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

Abstract

Several structurally related AsO and PO 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 Fe, 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 AsO, while minor bands for O 1s at 531.3eV and As 3d 5/2 at 44.8eV were due to AsO groups exposed on the surface possibly forming OH-groups. Mansfieldite showed, besides Al, As and O, a trace of Co. The PO 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 Fe for Al 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 and a broad band around 3082cm. The spectrum of mansfieldite was like that of scorodite with a sharp band at 3536cm and broader maxima at 3100cm and 2888cm. Substituting Al in the arsenate structure instead of Fe resulted in a shift of the metal-OH-stretching mode by 23cm towards higher wavenumbers due to a slightly longer H-bonding in mansfieldite compared to scorodite. The intense band for scorodite at 805cm was ascribed to the symmetric stretching mode of the AsO. The medium intensity bands at 890, 869, and 830cm were ascribed to the internal modes. A significant shift towards higher wavenumbers was observed for mansfieldite. The strengite Raman spectrum in the 900-1150cm shows a strong band at 981cm accompanied by a series of less intense bands. The 981cm band was assigned to the PO symmetric stretching mode, while the weak band at 1116cm was the corresponding antisymmetric stretching mode. The remaining bands at 1009, 1023 and 1035cm were assigned to υ(A) internal modes in analogy to the interpretation of the AsO 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 and was assigned to the symmetric stretching mode of the PO, while the corresponding antisymmetric stretching mode was observed at 1080cm. Due to the band splitting component bands were observed at 1059, 1046, 1013 and 940cm. The AsO symmetric bending modes for scorodite were observed at 381 and 337cm, while corresponding antisymmetric bending modes occurred at 424, 449 and 484cm. 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.

Mineralogy and Weathering of Realgar-Rich Tailings At a Former As-Sb-Cr Mine At Lojane, North Macedonia

“…S3) by the appearance of two Raman bands at 464 and 527 cm -1 , characteristic for the Sb 5þ -O antisymmetric and symmetric stretching vibrations, respectively (Bahfenne & Frost 2010a). Other Raman bands (given in italics) correspond quite well with the Raman spectra reported for natural scorodites (Filippi et al 2007, Kloprogge & Wood 2017 with major bands at~180 (181),~799 (804), and~893 (890) cm -1 . The very strong bands at~800 and~900 cm -1 are assigned to As-O stretching in scorodite (Savage et al 2005).…”

Enhancement of the piezoelectric effect in Fe-substituted GaAsO4: A combined XRD, Raman spectroscopy and first principles study

“…The AsO4 3groups occupy the C1 site symmetry in the crystal and the corresponding factor group analysis theoretically yields 9A modes, altogether 36 Raman active modes classified as Γ(AsO4 3-) = 9(Ag(Ra) + B1g(Ra) + B2g(Ra) + Au(Silent) + B1u(IR) +B2u(IR) + B3u(IR)). According to previous related research work 32,33 , the arsenate bending and stretching vibrations modes of the gallium(III) arsenate dihydrate (GaAsO4,2H2O) and iron(III) arsenate dihydrate (FeAsO4,2H2O, scorodite) can all be distinctively observed in the Raman spectrum between 327 and 484 cm -1 and the region from 800 to 900 cm -1 , respectively. However, the position and intensity of their Raman lines also exhibit obvious differences in these two regions.…”

“…45, respectively, which correspond to reflections of monoclinic FeAsO4 (See SI). This may suggest that the monoclinic FeAsO4 phase found by XRD analysis is mainly derived from the dehydration of the corresponding hydrated phase, which loses its structural water during the higher temperature treatment and finally leads to a nucleation and growth decomposition mechanism 31,32 . At the same time, it is evident that all diffraction peaks shift to lower 2θ values with increasing x in Ga1-…”

Machine learning algorithms applied to Raman spectra for the identification of variscite originating from the mining complex of Gavà

“…http://orcid.org/0000-0003-1676-4438 Phosphate bending [39] 392 0.233489443 397 0.273253664 PO 4 out of plane bends [38] phosphate bending [39] 452 0.258777425 465 0.215060576 ν 1 symmetric stretching mode [1,38,39] 982 [1,38] ;…”

“…It is sensitive to structural and compositional mineralogical variations by the observation of changes in Raman band positions or shapes such as width, intensity, and the presence of shoulders. Solid solutions, such as variscite/strengite, as well as polymorphous minerals (variscite/metavariscite) can be detected by changes in their Raman spectra [1,3,7] ; these changes can be quite subtle when the variations in the mineral composition or structure are also small. Furthermore, the reported differences in colour observed in variscite specimens could be related to small compositional and structural changes which may result in only minor changes in the shape or shift of the Raman bands.…”