First-principles calculations of the thermodynamic mixing properties of arsenic incorporation into pyrite and marcasite
Artículo
Open/ Download
Publication date
2006-01-31Metadata
Show full item record
Cómo citar
Reich Morales, Martín
Cómo citar
First-principles calculations of the thermodynamic mixing properties of arsenic incorporation into pyrite and marcasite
Author
Abstract
The thermodynamic mixing properties of As into pyrite and marcasite have been investigated using first-principles and Monte Carlo calculations in order to understand the incorporation of this important metalloid into solid solution. Using quantum-mechanical methods to account for spin and electron transfer processes typical of sulfide minerals, the total energies of different As-S configurations were calculated at the atomic scale, and the resulting As-S interactions were incorporated into Monte Carlo simulations. Enthalpies, configurational entropies and Gibbs free energies of mixing show that two-phase mixtures of FeS2 (pyrite or marcasite) and FeAsS (arsenopyrite) are energetically more favorable than the solid solution Fe(S,AS)(2) (arsenian pyrite or marcasite) for a wide range of geologically relevant temperatures. Although miscibility gaps dominate both solid solution series, the solubility of As is favored for X-As < 0.05 in iron disulfides. Consequently, pyrite and marcasite can host up to similar to 6 wt.% of As in solid solution before unmixing into (pyrite or marcasite)+arsenopyrite. This finding is in agreement with previously published HRTEM observations of As-rich pyrites (> 6 wt.% As) that document the presence of randomly distributed domains of pyrite+ arsenopyrite at the nanoscale. According to the calculations, stable and metastable varieties of arsenian pyrite and marcasite are predicted to occur at low (X-As < 0.05) and high (X-As > 0.05) As bulk compositions, respectively.
Quote Item
CHEMICAL GEOLOGY Volume: 225 Issue: 3-4 Pages: 278-290 Published: JAN 31 2006
Collections