Molybdic acid ionisation under hydrothermal conditions to 300 °C

doi:10.1016/j.gca.2010.04.054

Geochimica et Cosmochimica Acta

Volume 74, Issue 15, 1 August 2010, Pages 4365-4374

https://doi.org/10.1016/j.gca.2010.04.054 Get rights and content

Abstract

This UV spectrophotometric study was aimed at providing precise, experimentally derived thermodynamic data for the ionisation of molybdic acid (H₂MoO₄) from 30 to 300 °C and at equilibrium saturated vapour pressures. The determination of the equilibrium constants and associated thermodynamic parameters were facilitated by spectrophotometric measurements using a specially designed high temperature optical Ti–Pd flow-through cell with silica glass windows.

The following van’t Hoff isochore equations describe the temperature dependence of the first and second ionisation constants of molybdic acid up to 300 °C: $\begin{matrix} \log_{10} K_{1} = - 96.125 - 0.02875 \cdot T + 17.660 \cdot \ln (T) \\ \log_{10} K_{2} = - 30.082 - 0.02690 \cdot T + 5.9366 \cdot \ln (T) \end{matrix}$

The resulting ionisation constants of molybdic acid demonstrate that in low sulphur containing hydrothermal fluids in the Earth’s crust, the transport of molybdenum is favoured by the ${HMoO}_{4}^{-} / {MoO}_{4}^{2 -}$ species while the role of the associated H₂MoO₄ is of negligible importance at elevated temperatures above 200 °C.

Introduction

Molybdenum occurs in both the +4 and +6 oxidation states in minerals in and on the Earth’s crust. The most abundant molybdenum containing phase, molybdenite (MoS₂), precipitates from aqueous fluids over a wide range of temperatures and pressures from magmatic to lower temperature metamorphic environments and epithermal systems (more rarely). In the trap rocks of the Deccan basalts, powellite (CaMoO₄) occurs in association with numerous zeolite minerals and has precipitated from the moderate temperature zeolite facies fluids at t ⩽ 250 °C. Molybdenum is also a ubiquitous trace component of scheelite (CaWO₄) which is often associated with gold and quartz in hydrothermal vein deposits. It is known that molybdenum may occur in high concentrations in volcanic gas fumaroles (Wahrenberger et al., 2002, Williams-Jones and Heinrich, 2005, Rempel et al., 2006) as well as in hot spring precipitates such as at Waimangu in New Zealand (Seward and Sheppard, 1986) and geothermal waters (Arnorsson and Oskarsson, 2007). However, the chemistry of molybdenum in aqueous solutions at elevated temperatures and pressures is rather poorly known. The ambient temperature aqueous geochemistry of molybdenum is also of current interest given the use of molybdenum isotopes and their fractionation in delineating/detecting redox conditions in paleo-Earth surface environments.

It has been suggested that in magmatic hydrothermal fluids as well as geothermal waters, mononuclear molybdate and tungstate species (i.e. the hexavalant oxidation state) dominate the speciation of molybdenum and tungsten (Kolonin et al., 1975b, Candela and Holland, 1984, Arnorsson and Ivarsson, 1985, Stemprok, 1990, Keppler and Wyllie, 1991). In addition to molybdic acid ( $H_{2} {MoO}_{4}^{0}$ ) and its dissociation products ( ${MoO}_{4}^{2 -}$ and ${HMoO}_{4}^{-}$ ), Kudrin (1989) suggested that associated species such as ${NaHMoO}_{4}^{0} and {KHMoO}_{4}^{0}$ may be responsible for the transport of molybdenum and tungsten in hydrothermal fluids at high temperatures (⩾300 °C). Under reducing conditions, transport of molybdenum can be carried out in lower (+4) valency state (Kudrin, 1985). Experimental studies have also shown that the partitioning of molybdenum in magmatic systems is independent of the chlorine content of magmas and associated aqueous phases (Candela and Holland, 1984). Fluoride does not appear to be essential for the concentration of Mo and W in fluids evolving from granitic magma (Candela and Holland, 1984, Keppler and Wyllie, 1991, Lentz and Suzuki, 2000), although Tugarinov et al. (1973) considered the transport of molybdenum in the form of fluoride complexes in acid solutions at high temperatures to be important. More recently, Ulrich and Mavrogenes (2008) have measured the solubility of MoO₃ in water and elemental molybdenum in KCl solutions with the redox conditions and pH controlled by various mineral buffers at 500–800 °C and 150–300 Mpa. With the aid of XANES spectra, they suggested that “Mo-oxo-chloride” species such as ${MoO}_{2} {Cl}_{n}^{2 - n}$ were present at higher salinities and that simple ${MoO}_{4}^{2 -}$ species were present in more dilute solutions.

