The mechanism and kinetics of the transformation from marcasite to pyrite: in situ and ex situ experiments and geological implications

• Published in: Contributions to mineralogy and petrology Vol. 175; no. 3
• Main Authors: Yao, Xizhi, Xia, Fang, Deditius, Artur P., Brugger, Joël, Etschmann, Barbara E., Pearce, Mark A., Pring, Allan
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 02-03-2020; Springer; Springer Nature B.V
• Subjects: Crystallography; Diffraction; Earth and Environmental Science; Earth Sciences; Fluorescence; Galena; Geology; Inclusions; Kinetics; Low temperature; Low temperature environments; Mapping; Microparticles; Mineral deposits; Mineral Resources; Mineralogy; Mines and mineral resources; Nucleation; Organic chemistry; Orientation; Original Paper; Petrology; Phase changes; Phase transitions; Pyrite; Reaction kinetics; Sulfur compounds; Surface properties; Synchrotron radiation; Trace elements; Water vapor; Water vapour; X ray powder diffraction; X rays; X-ray diffraction; X-ray fluorescence; X-ray spectroscopy; Pyrite; Thermal expansion coefficient of marcasite; Reaction kinetics; Marcasite; Synchrotron X-ray florescence microscopy (SXFM); Trace elements; In situ powder X-ray diffraction (PXRD); Electron backscatter diffraction (EBSD)
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-020-1665-4

Despite the natural abundance of pyrite and marcasite and their intergrowth, and a wealth of information they can provide on the physical–chemical conditions of mineral deposits, a complete mechanistic and kinetic study on the phase transformation from the thermodynamically metastable polymorph marcasite to the stable polymorph pyrite is yet to be made. This limits the application of marcasite as an indicator mineral for low-temperature geological environments. Here, we report results from in situ synchrotron powder X-ray diffraction and ex situ anneal/quench experiments at 400–540 °C, demonstrating that the mechanism and kinetics of this transformation depend not only on temperature, but also on particle size, the presence of water vapor, and the presence of pyrite inclusions in marcasite. Under dry conditions, the transformation is limited by surface nucleation and occurs via epitaxial nucleation of pyrite on marcasite, with {100}
pyrite
//{101}
marcasite
and {001}
pyrite
//{010}
marcasite
. In contrast, in the presence of water vapor, there is little crystallographic orientation relationship between the two phases; the transformation is still limited by surface nucleation, but modification of the surface properties by water vapor results in a different nucleation mechanism, and consequently different kinetics. Kinetic analysis estimates a half-life of 1.5 Ma at 300 °C for the transformation under dry conditions with small and pyrite-free marcasite grains, but this estimation should be used with extreme caution due to the complexity of the transformation. From synchrotron X-ray fluorescence elemental mapping, trace elements (As and Pb) play an insignificant role in the transformation. However, the presence of a fluid phase changes the behavior of Pb. Under dry conditions randomly oriented particles of galena formed in pyrite, while under water vapor conditions arrays of nano- to microparticles of galena precipitated in pores. This study highlights that although the natural occurrence of marcasite can indicate low-temperature environments, precise estimation of temperature should not be made without considering the influences from various reaction parameters.