The solubility of molybdenum trioxide (MoO₃(s)) in aqueous solutions has been investigated experimentally at 250 degrees, 300 degrees, and 350 degrees C and saturated water vapor pressure, and total Na concentrations ranging from 0 to 3 molal (m). Results of these experiments show that the solubility of MoO₃(s) increases with increasing temperature and at 350 degrees C can reach several thousand parts per million at high salinity (>1 m NaCl). At low Na⁺ activity, MoO₃(s) dissolves dominantly as HMoO₄^-, whereas at high Na⁺ activity, the dominant species is NaHMoO₄⁰. The two dissolution reactions are

MoO₃(s) + H₂O = HMoO₄^- + H⁺ (1)

and

MoO₃(s) + H₂O + Na⁺ = NaHMoO₄⁰ + H⁺. (2)

The values of the logarithms of the equilibrium constants for reaction (1) are -5.20 +/- 0.12, -5.31 +/- 0.17, and -5.50 +/- 0.09 at 250 degrees, 300 degrees, and 350 degrees C, respectively, and for reaction (2) the values are -3.40 +/- 0.11, -3.25 +/- 0.19, and -2.97 +/- 0.09 for the same temperatures. In combination, these equilibrium constants yield equilibrium constants for the reaction relating the two aqueous species:

Na⁺ + HMoO₄^- = NaHMoO₄⁰. (3)

The values of the logarithms of the equilibrium constants for reaction (3) are 1.80 +/- 0.16, 2.06 +/- 0.25, and 2.53 +/- 0.13 at 250 degrees, 300 degrees, and 350 degrees C, respectively. Calculations, based on the results of this study and thermodynamic data available for other species, suggest strongly that in ore-forming hydrothermal systems, molybdenum is transported mainly as NaHMoO40 and deposits as molybdenite in response to cooling and possibly a reduction in fo₂.