Goethite stability fields

Figure 8.2 depicts the stability fields of goethite and hematite as a function of temperature and water pressure using data from several sources. The graph shows clearly that as the temperature increases, the stability field for hematite widens (see also Chap. 14). The goethite stability field broadens as Pnp increases. At PH2O = 0, the equilibrium temperature is 100 °C and rises to 300 °C at PH2O = 2 MPa. 

Fig. 8.2 Stability fields of goethite and hematite as a function of temperature and H2O pressure (Diakonov et al., 1994 with permission).

Fig. 8.4 Stability fields of Al-goethite and Al-hematite as a function of Al substitution.

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. 

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. 

Figure 12.19 Eh-pH diagram for the system Fe-02 S-H20 at 25°C showing stability fields of goethite (a-FeOOH), pyrite (FeS2), and monoclinic pyirhotite (Fe7Sg = Feo,877S) for SS(aq) = 10" mol/kg, and total carbonate lO" mol/kg. ZFe(aq) = 10 and 10 mol/kg at aqueous/ solid boundaries. The diagram shows that aqueous iron occurs chiefly in sulfate complexes. From Barnes and Langmuir (1979).

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 1.11. Sorption isotherm for Cr + reacted with poorly crystalline goethite (Cs = 10 kg m 5), illustrating movement from lower left to upper right (Manceau et al., 1992) in the stability field of Figure 1.10 (Schindler and Stumm, 1987). (After Sposito, 2004.)...

These kinetic differences reflect in part the ligand field stabilization of Cr(III), a ion, compared to Fe(lll), a 3d ion. It is observed that slower kinetics facilitate gel formation in Cr(III) systems where the average gel stoichiometry corresponds to [Cr(OH)3(OH2>3]. /JH2O [76]. Gelatinous precipitates form in Fe(IIl) systems having a composition between a-FeOOH (goethite) and a-Fe203 (hematite) [77,78],... 

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