An Experimental Study of the Solubility and Speciation of MoO3(s) in Hydrothermal Fluids at Temperatures up to 350°C

Abstract

The solubility of molybdenum trioxide (MoO3(s)) in aqueous solutions has been investigated experimentally at 250°, 300°, and 350°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°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°, 300°, and 350°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°, 300°, and 350°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.

One-Step Hydrothermal Synthesis of Phase-Engineered MoS2/MoO3 Electrocatalysts for Hydrogen Evolution Reaction

“…Under the hydrothermal conditions of high temperature and pressure used in this work, the MoO 3 was completely dissolved. Further confirmation is provided from previous experimental studies, , which have shown that the solubility of MoO 3 increases with increasing temperature, creating HMoO 4 – species (MoO 3 + H 2 O ↔ HMoO 4 – + H + ). It is worth noting that in an experiment, where we stopped the reaction after 1 h, it resulted in a transparent solution, suggesting the complete dissolution of MoO 3 bulk rodlike crystals.…”

An experimental investigation into the partition of Mo between aqueous fluids and felsic melts: Implications for the genesis of porphyry Mo ore deposits

“…2c and d). Molybdate is the main species of Mo in saline fluids (Cao, 1989;Kudrin, 1989;Bali et al, 2012;Zhang et al, 2012;Tattitch and Blundy, 2017;Shang et al, 2020). The higher salinity in our carbon dioxide series relative to the other experiments enhances the solubility of molybdate moieties such as Na 2 MoO 4 and K 2 MoO 4 in fluids and increased D Mo .…”

Genesis and fluid evolution of the Yuku porphyry Mo deposit, East Qinling orogen, China

“…The speciation of molybdenum in hydrothermal fluids has generally been considered as the Mo 6+ state, including H 2 MoO 4 , KHMoO 4 , NaHMoO 4 0 , and their dissociation products HMoO 4 − and MoO 4 2− (e.g., Minubayeva & Seward, 2010; Shang et al, 2020; Ulrich & Mavrogenes, 2008). The formation of molybdenite (MoS 2 ) requires abundant Mo 4+ in mineralizing fluids.…”

“…Cooling results in a dramatic decrease in molybdenite solubility in the Questa porphyry Mo deposit (Klemm et al, 2008; Ulrich & Mavrogenes, 2008). However, both HMoO 4 − and NaHMoO 4 0 are the dominant molybdenum species in NaCl‐bearing aqueous solutions at elevated temperatures (250–350°C; Shang et al, 2020). Our microthermometric results show the temperature of molybdenite formation was 309–360°C, suggesting a simple decrease in temperature cannot have destabilized Mo 6+ complexes to form Mo 4+ in the Yuku porphyry Mo deposit.…”

“…This process is an important mechanism for Mo enrichment (Audétat, Pettke, Heinrich, & Bodnar, 2008; Zajacz, Halter, Pettke, & Guillong, 2008), during which Mo preferentially enters the brine phase (Heinrich, Günther, Audétat, Ulrich, & Frischknecht, 1999; Williams‐Jones & Heinrich, 2005). Shang et al (2020) argued that the transport of Mo in hydrothermal fluids is strongly dependent on salinity. Ni et al (2015, 2017) proposed that intense fluid boiling can result in a substantial change in the salinity of the mineralizing fluids, destabilizing Mo 6+ complexes and leading to rapid precipitation of molybdenite.…”