Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations

Abstract

High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg 0.87Fe 0.13) 2SiO 4], bronzite [(Mg 0.8Fe 0.2) 2Si 2O 6] and diopside [Ca(Mg 0.8Fe 0.2)Si 2O 6] were collected before and after leaching in pH ∼2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca 2+ and Mg 2+) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH −, which arises from H + exchange with near-surface cations (Ca 2+ and Mg 2+) via electrophilic attack of H + on the M–O–Si moiety to produce the H 2Mg(M1)SiO 4(surf) complex at olivine surfaces, and two complexes, H 2Mg(M1)Si 2O 6(surf) and H 4Si 2O 6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H 2Mg(M1)SiO 4(surf) and H 2Mg(M1)Si 2O 6(surf) have been proposed previously, but the second pyroxene surface complex H 4Si 2O 6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2–O–Si moiety producing H 2Mg(M1)SiO 4(surf); while the second attacks the M1–O–Si moiety ultimately producing H 4SiO 4 which is released to solution. For pyroxenes, the first electrophilic reaction produces H 2Mg(M1)Si 2O 6(surf), while the second produces.H 4Si 2O 6(surf). These two reactions are followed by a nucleophilic attack of H 2O (or H 3O +) on Si of H 4Si 2O 6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si–O–Si moiety and release of H 4SiO 4 to solution. The intensity of the OH − peak for the leached pyroxenes is about double the OH − intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals. Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca 2+ and Mg 2+ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca–O and Mg–O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H 2Mg(M1)SiO 4, as indicated by high quality density functional calculations on an olivine where Mg 2+ in M2 is replaced by 2H +. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex.