The solubility of magnetite was measured in dilute aqueous solutions of HC1 at high temperature and pressure. At 390° C., 440 bars, in 0.0002 M HC1, 300 ppm ferrous iron is dissolved. Solubility decreases with acid concentration; in pure water at the same conditions solubility is less than 0.02 ppm. Calculation of real-gas equilibria from thermodynamic data shows that the contribution of volatile Fe2Cl6 molecules is insignificant in such dilute solutions. Comparison with room-temperature data suggests that ferrous iron is brought into true solution mainly as Fe2+ ion, by reaction with H+, although complexes such as FeCl2+ and FeOH2+ may participate.

Magnetite was synthesized by reaction of acidic iron chloride solutions with calcite at high temperature and pressure.

The amount of iron measured in solutions with HC1 concentrations less than a hundredth of that of natural fluids is great enough to be geologically significant.

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