Experimental study of dissolution rates of hedenbergitic clinopyroxene at high temperatures: dissolution in water from 25 °C to 374 °C
Zhang, Ronghua; Zhang, Xuetong; Guy, Bernard; Hu, Shumin; De Ligny, Dominique; Moutte, Jacques
European Journal of Mineralogy Volume 25 Number 3 (2013), p. 353 - 372
published: Jun 1, 2013
Open Access (paper can be downloaded for free)
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 Na0.04Ca0.95Mg0.3Fe2+ 0.64Fe3+ 0.06Al0.04Si1.97O6. 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 (rSi) 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 M i Zi+ )where r + is the far-from-equilibrium dissolution rate, R the gas constant, T the absolute temperature, Zi valence of metal M i and ai 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 i 2+, where M i 2+ 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 i 2+−O bonds in adjacent octahedral positions have been removed by proton exchange reaction, whereas, above 300 °C, the breaking of the octahedral M i 2+−O bonds occurs after adjacent tetrahedral Si–O bonds have been broken.