In reducing H₂S-containing solutions, molybdenum may precipitate as molybdenite or may coprecipitate with other sulphides. In addition, the oxygen of the molybdate ions may be successively replaced by sulphur to form thiomolybdates (Emerson and Huested, 1991, Barling et al., 2001, Arnorsson and Oskarsson, 2007). Arutyunyan (1966) has suggested that thiomolybdate complexes may play an important role in the transport of molybdenum in high temperature systems. Tugarinov et al. (1973) concluded that thiomolybdate complexes could not be responsible for transport of molybdenum due to insufficient concentrations of sulphur in hydrothermal solutions (according to their estimations, the necessary concentration of H₂S is about 1 mol/kg), whereas Kolonin and Laptev (1975a) concluded from spectrophotometric studies that the thiomolybdate species decompose at temperatures ⩾100 °C. More experimental studies are required.

A number of studies at elevated temperatures have been carried out on the solubility of MoO₂ (250–450 °C) (Kudrin, 1985), CaMoO₄ and Na₂MoO₄ (25–300 °C) (Zhidikova and Khodakovskii, 1971, Zhidikova et al., 1973), MoO₂ and Na₂MoO₄ (25–200 °C) (Ivanova et al., 1975), as well as on the hydrolysis of sodium molybdate (Maksimova et al., 1976) in the range from 15 to 90 °C. The free energies of formation of sodium molybdate and molybdate ion have also been determined by Graham and Hepler, 1956, Urusov et al., 1967 and Zhidikova and Kuskov (1971). Ivanova et al. (1975) derived the empirical equations for the temperature dependence of the first and second dissociation constants of molybdic acid based on the assumption that $Δ S_{diss}^{o}$ for most of the weak acids have similar values (average of −79.496 J mol⁻¹ K⁻¹ and −125.52 J mol⁻¹ K⁻¹ for the first and second dissociation step, respectively). Arnorsson and Ivarsson (1985) provide estimation of the second ionisation constant of molybdic acid using an HKF model approach. However, there has been no previous systematic experimental study of the ionisation equilibria of molybdic acid at elevated temperatures and hence, no reliable experimentally based thermodynamic data for these reactions are available. Some data could be estimated from above mentioned works but the speciation of molybdic acid in aqueous solution is very much dependent on the total molybdenum concentration (because of the formation of polyanions). The composition of the solution (i.e. ionic strength) can also favour polymerisation (Tytko et al., 1985).

The aim of this study has therefore been to obtain reliable experimental thermodynamic data for the first and second ionisation constants of molybdic acid as a function of temperature from 30 to 300 °C at the equilibrium saturated water vapour pressure. The two relevant deprotonation reactions for molybdic acid are, $H_{2} {MoO}_{4}^{0} \leftrightarrow {HMoO}_{4}^{-} + H^{+}$ ${HMoO}_{4}^{-} \leftrightarrow {MoO}_{4}^{2 -} + H^{+}$

Such data are a fundamental prerequisite to understanding the transport and deposition chemistry of molybdenum in aqueous systems over wide ranges of temperature and pressure. In addition, reliable thermodynamic data for molybdic acid ionisation comprise the necessary background chemistry for further studies of the stability of other anionic molybdenum species such as the halogenido- and thiomolybdates as well as poly- and heteropolyanions of molybdenum in aqueous media pertinent to natural systems.

