicates in environments close to neutral occurs in the stabihty field of magnetite. Magnetite is not formed in primary sediments in the presence of active forms of Si02. As the sulfur content in the waters decreases, the boundaries between the iron sulfide and iron sihcate fields shift toward neutral environments. In waters with pH = 8 pyrrhotite begins to be replaced by silicates at < 10and pyrite at Og 10 g-ion/1. 

The relationships between iron carbonate and sulfide minerals formed in the course of diagenesis are determined mainly by the concentrations of active forms of sulfur in the waters. The existence of a very narrow wedge of siderite field between the fields of pyrite and goethite in the region of sulfate-ion stabihty indicates that siderite can form in association with pyrite and goethite in environments with Eh values close to zero. 

Walter and co-workers (Walter and Burton, 1990 Walter et al., 1993 Ku et al., 1999) have made extensive efforts to demonstrate the importance of dissolution of calcium carbonate in shallow-water carbonate sediments. Up to — 50% carbonate dissolution can be driven by the sulfate reduction-sulfide oxidation process. In calcium carbonate-rich sediments there is often a lack of reactive iron to produce iron sulfide minerals. The sulfide that is produced by sulfate reduction can only be buried in dissolved form in pore waters, oxidized, or can diffuse out of the sediments. In most carbonate-rich sediments the oxidative process strongly dominates the fate of sulfide. Figure 6 (Walter et al., 1993) shows the strong relationship that generally occurs in the carbonate muds of Florida Bay between total carbon dioxide, excess dissolved calcium (calcium at a concentration above that predicted from salinity), and the amount of sulfate that has been reduced. It is noteworthy that the burrowed banks show much more extensive increase in calcium than the other mud banks. This is in good agreement with the observations of Aller and Rude (1988) that in Long Island Sound siliciclas-tic sediments an increased bioturbation leads to increased sulfide oxidation and carbonate dissolution. 

This mechanism of oxidative attack has two facets, namely the regeneration of ferric ions by the organism and the chemical interaction of ferric ions with the sulfide mineral. Singer and Stumm (1970) have shown that the rate of oxidation of ferrous iron by oxygen in abiotic systems is a function of pH. At pH values greater than 4.5, the kinetic relationship is described by eqn. (13) ... 

In this paper, we present data for total organic carbon (TOC), sulfur and iron contents in sediment samples from piston and box cores taken from the shelf and slope off southwestern Taiwan. We also present data of acid soluble iron, sulfidic iron and porewater sulfate in selected samples. The goal of this study is to illuminate the unusual inter-relationships among carbon, sulfur and iron in the rapidly accumulating sediments off southwestern Taiwan, which may serve as a typical example of sedimentation environments around high-standing islands with rapid denudation rates. 

The first consideration was the speciation and distribution of the metal in the sediment and water. Benthic organisms are exposed to surface water, pore water and sediment via the epidermis and/or the alimentary tract. Common binding sites for the metals in the sediment are iron and manganese oxides, clays, silica often with a coating of organic carbon that usually accounts for ca. 2% w/w. In a reducing environment contaminant metals will be precipitated as their sulfides. There is not necessarily a direct relationship between bioavailability and bioaccumulation, as digestion affects the availability and transport of the metals in animals, in ways that differ from those in plants. 

The relationship of the stirring rate in these experiments to the rates of hydrolysis reactions of basalt phases is indicative of surface-reaction controlled dissolution (21). First order kinetics are not inconsistent with certain rate-determining surface processes (22). Approximate first order kinetics with respect to dissolved oxygen concentration have been reported for the oxidation of aqueous ferrous iron (23) and sulfide (24), and in oxygen consumption studies with roll-type uranium deposits(25). 

Fig. 9. Sketch of facies relationships of the paragenetic associations of rocks of the leptite-porphyry cherty iron-formation (after Chernov et al.) / = plagioporphyries. halleflintas, volcanic breccias 2 = quartz-biotite tuffogenic schists with intercalations of amphibole-gamet and biotite-garnet schists 3 = graphitic quartz-biotite schists rich in sulfides 4 = iron cherts. Numerals on map—rock associations /= porphyry -iron chert //= tuff-porphyry paragenetic [11= tuff aluminous-iron chert paragenetic [V= tuff schist-iron chert paragenetic.

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