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An Experimental Study of the Solubility and Speciation of MoO3(s) in Hydrothermal Fluids at Temperatures up to 350 degrees C TEXT SIZE: A A A
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.
 

Publication name

 ECONOMIC GEOLOGY Volume: 115 Issue: 3 Pages: 661-669 DOI: 10.5382/econgeo.4715 Published: MAY 1 2020

Author(s)

 Shang, Linbo; Williams-Jones, Anthony E.; Wang, Xinsong; Timofeev, A.; Hu, Ruizhong; Bi, Xianwu

Corresponding author(s) 

 SHANG Linbo 
 shanglinbo@vip.gyig.ac.cn  
 -Chinese Acad Sci, Inst Geochem, State Key Lab Ore Deposit Geochem, Guiyang 550081, Peoples R China
 -McGill Univ, Dept Earth & Planetary Sci, 3450 Univ St, Montreal, PQ H3A 0E8, Canada

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