Section snippets

Experimental method

All solutions were prepared on a molal scale with Nanopure Millipore water (resistivity >18 MΩ/cm). Perchloric acid stock solution was diluted from concentrated acid (HClO₄, 60%, p.a., Merck) and standardized by colorimetric titration against dried Trisma-base (Tris(hydroxymethyl)aminomethane, 99+%, Aldrich) using methyl red as indicator and potentiometric titration (using a glass combination electrode (Metrohm). Sodium hydroxide solution was prepared from saturated sodium hydroxide solution

Data treatment

The measured spectra (background corrected) were stored in an absorbance matrix, A_i×j, where i = number of wavelengths, j = number of analysed solutions. Only the absorbances in the wavelength range from 210 to 280 nm were employed in the data treatment. In order to determine the number of absorbing species (i.e. the rank or number of principal components) required for a chemical model, we used a singular value decomposition (SVD) approach, such that, $A_{i \times j} = U_{i \times n} \times S_{n \times n} \times V_{j \times n}^{T}$ where the matrices U, S, V

Results and discussion

The spectra of three molybdate containing solutions of different pH over the whole range of studied temperatures and corrected for background absorbance (i.e. windows + solvent) are shown in Fig. 2. It can be seen that increasing temperature causes significant changes in the absorption spectrum. The overall absorbance decreases because of the effect of decreasing molar concentration due to changes in water density with the temperature. In addition, the shape of the spectra changes significantly

Acknowledgements

This research was funded by a Swiss National Science Foundation (Schweizerischer Nationalfonds) grant awarded to T.M. Seward. We are grateful for the constructive reviews of our manuscript by one anonymous reviewer as well as by Dave Wesolowski and Anthony Williams-Jones.

