Siderite FeCO

Figure 3.7 Siderite (FeCOs) nodules inside a tubercle revealed by breaking open the magnetite shell. (Courtesy of National Association of Corrosion Engineers, Corrosion 89 Paper No. 197 by H. M. Herro.)...

Carbon dioxide was once thought essential for the rusting of ferrous metals (viz. the carbonic acid theory of rusting) but is now considered of relatively minor importance . However, basic zinc carbonate is frequently found in the corrosion products of zinc and small amounts of siderite (FeCOs) are found in ferrous rusts. 

The major cations found in the carbonate minerals in coals are calcium, magnesium, and iron. The rather pure end member calcite (CaC03) is dominant in some coals, whereas siderite (FeCO ) is dominant in others. Calcite and ankerite (a mixed crystal composed of Ca, Mg, and Fe carbonates) are abundant in some coals. 

Jensen D. L., Boddum J. K., Tjell J. C., and Christensen T. H. (2002) The solubihty of rhodochrosite (MnCOs) and siderite (FeCOs) in anaerobic aquatic environments. Appl. Geochem. 17, 503-511. 

Although the Ca-Mg carbonates constitute the most frequently encountered carbonate minerals, other metal carbonates can also be important. Of these, the most abundant are siderite (FeCO ) and rhodocrosite (MnC03). The divalent metal carbonates typically occur as. solid. solutions and so contain trace to major amounts of one or more secondary metal cations (cf. Reeder 1983). Solubilities of some binary metal-carbonate solid solutions are known (cf. Glynn 1990). 

After alnminnm, iron is the most abundant metal in Earth s crust (6.2 percent by mass). It is found in many ores some of the important ones are hematite, Fc203 siderite, FeCOs and magnetite, Fc304 (Figure 22.5). 

Fe Equilibria. In the case of Fe, pore waters at all stations are generally distinctly undersaturated with respect to siderite (FeCOs) although near saturation may occur in a thin band in the upper 5 cm corresponding to the zone where Fe " concentrations are high or, at the FOAM station, below 40 cm (Fig. 15). Because total alkalinity was used, these undersaturations are minima. 

The reaction of atmospheric carbon dioxide with water (the reverse of the second equation) is important in the weathering of rocks, and the formation of large carbonate deposits such as caldte, dolomite (CaMg(C03)2), and siderite (FeCOs). 

Minerals containing the carbonate ion are plentiful. The principal carbonate minerals are caldte (CaCOs), magnesite (MgCOs), dolomite [MgCa(C03)2], and siderite (FeCOs). Calcite is the principal mineral in limestone rock, large deposits of which occur in many parts of the world. It is also the main constituent of marble, chalk, pearls, coral reefs, and the shells of marine animals such as clams and oysters. Although CaCOs has low solubility in pure water, it dissolves readily in acidic solutions with evolution of CO2. 

McCollom, T.M. Formation of meteorite hydrocarbons from thermal decomposition of siderite (FeCOs). Geochim. Cosmochim. Acta 2003, 67(2), 311-317. 

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