The solubility of molybdenum dioxide and trioxide in HCl-bearing water vapour at 350 °C and pressures up to 160 bars

The solubility and speciation of the assemblage MoO2–MoO3 in water vapour were investigated in experiments conducted at 350 °C, Ptotal from 59 to 160 bar and fHCl from 0 to 3.4 bar (0–2.0 mol%). Measured solubility at these conditions ranges from 22 to 2500 ppm (∑fMo from 4.4 × 10−4 to 6.5 × 10−2 bar). The concentration of Mo in the vapour at fHCl below 0.1 bar is similar to that in pure water vapour, but increases by two orders of magnitude at fHCl above 0.1 bar. The fugacity of gaseous Mo species is independent of chloride concentration at fHCl below 0.1 bar, but increases with increasing fHCl above this pressure. The dominant Mo species at fHCl below 0.1 bar is interpreted to be the same as it is in pure water vapour, and to form as a result of the reactionequation(A1)MoO3(s)+nH2O(g)⇔MoO3·nH2O(g)MoO3(s)+nH2O(g)⇔MoO3·nH2O(g)where the hydration number, n, is equal to 2.3. The value of log K for reaction ( A1), which was calculated at 350 °C and 1 bar of total pressure, is −7.1. The dependence of ∑fMo on fHCl at higher acidity points to the formation of the Mo oxychloride, MoO2Cl2, via the reactionequation(A2)MoO3(g)+2HCl(g)+⇔MoO2Cl2(g)+H2O(g)MoO3(g)+2HCl(g)+⇔MoO2Cl2(g)+H2O(g)The observed change in Mo speciation from hydrated oxide to oxychloride with increasing fHCl may explain the variable association of molybdenite with either sericitic or potassic alteration in porphyry Mo ore deposits.