The Formation of Siderite

In the marine environment siderite (FeCOj) is hardly found relative to iron sulfides because it is thermodynamically not stable in the presence of even low dissolved sulfide activities. Postma 

In fully marine systems siderite formation is probable to occur below the sulfate reduction zone where dissolved sulfide is absent, if reactive iron is still present and the Fe/Ca-ratio of pore water is high enough to stabilize siderite over calcite (Berner 1971). The coexistence of siderite and pyrite in anoxic marine sediments was shown by Ellwood et al. (1988) and Haese et al. (1997). Both studies attribute this observation to the presence of microenvironments resulting in different characteristic early diagenetic reactions next to each other within the same sediment depth. It appears that in one microenvironment sulfate reduction and the formation of pyrite is predominant, whereas at another site dissimilatory iron reduction and local supersaturation with respect to siderite occurs. Similarly, the importance of microenvironments has been pointed out for various other processes (Jorgensen 1977 Bell et al. 1987 Canfield 1989 Gingele 1992). 

Apart from microenvironments, an explanation for the concurrent dissimilatory sulfate and iron reduction was provided by Postma and Jakobsen (1996). They demonstrated that the stabilities of iron oxides are decisive with respect to iron and/or sulfate reduction assuming that the fermentative step and not the overall energy yield is overall rate limiting. Additionally, it shall be noted that the typical sulfate reducing bacteria Desulfovibrio desulfuricans was found to reduce iron oxide enzymatically contemporarily or optionally (Coleman et al. 1993). When only very small concentrations of as sole electron donor were available iron oxide instead of sulfate was used as electron acceptor by D. desulfuricans. 

---

The lower limit of partial pressure of COj at which the formation of siderite in oxide-carbonate sediments is possible also is limited ... 

The (microbial) dissimilatoiy iron reduction was shown in the previous section. In this section the reactions with major oxidants and reductants will be introduced. Additionally, the interactions between iron and phosphorus, as well as the formation of siderite and iron-bearing sheet silicates will be pointed out briefly to show to the variety of reactions in marine sediments coupled the reactivity of iron. 

In the wetlands of Idaho, the formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots (Typha latifolia, cat tail and Phalaris arundinacea, reed canary grass) may provide a means of attenuation and external exclusion of metals and trace elements (Hansel et al, 2002). Iron oxides were predominantly ferrihydrite with lesser amounts of goethite and minor levels of siderite and lepidocrocite. Both spatial and temporal correlations between As and Fe on the root surfaces were observed and arsenic existed as arsenate-iron hydroxide complexes (82%). 

The effect of the partial pressure of oxygen and of CO2 is also very important for the decomposition behaviour of siderite and rhodochrosite. The formation of the iron oxides was followed by TG and by high temperature X-ray diffraction. Below 10-6 mm Hg oxygen pressure only Fe304 was formed. 

Figure 10. Comparison of isotopic fractionations determined between Fe(II)aq and Fe carbonates relative to mole fraction of Fe from predictions based on spectroscopic data (Polyakov and Mineev 2000 Schauble et al. 2001), natural samples (Johnson et al. 2003), DIR (Johnson et al. 2004a), and abiotic formation of siderite under equilibrium conditions (Wiesli et al. 2004). Fe(II)aq exists as the hexaquo complex in the study of Wiesli et al. (2004) hexaquo Fe(II) is assumed for the other studies. Total cations normalized to unity, so that end-member siderite is plotted at Xpe = 1.0. Error bars shown reflect reported uncertainties analytical errors for data reported by Johnson et al. (2004a) and Wiesli et al. (2004) are smaller than the size of the symbol. Fractionations measured on bulk carbonate produced by DIR are interpreted to reflect kinetic isotope fractionations, whereas those estimated from partial dissolutions are interpreted to lie closer to those of equilibrium values because they reflect the outer layers of the crystals. Also shown are data for a Ca-bearing DIR experiment, where the bulk solid has a composition of q)proximately Cao.i5Feo.85C03, high-Ca and low-Ca refer to the range measured during partial dissolution studies (Johnson et al. 2004a). Adapted from Johnson et al. (2004a).

