X-ray Photoelectron Spectroscopic and Raman microscopic investigation of the variscite group minerals: Variscite, strengite, scorodite and mansfieldite

Abstract

Several structurally related AsO4 and PO4 minerals, were studied with Raman microscopy and X-ray Photoelectron Spectroscopy (XPS). XPS revealed only Fe, As and O for scorodite. The Fe 2p, As 3d, and O 1s indicated one position for Fe2+, while 2 different environments for O and As were observed. The O 1s at 530.3eV and the As 3d 5/2 at 43.7eV belonged to AsO4, while minor bands for O 1s at 531.3eV and As 3d 5/2 at 44.8eV were due to AsO4 groups exposed on the surface possibly forming OH-groups. Mansfieldite showed, besides Al, As and O, a trace of Co. The PO4 equivalent of mansfieldite is variscite. The change in crystal structure replacing As with P resulted in an increase in the binding energy (BE) of the Al 2p by 2.9eV. The substitution of Fe3+ for Al3+ in the structure of strengite resulted in a Fe 2p at 710.8eV. An increase in the Fe 2p BE of 4.8eV was found between mansfieldite and strengite. The scorodite Raman OH-stretching region showed a sharp band at 3513cm−1 and a broad band around 3082cm−1. The spectrum of mansfieldite was like that of scorodite with a sharp band at 3536cm−1 and broader maxima at 3100cm−1 and 2888cm−1. Substituting Al in the arsenate structure instead of Fe resulted in a shift of the metal-OH-stretching mode by 23cm−1 towards higher wavenumbers due to a slightly longer H-bonding in mansfieldite compared to scorodite. The intense band for scorodite at 805cm−1 was ascribed to the symmetric stretching mode of the AsO4. The medium intensity bands at 890, 869, and 830cm−1 were ascribed to the internal modes. A significant shift towards higher wavenumbers was observed for mansfieldite. The strengite Raman spectrum in the 900–1150cm−1 shows a strong band at 981cm−1 accompanied by a series of less intense bands. The 981cm−1 band was assigned to the PO4 symmetric stretching mode, while the weak band at 1116cm−1 was the corresponding antisymmetric stretching mode. The remaining bands at 1009, 1023 and 1035cm−1 were assigned to υ1(A1) internal modes in analogy to the interpretation of the AsO4 bands for scorodite and mansfieldite. The variscite spectrum showed a shift towards higher wavenumbers in comparison to the strengite spectrum with the strongest band observed at 1030cm−1 and was assigned to the symmetric stretching mode of the PO4, while the corresponding antisymmetric stretching mode was observed at 1080cm−1. Due to the band splitting component bands were observed at 1059, 1046, 1013 and 940cm−1. The AsO4 symmetric bending modes for scorodite were observed at 381 and 337cm−1, while corresponding antisymmetric bending modes occurred at 424, 449 and 484cm−1. Comparison with other arsenate and phosphate minerals showed that both XPS and Raman spectroscopy are fast and non-destructive techniques to identify these minerals based on their differences in chemistry and the arsenate/phosphate vibrational modes due to changes in the symmetry and the unique fingerprint region of the lattice modes.