A Fe K-edge XAS study of amethyst

作者：Di Benedetto Francesco^*; D'Acapito Francesco; Fornaciai Gabriele; Innocenti Massimo; Montegrossi Giordano; Pardi Luca A; Tesi Silvia; Romanelli Maurizio

来源：Physics and Chemistry of Minerals, 2010, 37(5): 283-289.

DOI：10.1007/s00269-009-0332-0

• Amethyst
• Fe
• XANES
• EXAFS
• DFT calculations
• Fe bioavailability

An X-ray absorption spectroscopy (XAS) study of the Fe local environment in natural amethyst (a variety of alpha-quartz, SiO(2)) has been carried out. Room temperature measurements were performed at the Fe K-edge (7,112 eV), at both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. Experimental results were then compared with DFT calculations. XANES experimental spectra suggest Fe to occur mainly in the trivalent state, although a fraction of Fe(2+) is identified. EXAFS spectra, on the other hand, reveal an unusual short distance for the first coordination shell: < Fe-O > = 1.78(2) , the coordination number being 2.7(5). These results allow to establish that Fe replaces Si in its tetrahedral site, and that numerous local distortions are occurring as a consequence of the presence of Fe(3+) variably compensated by protons and/or alkaline ions, or uncompensated. The formal valence of Fe, on the basis of both experimental and DFT structural features, can be either 4+ or 3+. Taking into account the XANES evidences, we suggest that Fe mainly occurs in the trivalent state, compensated by protons, and that a minor fraction of Fe(4+) is stabilised by the favourable local structural arrangement.

• 出版日期2010-5