Hematite stability fields

Figure 5.51 fo2 stability field of biotite. (A) Pseudobinary phlogopite-annite mixture. Numbers at experimental points indicate observed Fe/(Fe + Mg) atom ratio. (B) Annite. HM = hematite-magnetite buffer NNO = Ni-NiO buffer MW = magnetite-wuestite buffer QFM = quartz-fayalite-magnetite buffer. From Wones and Eugster (1965). Reprinted with permission of The Mineralogical Society of America. 

Figure 8.22A shows the Eh-pH diagram of iron in the Fe-O-H system at T = 25 °C and P = 1 bar. The diagram is relatively simple the limits of predominance are drawn for a solute total molality of 10 . Within the stability field of water, iron is present in the valence states 1+ and 3-I-. In figure 8.22A, it is assumed that the condensed forms are simply hematite Fe203 and magnetite Fe304. Actually, in the 3-1- valence state, metastable ferric hydroxide Fe(OH)3 and metastable goe-thite FeOOH may also form, and, in the 1+ valence state, ferrous hydroxide Fe(OH)2 may form. It is also assumed that the trivalent solute ion is simply Fe ", whereas, in fact, various aqueous ferric complexes may nucleate [i.e., Fe(OH), Fe(OH)2+, etc.]. 

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. 

Robins (1967) plotted the stability domains of these two oxides as a function of temperature, pH and [Fe ]. Hematite predominated over the pH range 0-3 at temperatures above 150-200 °C the stability field of hematite widened as [Fe ] increased. 

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 7.25 Log/02 vs pH diagram for sulphur species showing the stability fields of pyrite, pyrrhotite, magnetite and hematite. The boundaries are for ji = 1.0, and molalides for totaJ sulphur = 0,01, JC " 0.1, Na" = 0.9, Ca " — 0.01. The contours show deviations of 6 ShjS from at 250 C under equilibrium conditions (after Ohmoto and Rye, 1979).

Bur] Burdese, A., Lucco Borlera, M., Caleium Ferrite-Magnetite-Hematite System. Field of Stability, Composition and Most Probable Strueture of the Phase Ineorreetly Indieated with Ca0 2Fe203 Formula (in Italian), Metall. Ital, 52(11), 710-715 (1960) (Crys. Strueture, Experimental, Phase Diagram, Phase Relations, 24)... 

Area three lies within the sericite and chlorite stability fields, so either one or both of these minerals will be associated with uranium oxide in deposits formed under these conditions of/02 and pH. The iron mineral associated with this assemblage would be hematite. Similarly, in area four adularia is the stable potassium silicate, so the assemblages associated with uranium in this area may be adularia-hematite or adularia-chlorite-hematite. 

Figure 1 illustrates that in reduced aquifers uranium would not be mobile. It also shows that uranium can be precipitated well into the field of stability of hematite under oxidizing conditions. This common association between uranium mineralization and hematite needs to be taken into account in any exploration program. 

Laberty and Navrotsky (1998) determined the enthalpies of formation of a number of iron oxide and oxyhydroxide polymorphs. Data are listed in Table 2 which also compares the enthalpy relations among aluminum, iron, and manganese. It is evident that the Fe oxyhydroxide phases are much less stable relative to the anhydrous ferric phase (hematite) than are the aluminum oxyhydroxides relative to corundum. This is consistent with the much more frequent observation of hematite than of corundum in the field. It is also evident that the iron phases are as rich in polymorphism as the aluminum phases. It is clear that the enthalpy differences for both anhydrous (AI2O3, Fe203, Mn02) and hydrous (AlOOH, FeOOH, MnOOH) polymorphs are small, setting the stage for nanoscale stability crossovers. 

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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