Ferrous hydroxide precipitated into solution by mixing a ferrous salt comprising anion An− with caustic soda. In most studies, initial conditions were given by a ratio {[Fe2+]/[OH−]}={[FeA(2/n)]/[NaOH]} of the concentration of ferrous salt over that of NaOH most often comprised between 0.2 and 0.4 mol l⁻¹. It is equivalent to R−1 as named in the coprecipitation method where R was defined as {[OH⁻¹]/[Fe_total]} [3,22,23]. Then, the solution was stirred in the presence of air at constant velocity in a beaker with a magnetic rod and two electrodes recorded pH and potential using a calomel electrode as a reference, even though the potential Eh was reported with respect to the standard hydrogen electrode (SHE). Ferrous chloride FeCl₂⋅4 H₂O and melanterite FeSO₄⋅7 H₂O were appropriate for Cl⁻ and SO42−, but it became much less easier for CO32− anions, since carbonate salts were not soluble; Fe(OH)₂ precipitated with ferrous sulphate before adding sodium carbonate to obtain GR1(CO32−) [5]. The same was done for GR1(SO32−) [24], GR2(S2O32−) [17], and GR2(SeO42−) [18]. In presence of an excess of ferrous salt to precipitate Fe(OH)₂, typical curves of Eh and pH vs. time curves display three stages (Fig. 1). A first one (A) ends at the inflection point tg where all ferrous hydroxide is transformed into GR incorporating present anions An−. During a second stage B ending at inflection point tf, the GR oxidises into the usual ferric rusts and disappears completely. A third stage C from tf to t∞ corresponds to the oxidation of Fe^II ions left within the solution. Fig. 1 reports specifically the case of the sulphate medium, which is chosen here since it was the most widely studied system, setting the initial concentrations at R=Rg=(5/3)=1.67 to obtain solely GR2(SO42−) [6]. Samples of transient compounds during oxidation were analysed by Mössbauer spectroscopy at 15 K (Fig. 2) where they were set in a holder inside the cryostat only a few seconds after filtration.

2.2.1 Formation of green rust (Rg=1.67)

Interpretation of the Eh and pH versus time curves is easy within the frame of the mass-balance diagram where R={[OH⁻]/[Fe_total]} is plotted versus x={[Fe^III]/[Fe_total]} (Fig. 3) [22,23]. Initial conditions are shown in the diagram by a point on the co-ordinate axis that corresponds to a mixture of the initial precipitate of Fe(OH)₂ at point G, (R,x)=(0,2), and Fe^II within solution at F, (0,0). During the overall oxidation process, Fe(OH)₂ transforms into FeOOH and reaction paths follow straight lines with a slope of 1 (Fig. 3). Therefore, the straight line that goes directly through the point representing GR2(SO42−) at (0.33,2) has for co-ordinate at origin (0,1.67); Rg=(5/3)=1.67 is the initial value of R to obtain 100% of GR with an excess of Fe^II with respect to Fe(OH)₂ at R=Rh=2. Stage A that starts at point A (t0g) to end at B (tg) is thus the oxidation of Fe(OH)₂ (cf. Fig. 2a) into GR2(SO42−) (cf. Fig. 2d) using this initial excess of Fe^II and anions at R=Rg:

5FeII(OH)2+SO42−+Fe2++(1/2)O2+H2O⇌FeII4FeIII2(OH)12SO4

(1a)

2.2.2 Formation of magnetite (Rh=2)

2.2.3 Trigger value Rc=1.714

2.3.1 Lepidocrocite and goethite for GR1(Cl⁻) and GR2(SO42−) in acidic medium

The determination of the products of oxidation of GRs are of major importance for some technological issues involving corrosion and environmental science as well. It is also a topic by itself for explaining the genesis of minerals. Early works were devoted to the final products of oxidation of Fe(OH)₂ when varying the initial ratio R={[OH⁻]/[Fe_total]}. This is illustrated for the chloride-containing medium where R′={[Cl−]/[OH−]}=2R−1 [16]. Mössbauer spectroscopy was used for analysing the final products (Fig. 4). Magnetite is obtained for R′∼1, i.e. R∼2, then goethite α-FeOOH is maximum for R′∼1.67. Then lepidocrocite γ-FeOOH is overwhelming around R′=3.5, whereas akaganeite β-FeOOH begins to appear for R′=4 and is alone for R′=8. A comparable result is observed for the sulphate containing medium, i.e. magnetite for R∼2, then an increase of goethite that drops suddenly at R=Rc=1.714 where lepidocrocite appears at 75% and decreases slowly along with a new increase of goethite. In both examples, lepidocrocite appears at the end of stage B before it transforms into α-FeOOH according to reaction (1c) in acidic conditions if GRs form as its precursor in the pathway.

2.3.2 Ferrihydrite and goethite for GR1(CO32−) in basic medium

The carbonate-containing medium behaves differently at Rg=5/3; GR1(CO32−) transforms into ferrihydrite, which itself transforms into goethite later on (Fig. 5) [1]. It was demonstrated that the evolution could be stopped by the adsorption of anions such as phosphate onto ferrihydrite. Field observations that ferrihydrite of reddish brown colour forms in waterlogged soils where bluish green fougerite lies are a good indication that the hydroxycarbonate is indeed concerned.

