Surface states and reactivity of pyrite and marcasite

The sulphur surface sites of pristine marcasite surfaces and their reactivi ty during initial air oxidation were investigated for the first time using synchrotron radiation excited photoelectron spectroscopy (SRXPS). Both S 2p and Fe 2p spectra were collected and compared to those of the polymorph py rite. A new sulphur surface component is found additionally to the two know n pyrite surface signals. Due to the non-isotropic character of the orthorh ombic marcasite structure, its peak intensity depends strongly on the surfa ce orientation. Its binding energy is close to the region of short-chained polysulphides. The signal is assigned to sulphur trimers produced by electr on transfer between surface near sulphur dimers and S- formed after rupture s of S-S bonds which results in S2- and S-3(2-). In the cubic crystal arran gement of pyrite the stabilisation of S- to S2- realised by an electron tra nsfer from iron to sulphur only. A confirmation of this assumption is found in the Fe 2p spectra. The contribution of Fe3+ species - a result of the e lectron transfer process - is considerably less pronounced for marcasite. The mechanism of initial oxidation is different for marcasite and pyrite. T he sulphur species responsible for the new spectroscopic component are the most reactive sites at the marcasite surface, while S2- is oxidised first i n case of pyrite. The signals indicating the products of marcasite oxidatio n in the S 2p spectrum are less intense as expected from the intensity decr easing of the trimer peak. Therefore, an oxidation of sulphur trimers to el emental Sulphur is assumed which could not be detected because of its volat ility in the vacuum of the air lock system. (C) 2001 Elsevier Science B.V. All rights reserved.