Experimental study of dissolution rates of hedenbergitic clinopyroxene at high temperatures: dissolution in water from 25°C to 374°C

Ronghua Zhang (1) , Xuetong Zhang (1) , Bernard Guy (2, 3, 4) , Shumin Hu (1) , Dominique de Ligny (5, 6) , Jacques Moutte (2, 3, 4)

1 Institute of Mineral Resources
2 SPIN-ENSMSE - Centre Sciences des Processus Industriels et Naturels
3 GSE-ENSMSE - Département GéoSciences et Environnement
4 EVS - Environnement, Ville, Société
5 LPCML - Laboratoire de Physico-Chimie des Matériaux Luminescents
6 ILM - Institut Lumière Matière [Villeurbanne]

Steady-state pyroxene dissolution rates in aqueous solutions have been measured at temperatures from 25 to 374°C at a pressure of 23 MPa and at neutral pH. The pyroxene is hedenbergitic clinopyroxene, of composition Na_0.04Ca_0.95Mg_0.3Fe²⁺_0.64Fe³⁺_0.06Al_0.04Si_1.97O₆. All experiments were performed at conditions far from equilibrium in Ti-alloy mixed-flow reactors. In most runs, the reactive solutions were undersaturated with respect to pyroxene and secondary minerals were rarely found at the reacted surface. The dissolution is non-stoichiometric in most cases, while the different chemical elements of the pyroxene are released at different rates. Stoichiometric steady-state dissolution was obtained in neutral solution at 100°C. The release rates of the different elements vary with temperature and solution chemistry. The dissolution rates (r_Si) in neutral pH conditions increase with temperature from 25 to 300°C, reach a maximum at 300°C, and then decrease with continued temperature increase. At a given temperature, the rates decrease significantly with increasing pH of the reactive fluid and are also affected by the activities of Ca, Mg, Fe in the solution. At neutral pH, the dependence of the pyroxene dissolution rates on activities of Ca, Mg, Fe and H⁺ in the fluid can be expressed by the relation: log r₊(T, _ai) = log (A - E_a/(2.303 RT) + α log (a_H+)^Zi / a_Mi^Zi+) where r₊ is the far-from-equilibrium dissolution rate, R the gas constant, T the absolute temperature, Z_i valence of metal M_i and a_i represents the activity of the subscript aqueous species. E_a equals 22.667 kJ/mole/K and A = 2.011 × 10⁻⁷ mole/cm²/s; α is the empirical reaction rate order, which can be derived from the experimental results. At temperatures below 300°C, the exchange reactions 2H⁺ ↔ M_i²⁺, where M_i²⁺ refers to divalent cations Mg²⁺, Fe²⁺ or Ca²⁺, dominate in the dissolution. The following evolution of the dissolution with temperature is proposed: at 300°C, the tetrahedral Si-O bonds break after the M_i²⁺−O bonds in adjacent octahedral positions have been removed by proton exchange reaction, whereas, above 300°C, the breaking of the octahedral M_i²⁺−O bonds occurs after adjacent tetrahedral Si-O bonds have been broken.

Mots clés: clinopyroxene; critical state; dissolution; hedenbergite; high temperature; kinetics; mineral surface; rate

Domaines: Géochimie; Minéralogie; Chimie inorganique

Ronghua Zhang, Xuetong Zhang, Bernard Guy, Shumin Hu, Dominique de Ligny, et al.. Experimental study of dissolution rates of hedenbergitic clinopyroxene at high temperatures: dissolution in water from 25°C to 374°C. European Journal of Mineralogy, 2013, 25 (13), pp.353-372. ⟨10.1127/0935-1221/2013/0025-2268⟩. ⟨hal-00908774⟩