Fig. 6 allows comparing the reaction path in the case of the carbonated medium with those of chloride and sulphate media. The two first stages are similar and correspond to the same reactions: stage A is the oxidation of Fe(OH)₂ into the GR at tg incorporating the afferent anion and stage B the oxidation of GR giving FeOOH (ferrihydrite or lepidocrocite) by releasing Fe^II ions and intercalated anions into solution at tf. However, stage C is quite different, since the solution is now buffered by the released HCO3− ions; CO32− ions within the solid GR become HCO3− ions by consuming protons of the solution and thus the pH increases. This is illustrated by the pH curve that remains basic around 9 (inset of Fig. 6). Consequently, Fe^II ions can only get oxidised into Fe^III ions that immediately precipitate and the path is line CE that reaches 100% of α-goethite. Finally the three stage reactions become:

The fast oxidation of GRs by hydrogen peroxide or an aerial oxidation of previously dried GRs gives rise to the fully deprotonated ‘ferric GR’. For instance, in the case of GR1(Cl⁻), instead of getting the XRD pattern of lepidocrocite or ferrihydrite as for the aerial oxidation in solution, the oxidised product has a pattern quite similar to that initially observed and only lattice parameters are slightly different with blurred and broadened lines [19]; however, its Mössbauer spectrum displays only ferric sextets at 12 K and no ferrous quadrupole doublet at 78 K. Thus, its formula is [FeIII4O8H5]+⋅[Cl−⋅∼2H2O]2−. Moreover, the compound has a bright orange colour [19]. In the case of GR1(CO32−), we obtain also a ‘ferric GR1(CO32−)^∗’, [FeIII6O12H8]2+⋅[CO32−⋅∼3H2O]2−. How to represent this compound in the mass-balance diagram? The initial GR1(CO32−) was represented by point B [1/3,2] and GR1(CO32−)^∗ by point D [1,8/3] (Fig. 7). The oxidation path is consequently line BD of slope 1 that joins these two points. A partial deprotonation of GR can be obtained by adjusting precisely the amount of H₂O₂ to aim at an intermediate value of x in the way between 1/3 and 1. Mössbauer spectra have been shown and any point in line BD (Fig. 7) represents such a GR of formula FeII6(1−x)FeIII6xO12H2(7−3x)CO3 [9].

The same type of phenomenon occurs if oxidising violently Fe(OH)₂ with H₂O₂. A phase named δ-FeOOH forms. Its mineral counterpart is feroxyhyte [4]. In both cases, the original crystal lattice is conserved; each Fe^II(OH)₆ octahedron becomes a Fe^III(OOH)₃ octahedron while the Fe^II gets oxidised. Reactions are quite similar as shown in the mass-balance diagram (Fig. 7). Starting at point G [0,2], Fe(OH)₂ for ferrous hydroxide, one ends at point E [1,3], δ-FeOOH for feroxyhyte, by consuming some OH⁻ and following path GE according to:

For its part, the fast oxidation of GR1(CO32−) that starts at B [1/3,2] and ends at D [1,8/3] with also a consumption of OH⁻ follows accordingly path BD:

Voltammetric curves have been drawn to form GR1(CO32−) and obtain electrode potential data by using an iron foil [11]. The curve is reversible and a hysteresis occurs in situ during oxidation and reduction (Fig. 8a). SEM picture displays hexagonal orange ferric GR1(CO32−)^∗ crystals (Fig. 8b); a continuum {FeII6(1−x)FeIII6xO12H2(7−3x)}2+⋅[CO32−⋅∼3H2O]2− exists where x lies all over the [1/3,1] interval. Using voltammetric curves is common in corrosion science giving valuable information. In this extremely important case – since it deals with the corrosion of iron-made materials and steels –, products and processes can be quite different, due to the various conditions of oxidation that can be encountered. This renders any prediction more difficult.

The second pathway EF is followed during the dissimilatory reduction of FeOOH by bacteria (Fig. 9) [12]. The practical role of DIRB is to reduce ferric species from solid state into Fe²⁺ ions that are dissolved in circumneutral medium by enzymatic activity [20,21]. This produces a mixture of Fe²⁺ and FeOOH and the running point migrates from E to F (Fig. 9).

The precipitate that forms depends on the position at which the mixture of FeOOH and Fe^II is located before a coprecipitation occurs in relation with the pH of the solution through the value of R. From E to C^∗ where x=2/3, only off-stoichiometric magnetite may form following line C′ to B′ (Fig. 9). From C^∗ to C^∗∗, only GR1(CO32−)^∗ forms by coprecipitation (C″B″) in circumneutral conditions. The domain of biogenerated hydroxycarbonate corresponds to [1/3,2/3] and the reaction is:

Occurrences of GRs in the Gr horizon of a gley soil are consistent with the previous scheme. The mineral is formed by bacterial reduction of ferric oxyhydroxides [8]. Carbonate and Fe^II ions come from the consumption and decomposition of humic substances, and the respiration of bacteria. Route EC″B″ of Fig. 9 is followed, giving rise to partially deprotonated GR1(CO32−)^∗ where x belongs to the [1/3,2/3] interval. All fougerite minerals identified up today display Mössbauer spectra where x belongs to this interval.

Therefore, fougerite is [FeII6(1−x)FeIII6xO12H2(7−3x)]2+⋅[CO32−⋅∼3H2O]2− where x∈[1/3,2/3]. Some minor substitution of Fe^II and Fe^III by Mg^II and Al^III in Nature cannot be excluded.