Postma D (1981) Formation of siderite and vivianite and the pore-water composition of a recent bog sediment in Denmark. Chem Geol 31 225-244... 

The siderite model of the formation of corrosion-resistant scales. Am. Waterworks Assoc. J. 73 572-579... 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

The reductive dissolution of solid compounds in anaerobic soils, sediments, and waters begins with the reduction of prominent cations within the compounds. Many Fe(III) (oxy)(hydr)oxide compounds are especially susceptible to reductive dissolution. The reduction process converts Fe(III) into more water-soluble Fe(II). The formation of Fe(II) causes the (oxy)(hydr)oxides to decompose in water. In some cases, the Fe(II) rapidly precipitates as new solid compounds, such as siderite (FeCCT) or magnetite (FesCL). 

Taking into account the established relationship of the values of pH, Fcoj and Opg in the water, it must be presumed that in present conditions formation of siderite is practically possible only in a narrow range of nearly neutral environments. From Fig. 39 it follows that the redox potential of such environments would have negative values. 

Formation of siderite in the system Fe-H20-C02 does not change the relationships between oxide and hydroxide sediments. 

The formation of carbonates in the system is limited mainly by the presence of active forms of Si02 and by the concentration of S. On the composite diagram (Fig. 40) the siderite field is represented by a sector of small area in environments with a pH of 5.5 to 6.2 and Eh from +0.05 to — 0.05 V, even at high partial pressure of carbon dioxide (1.0-0.01 bar) with other parameters siderite is replaced by Fe(OH)3 and iron silicates or sulfides. 

Transport of iron in carbonate waters, mainly in the form of Fe " bicarbonate, is more common. The decrease in COj due to the overall reduction in pressure when ground waters come to the surface, when carbon dioxide is consumed as a result of photosynthetic activity of plants or even, as Mokiyevskaya (1959) mentions, when the temperature rises, leads to deposition of FeCOj. In Strakhov s opinion such a process could lead to the formation of oolitic hydrogoethite-chamosite-siderite ores. The iron migrated in mobile form as Fe, which accumulated in solution in a reducing environment. Formation of the ores was related to the draining of high-iron waters formed in swampy regions. The near-shore parts of the sea with... 

The formation of lepidocrocite is possible only in the case of slow oxidation, in particular in solutions with a high carbon dioxide content, in the absence of iron bacteria and significant amounts of free silica. Such conditions are very common in the weathering of siderite. 

Fig. 64. Scheme of vertical zoning and biogeochemical cycle of iron in the formation of iron-rich sediments (pH = 6 siderite, greenalite, and hydrogoethite are diagenetic, magnetite crystalline). 

There are enough petrographic observations confirming the reality of formation of magnetite at the expense of siderite by one of the schemes given (La Berge, 1964, 1973 Klein, 1973 French, 1973). 

Both reactions were accomplished experimentally. It actually turned out (Mel nik, 1966a, c) that in a stream of water vapor the dissociation of siderite to magnetite with liberation of carbon dioxide and hydrogen takes place at a lower temperature (by 60-80°C) than in a stream of nitrogen with liberation of CO2 and CO. The experimental works of other authors (Baykov and Tumarev, 1937 Berg and Buzdov, 1961) have shown that in a neutral atmosphere or in the presence of CO2, siderite dissociates according to reaction (4.18) with the formation of CO. 

In the stability field of this association the formation of grunerite depends on the ratio of CO2 and H2O in the fluid, rather than on temperature. For instance, at T = 427°C and P — 5000 bar the siderite + grunerite association... 

A change in this ratio as a result of introduction of water or COj leads to the formation of grunerite or siderite, respectively. Thus the main reason for the formation of grunerite is the presence in the rocks being metamorphosed of an amount of water sufficient to maintain a high Ph o Ph o- coj ratio when water is consumed in reaction (4.36). 

Diagenetic magnetite in the schists of the BIF and mixed chemogenic-clastic rocks, in part siderite in schists sulfides in schists and carbonate-silicate BIF. The formation of diagenetic ore minerals was accompanied by reduction of iron and the appearance of silicates such as greenalite or even of... 

---