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Citation Excerpt :
To determine the Mo speciation in the fluid phase, most previous studies were conducted at relatively low temperature (<600 °C, Rempel et al., 2006, 2008; Shang et al., 2020). Since Mo speciation has been shown to be influenced by temperature (Bernard et al., 1990; Minubayeva and Seward, 2010; Borg et al., 2012; Zhang et al., 2012), these low-temperature experimental data are not directly comparable to this study yet do provide some guidelines for determining what ligands tend to bond with Mo. The solubility data and in situ spectroscopy experiments in these studies at relatively low-temperatures suggest that Mo is mainly present as H2MoO4, HMoO4- or MoO42- in pure water and low-Cl aqueous fluid (Rempel et al., 2006, 2008; Shang et al., 2020) and complex Mo-Cl species in high-Cl fluid such as MoO2(OH)2Cl- and MoO2Cln2-n (n = 1–3, Yokoi et al., 1993; Rempel et al., 2008; Dadze et al., 2018).
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Molybdenum isotope variation mechanism and ore-genesis of Niutoushan Pb–Zn sulfide orebodies in the Xiangshan volcanic basin, South China
2022, Journal of Geochemical Exploration
Citation Excerpt :
As mentioned above, Mo in the sulfide minerals is likely to be distributed as nanosized molybdenite inclusions. In general, molybdate is the most important Mo species for Mo transport, even in a relatively reduced hydrothermal fluid (e.g., a magmatic-hydrothermal fluid with fO2 between the FMQ and nickel–nickel oxide [NNO] buffer) of porphyry Mo systems (Minubayeva and Seward, 2010; Tattitch and Blundy, 2017; Keppler and Wyllie, 1991) and hydrothermal-vein-type Mo–Pb–Zn mineralization (Li et al., 2013). Furthermore, Mo–S species were not found in multimetal solubility experiments in the solution with fO2 in a pyrite–pyrrhotite–magnetite buffer and at temperature of 200–350 °C (Wood et al., 1987).
Niutoushan Pb–Zn sulfide orebodies are found in the western part of the Xiangshan volcanic basin; however, genesis of the mineralization is highly debated. To appropriately constrain the mineralization process and genesis of the Niutoushan Pb–Zn sulfide orebodies present in the volcanic rocks in the Xiangshan volcanic basin, we investigate Mo isotope compositions of sulfide minerals (pyrite and sphalerite) from the Niutoushan Pb–Zn sulfide orebodies and those of the volcanic and subvolcanic rocks (the least-altered porphyritic lava and granitic porphyry rocks) from the Xiangshan volcanic-subvolcanic complex.
The δ⁹⁸Mo values (normalized to NIST3134 of 0.25 ‰) of the Xiangshan volcanic-subvolcanic rocks, including the least-altered granitic porphyry (0.20 ‰–0.26 ‰) and porphyritic lava rocks (0.40 ‰–0.51 ‰), are similar to those of the bulk continental crust (0.1 ‰–0.35 ‰, Willbold and Elliott, 2017). The δ⁹⁸Mo values of sulfides from the Niutoushan Pb–Zn sulfide orebodies (pyrite: −0.95 ‰ to 0.30 ‰, sphalerite: −0.94 ‰ to −0.59 ‰) are mostly lower than those of the continental crust and span a wide range. After examining potential factors that may cause δ⁹⁸Mo variation, it is concluded that the wide δ⁹⁸Mo range of the Niutoushan Pb–Zn sulfide orebodies resulted mainly from the hydrothermal fluid evolution through Rayleigh fractionation in the mineralization process than from changes in other factors of the fluid, such as temperature, redox, and fluid boiling process.
Further, based on this conclusion, the δ⁹⁸Mo value of the initial hydrothermal fluid finally forming the Niutoushan sulfide orebodies was estimated to be 0.46 ‰ by combining the lowest δ⁹⁸Mo value of the sulfides and the Mo isotope fractionation between the fluid and molybdenite. The initial hydrothermal ore-forming fluid with such a δ⁹⁸Mo value was most likely derived from the Xiangshan subvolcanic magmas. This study highlights the potential of Mo isotope composition as a novel tool for tracking origin and evolution of ore-forming hydrothermal fluids and metals in case of Pb–Zn sulfide mineralization associated with igneous activities.

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Molybdic acid ionisation under hydrothermal conditions to 300 °C

Abstract

Introduction

Section snippets

Experimental method

Data treatment

Results and discussion

Acknowledgements

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Mar. Chem.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Molybdenum in Icelandic geothermal waters

Contrib. Mineral. Petrol.

The stability of water-soluble forms of molybdenum in sulphur-containing solutions at high temperatures

Geokhimiya

Extraction of chemical speciation and molar absorption coefficients with well-posed solutions of Beer’s law

J. Solution Chem.

Dissociation constant of bisulfate ion in aqueous sodium chloride solutions to 250 °C

J. Phys. Chem.

A formulation for the static permittivity of water and steam at temperatures from 238 K to 873 K at pressures up to 1200 MPa, including derivatives and Debye-Hueckel coefficients

J. Phys. Chem. Ref. Data

Heats of formation of sodium molybdate, molybdic acid, and aqueous molybdate ion

J. Am. Chem. Soc.

A unified fundamental equation for the thermodynamic properties of water

J. Phys. Chem. Ref. Data

Ion association of dilute aqueous sodium hydroxide solutions to 600 °C and 300 MPa by conductance measurements

J. Solution Chem.

Equilibria in the molybdenum trioxide–water system in the 25–300 °C range

Geokhimiya

Partitioning of copper, tin, molybdenum, tungsten, uranium and thorium between melt and aqueous fluid in the systems haplogranite–water–hydrogen chloride and haplogranite–water–hydrogen fluoride

Contrib. Mineral. Petrol.