Journal
AMERICAN MINERALOGIST
Volume 96, Issue 11-12, Pages 1878-1893Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am.2011.3691
Keywords
Hydrothermal replacement; pyrrhotite alteration; pyrite; and marcasite; dissolution-reprecipitation reaction; crystallographic relationship; textures
Categories
Funding
- Australian Research Council [DP0772299, DP1095069]
- South Australian Museum
- University of South Australia
- Ian Wark Research Institute
- Australian Research Council [DP1095069] Funding Source: Australian Research Council
Ask authors/readers for more resources
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 degrees C, vapor-saturated pressures), Sigma 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 mu m in size and randomly oriented. In comparison, marcasite crystals were <1 mu m in size, and no significant overgrowth was observed. At pH(21 degrees C) <3, the marcasite nanocrystals showed the well-known crystallographic relationship with pynhotite, but at pH(21 degrees 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.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available