X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY OF WATER-ADSORPTION ON IRON SULFIDE MINERALS

Samples of natural pyrrhotite and pyrite were fractured within the analytical chamber of an X-ray photoelectron spectrometer. The pristine mineral surfaces were then exposed, in the absence of oxygen, to total doses of 100, 200, 400, 800, 1400, 28,000, and 300,000 Langmuirs (L) of D2O. X-ray photoelectron spectroscopic (XPS) analyses were performed between each water dose, to investigate the interaction of these iron sulphide surfaces with water vapour. Recorded Fe and S photoelectron spectra showed no evidence of oxidation products on either mineral, even at highest D2O doses, nor could an oxide oxygen signal be fitted in the spectra for either mineral. On pyrrhotite, the O 1s spectra are composed of contributions from dominantly hydroxyl (at 532.0 +/- 0.2 eV) and subordinate chemisorbed water (at 533.5 +/- 0.2 eV) signals. The main O 1s peak on pyrite is also formed from hydroxyl (531.0 +/- 0.3 eV) and adsorbed water/hydroxyl (at 532.3 eV) signals. However, some O 1s spectra recorded on pyrite have peaks at anomalously high binding energies (>535 eV). The anomalous high binding energy species are attributed to electrically-isolated OH/H2O, as reported elsewhere, and to liquid-like water, which has not previously been described in the literature. Pyrrhotite and pyrite interact with water via fundamentally different processes. Pyrrhotite reaction involves the donation of electron charge through Fe vacancies, whereas the water species detected on pyrite interact with the Fe 3d (e(g)) molecular orbital, and it is suggested that hydrogen bonding with the disulphide moiety may be important.