Siderite stability field

Figure 7 compares predictions for 2.75 Ga by two families of models to the local temperature and PCO2 limits obtained by Rye et al. (1995). The region below the siderite stability field is... 

In the case of equality of concentration of sulfide and sulfate ions the equilibrium values of partial pressure of oxygen are 10 and bar, respectively. These values fall in the stability field of siderite and ferrous-iron silicate (see Fig. 22). In the stability field of siderite plus goethite or hematite, the sulfate ion already predominates. Thus sedimentary sulfides in ancient rocks (Archean, or Azoic) evidently contain juvenile sulfur, introduced in the form of hydrogen sulfide. The sulfides in iron-formations could have been formed both by way of chemogenic deposition, and by way of diagenetic reduction of sulfates. In the presence of free oxygen only sulfate ions are stable. 

Before the beginning of regional metamorphism (T< 250-280°C) only greenalite or minnesotaite and siderite, the stability fields of which are determined by the reaction ... 

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... 

When the diagrams are examined, the broad field of siderite stability, which increases sharply as m and total pressure (P ) increase, is striking. Metamorphism of the siderite-quartz association can go in two directions,... 

As we increase the thermodynamic stability (i.e., the pA p) of the ferric oxyhydroxide considered in our Eh-pH diagram, the size of its stability field increases. This is evident from Fig. 12.12, which shows the very large stability field of goethite (pK = 44.2) relative to that of the amorphous phase (p/Tsp = 37.1). The field of siderite practically disappears in equilibrium with goethite, suggesting that siderite and well-crystallized goethite (or hematite with a similar stability) should rarely be found together. 

HS . The lack of a stability field for siderite reflects the low carbonate and high sulfur concentrations chosen for the diagram, and the large sizes of both pyrite and goethite fields are consistent with the remarkable insolubilities of both minerals over a wide pH range. 

Figure 12.24 Eh-pH diagram showing the stability field of Fe(OH)i(s) assuming = 38.5, and siderite, FeCOj.at 10°C. Mineral-solution boundaries are drawn for a dissolved IFe activity of 10 mol/kg (100 mg/kg as Fe), and a total alkalinity of 10 " mol/kg (24 mg/kg as HCOj). Field-measured Eh and pH values for 16 streams and 19 groundwaters from coal strip-mined areas of northwestern Pennsylvania are circled. From M.A. Gang and D. Langmuir (1974). Copyright 1974 by the National Coal Association. Used by permission.

Ferroan carbonate formed during mesodiagenesis may be siderite rather than ankerite if the formation waters have a sufficiently high Fe/Ca activity ratio. For instance, formation waters in Triassic reservoir sandstones in southern Tunisia (T x 80°C) have log (ape +/aca-+) values between -3 and -2.5, and thus fall within the stability field of siderite, which agrees well with the petrographic observations of Morad et al. (1994). Probably due to kinetic reasons, siderite... 

Natural chlorites may contain significant amounts of iron, and their stability will therefore depend on both pH and /02. The lack of thermodynamic data on chlorites of variable composition prevents the calculation of a stability field for these minerals. Qualitative relationships suggest, however, that this field would superimpose over the siderite field and be slightly larger. This would further restrict the conditions of formation of deposits with iron-containing chlorite. 

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