Ferrous chloride species is considered to be the main factor of iron transportation by hydrothermal fluids at high
T-P parameters of the Earth's crust (Henrichi and Seward, 1990; Ohmoto et al., 1994). The stability constants of FeCl+ are well known up to 300°C (Henrichi and Seward, 1990). At the same time the literature data on thermodynamic properties of FeCl20 species which became more important as the temperature raises differs much from each other. This is the reason for the aim of the present study was to estimate ferrous speciation in iron-bearing chloride fluids at temperatures of 300-450°C and pressures of 500-1000 bars.
The solubility of natural magnetite was studied in the HCl solutions (6*10-4 - 0.2m) with control of hydrogen pressure. The experiments were carried out in titanium autoclaves passivated with 1M HNO3. Temperatures being controlled within ±2°C and pressures by filling coefficients of autoclave. Hydrogen fugacity was controlled by introducing into the system a weighed amount of aluminum metal and reacting it to evolve H2. After equilibration the autoclaves were rapidly quenched in the cold water. The pH value of the samples was measured immediately after opening of the autoclaves. Preliminary runs shown that aqueous iron present exclusively in the ferrous state. Therefore only total iron concentrations were determined with potassium permanganate or spectrophotometrically after filtering the solutions using a 0.05mm filter. Experimental error in pH value was within ±0.03 un. and in iron concentration ±5%.
Calculation of the compositions of the system was performed using GIBBS code (Shvarov, 1995) with respect to thermodynamic functions for aqueous species, exclusive of ferrous ones, from SUPCRT92 database, Fe(OH)+ and Fe(OH2)0 from Diakonov (1995), HCl0 from Tagirov and Zotov (1995), Fe3O4(k) -from Hemingway (1990). Influence of aluminum species on the measured pH in low-acid solutions was accounted by using data of Diakonov (1995).
The dominant equilibria of the system under investigation may be expressed:
1/3Fe3O4 + 2H + + 1/3H2(g) =
Fe2+ + 4/3 H2O (1)
1/3Fe3O4 + 2H+ + nCl- + 1/3 H2(g) =
FeCln2-n + 4/3H2O (2)
1/3Fe3O4 + 2HCl0 + 1/3 H2(g) =
FeCl20 + 4/3H2O (2')
1/3Fe3O4 + (2-m)H+ + 1/3H2(g) =
Fe(OH)m2--m + 4/3H2O (3)
Fe2+ + Cl- = FeCl+ (4)
Fe2+ + 2Cl- = FeCl20 (5)
It is reasonable to suggest that such uncharged species as FeCl20(aq) would predominant at parameters of runs (Fein et al., 1992). But the results obtained rectify that it is necessary to take into account presence of Fe2+, FeCl+ and Fe(OH)+ (at low pH) as the slope of the fitted lines in coordinates logC(Fe2+)/f(H2)1/3 vs. log[HCl0] is less then two in any run (reaction (2')). The results obtained showed it is Fe2+ ion that predominate in the solution at 300°C , 600 bars and HCl concentrations used instead of Fe(OH)+ as suggested by Zeng et al. (1989) (log K01=3.83±0.11). The species Fe+2 and FeCl+ both present in the solution in considerable amounts at 450°C, 500 bars (log K04 =4.50±0.15). An excellent agreement of the constants of the reactions involving these species obtained in the present study with those obtained by extrapolation of literature data (SUPCRT92 for Fe+2 and Henrichi and Seward (1990) for FeCl+) permits us to calculate stability constant for FeCl20 at 450°C and 1000 bars, when this species is able to make a large contribution in magnetite's solubility: log K05= 6.4±0.15 (log K05= 7.82 (Fein et al., 1992)) and 4.18 (Boctor et al., 1980)). As a result an internally consistent set of aqueous species of iron's (II) thermodynamic functions is calculated on the base of HKF model (Tanger and Helgeson, 1988).
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