X-ray photoelectron spectroscopic and Raman microscopic investigation of the variscite group minerals: variscite, strengite, scorodite and mansfieldite

Kloprogge, J. Theo and Wood, Barry J. (2017) X-ray photoelectron spectroscopic and Raman microscopic investigation of the variscite group minerals: variscite, strengite, scorodite and mansfieldite. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 185 163-172. doi:10.1016/j.saa.2017.05.042

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Author Kloprogge, J. Theo
Wood, Barry J.
Title X-ray photoelectron spectroscopic and Raman microscopic investigation of the variscite group minerals: variscite, strengite, scorodite and mansfieldite
Journal name Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy   Check publisher's open access policy
ISSN 1386-1425
1873-3557
Publication date 2017-05-30
Sub-type Article (original research)
DOI 10.1016/j.saa.2017.05.042
Open Access Status Not yet assessed
Volume 185
Start page 163
End page 172
Total pages 10
Place of publication Amsterdam, Netherlands
Publisher Elsevier
Language eng
Subject 1602 Analytical Chemistry
3107 Atomic and Molecular Physics, and Optics
3105 Instrumentation
1607 Spectroscopy
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 530.3 eV and the As 3d 5/2 at 43.7 eV belonged to AsO, while minor bands for O 1s at 531.3 eV and As 3d 5/2 at 44.8 eV 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 2.9 eV. The substitution of Fe for Al in the structure of strengite resulted in a Fe 2p at 710.8 eV. An increase in the Fe 2p BE of 4.8 eV was found between mansfieldite and strengite. The scorodite Raman OH-stretching region showed a sharp band at 3513 cm and a broad band around 3082 cm. The spectrum of mansfieldite was like that of scorodite with a sharp band at 3536 cm and broader maxima at 3100 cm and 2888 cm. Substituting Al in the arsenate structure instead of Fe resulted in a shift of the metal-OH-stretching mode by 23 cm towards higher wavenumbers due to a slightly longer H-bonding in mansfieldite compared to scorodite. The intense band for scorodite at 805 cm was ascribed to the symmetric stretching mode of the AsO. The medium intensity bands at 890, 869, and 830 cm were ascribed to the internal modes. A significant shift towards higher wavenumbers was observed for mansfieldite. The strengite Raman spectrum in the 900–1150 cm shows a strong band at 981 cm accompanied by a series of less intense bands. The 981 cm band was assigned to the PO symmetric stretching mode, while the weak band at 1116 cm was the corresponding antisymmetric stretching mode. The remaining bands at 1009, 1023 and 1035 cm 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 1030 cm and was assigned to the symmetric stretching mode of the PO, while the corresponding antisymmetric stretching mode was observed at 1080 cm. Due to the band splitting component bands were observed at 1059, 1046, 1013 and 940 cm. The AsO symmetric bending modes for scorodite were observed at 381 and 337 cm, while corresponding antisymmetric bending modes occurred at 424, 449 and 484 cm. 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.
Formatted 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 530.3 eV and the As 3d 5/2 at 43.7 eV belonged to AsO4, while minor bands for O 1s at 531.3 eV and As 3d 5/2 at 44.8 eV 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 2.9 eV. The substitution of Fe3 + for Al3 + in the structure of strengite resulted in a Fe 2p at 710.8 eV. An increase in the Fe 2p BE of 4.8 eV was found between mansfieldite and strengite. The scorodite Raman OH-stretching region showed a sharp band at 3513 cm− 1 and a broad band around 3082 cm− 1. The spectrum of mansfieldite was like that of scorodite with a sharp band at 3536 cm− 1 and broader maxima at 3100 cm− 1 and 2888 cm− 1. Substituting Al in the arsenate structure instead of Fe resulted in a shift of the metal-OH-stretching mode by 23 cm− 1 towards higher wavenumbers due to a slightly longer H-bonding in mansfieldite compared to scorodite. The intense band for scorodite at 805 cm− 1 was ascribed to the symmetric stretching mode of the AsO4. The medium intensity bands at 890, 869, and 830 cm− 1 were ascribed to the internal modes. A significant shift towards higher wavenumbers was observed for mansfieldite. The strengite Raman spectrum in the 900–1150 cm− 1 shows a strong band at 981 cm− 1 accompanied by a series of less intense bands. The 981 cm− 1 band was assigned to the PO4 symmetric stretching mode, while the weak band at 1116 cm− 1 was the corresponding antisymmetric stretching mode. The remaining bands at 1009, 1023 and 1035 cm− 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 1030 cm− 1 and was assigned to the symmetric stretching mode of the PO4, while the corresponding antisymmetric stretching mode was observed at 1080 cm− 1. Due to the band splitting component bands were observed at 1059, 1046, 1013 and 940 cm− 1. The AsO4 symmetric bending modes for scorodite were observed at 381 and 337 cm− 1, while corresponding antisymmetric bending modes occurred at 424, 449 and 484 cm− 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.
Keyword Raman spectroscopy
Scorodite
Strengite
Variscite
Mansfieldite
X-ray Photoelectron Spectroscopy
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: HERDC Pre-Audit
Centre for Microscopy and Microanalysis Publications
School of Earth and Environmental Sciences
 
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Created: Tue, 30 May 2017, 15:26:11 EST by Ashleigh Paroz on behalf of School of Earth and Environmental Sciences