Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 C: An experimental study of reaction textures and mechanisms

G. Qian; F. Xia; J. Brugger; W.M. Skinner; J. Bei; G. Chen; A. Pring

doi:10.2138/am.2011.3691

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Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 C: An experimental study of reaction textures and mechanisms

Journal article

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Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 C: An experimental study of reaction textures and mechanisms

G. Qian, F. Xia, J. Brugger, W.M. Skinner, J. Bei, G. Chen and A. Pring

American Mineralogist, Vol.96(11-12), pp.1878-1893

2011

DOI: https://doi.org/10.2138/am.2011.3691

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Abstract

The transformation of pyrrhotite to Fe disulfide (pyrite and/or marcasite) under hydrothermal conditions was studied experimentally by probing the effects of temperature (up to 220 °C, vapor-saturated pressures), ∑S(-II) concentrations, pH, and availability of oxygen on reaction progress and on the resulting textures. The pyrrhotite to Fe disulfide reaction proceeded by a dissolution-reprecipitation mechanism under all conditions. Marcasite and pyrite formed under both oxic and anaerobic conditions, which is inconsistent with the traditionally assumed polysulfide route for FeS2 formation (oxidants required for polysulfide formation). The nature of the products was controlled by the level of supersaturation of the solution with respect to Fe disulfide minerals. Marcasite formed preferentially at low pH or S(-II)-deficient solutions (saturation index << 1000), while pyrite was the main product at saturation indices >1000. Different textures were obtained for the replacement of pyrrhotite by either pyrite or marcasite. Pyrite formation proceeded by direct replacement of pyrrhotite and, simultaneously, by overgrowth from solution. The pyrite crystals were >10 μm in size and randomly oriented. In comparison, marcasite crystals were <1 μm in size, and no significant overgrowth was observed. At pH21°C <3, the marcasite nanocrystals showed the well-known crystallographic relationship with pyrrhotite, but at pH21°C 3.96, the marcasite crystallites were randomly oriented. These experimental results confirm that the preservation of the crystallographic orientation is not a distinguishing feature between dissolution-reprecipitation and solid-state reactions. The different textures among pyrite and marcasite reflect the dominance of crystal growth (pyrite) vs. nucleation (marcasite) as a precipitation mechanism.

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Title: Replacement of pyrrhotite by pyrite and marcasite under hydrothermal conditions up to 220 C: An experimental study of reaction textures and mechanisms
Authors/Creators: G. Qian (Author/Creator)
F. Xia (Author/Creator)
J. Brugger (Author/Creator)
W.M. Skinner (Author/Creator)
J. Bei (Author/Creator)
G. Chen (Author/Creator)
A. Pring (Author/Creator)
Publication Details: American Mineralogist, Vol.96(11-12), pp.1878-1893
Publisher: Mineralogical Society of America
Identifiers: 991005545325607891
Copyright: © 2011 Mineralogical Society of America
Murdoch Affiliation: Murdoch University
Language: English
Resource Type: Journal article

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Collaboration types: Domestic collaboration; International collaboration
Citation topics: 8 Earth Sciences; 8.8 Geochemistry, Geophysics & Geology; 8.8.1 Geochemical Petrogenesis
Web Of Science research areas: Geochemistry & Geophysics; Mineralogy
ESI research areas: Geosciences