The kinetics and mechanism of pyrite thermal decomposition

Abstract

Pyrite (FeS2) decomposition in He, N2, CO2‐CO‐SO2, O2‐CO2 and H2S‐H2 gas mixtures was studied from 400–590°C using optical microscopy, X‐ray diffraction, electron microprobe analyses, and gas chromatography. The rate constants were determined from the weight loss and the thickness change of pyrite in reacted samples. The temperature dependent composition of the product, pyrrhotite (Fe1‐xS), was measured by electron microprobe analysis and X‐ray diffraction. Pyrite decomposition to pyrrhotite and sulfur vapor (dominantly S2) was observed in all gases and follows linear kinetics. The apparent activation energy of pyrite thermal decomposition is 297 + 34 kJ mol−1 in inert gases (e.g. He, N2), and 275 + 20 kJ mol−1 in purified CO2 and in low concentration CO‐SO2 gas mixtures with CO2. The rate constants are the same regardless of gas composition at a given temperature. In oxidizing (O2‐CO2) or reducing (H2S‐H2) gas mixtures, the rate constants are larger than that in inert gases because additional reactions between pyrite and oxidizing/reducing gases accelerate the overall process while reducing the apparent activation energies. A mechanism of pyrite decomposition, which explains our experimental results, as well as the discordant activation energy data in the literature, is proposed.

Dynamic pyrometamorphism during atmospheric entry of large (˜10 micron) pyrrhotite fragments from cluster IDPs

“…However, pyrite decomposition experiments showed that not only would the nature of released sulfur allotropes affect the overall energy distribution of the decomposition process, the efficiency of vaporization of liquid sulfur produced would also be critical (Hong and Fegley 1997). The vesicular pyrrhotite textures and correlation of vesicle size with increasing Fe-oxide formation suggest that efficient pathways for loss of sulfur existed during flash-heating of sulfur IDPs described here.…”

“…9). Should liquid sulfur vaporization have been fast enough, pyrrhotite decomposition might have followed the one-step release of a S 2 molecule as was experimentally verified by Hong and Fegley (1997) during pyrite decomposition.…”

“…Both are probably low but depending on the ability to develop vesicles for sulfur loss the S 2 pressure could build up as a function of time during flash heating to the peak-heating temperature. Assuming for the moment that pyritic sulfide formed as described above during flash heating of the sulfide IDPs, subsequent low-temperature (<500 °C) pyrite decomposition could yield hematite plus SO 2 (Hong and Fegley 1997).…”

“…Experimental studies (Condit et al 1974;Hong and Fegley 1997) offer a coherent framework for crystallographically controlled formation of Fe-oxides and vesicles, a double Fe-oxide rim of different grain sizes, and the range of pyrrhotite compositions established during atmospheric entry of the sulfide IDPs described here. In nmto micron-sized grains of these particles diffusion is not a rate-controlling process.…”

Iron Mineralogy of Venus' Surface Investigated by M�ssbauer Spectroscopy

“…sulfur gases in the Venus sulfur cycle (Fegley and Treiman 1992, Fegley et al 1995b, Hong and Fegley 1997, 1998. Thus, determination of the nature and the abundance of Febearing minerals in rocks and soil will provide important information about atmospheric weathering processes.…”