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Spectrochim Acta A Mol Biomol Spectrosc. 2013 Dec;116:204-8. doi: 10.1016/j.saa.2013.07.015. Epub 2013 Jul 23.

Raman and infrared spectroscopic study of the mineral goyazite SrAl3(PO4)2(OH)5·H2O.

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1
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia.

Abstract

We have studied the mineral goyazite using Raman and infrared spectroscopy. Goyazite is a member of the crandallite subgroup of the alunite supergroup. The crystal structure is of the alunite-type and consists of sheets of corner-sharing AlO6 octahedra parallel to (0001). The octahedrally coordinated Sr(2+) cations occupy cavities between pairs of octahedral sheets and are surrounded by six oxygen atoms from the Al(3+)O6 octahedra. The very intense sharp band at 983 cm(-1) is assigned to the ν1PO4(3-) symmetric stretching mode. The observation of a single band supports the concept that all the phosphate units are equivalent in the structure of goyazite. Raman bands observed at 1029 cm(-1) and 1037 cm(-1) are assigned to the ν3PO4(3-) antisymmetric stretching vibrations. Two Raman bands at 895 and 927 cm(-1) are attributed to the stretching vibrations of H2PO4; thus indicating some hydrogen phosphate units in the structure of goyazite. Raman bands at 556, 581, 596 and 612 cm(-1) are assigned to the ν4PO4(3-) bending modes, suggesting a reduction of symmetry of phosphate units. Two sharp Raman bands at 3609 and 3631 cm(-1) are attributed to OH stretching vibrations from the goyazite hydroxyl units. Broad Raman bands at 2924, 3043, 3210, 3429 and 3511 cm(-1) are assigned to water stretching vibrations. This research shows that from a vibrational spectroscopic point of view, the formula SrAl3(H PO4,PO4)2(OH)6 · H2O is a better formulation for the mineral goyazite. Vibrational spectroscopy enables subtle details of the molecular structure of goyazite to be determined.

KEYWORDS:

Crandallite; Goyazite; Phosphate; Raman spectroscopy

PMID:
23933557
DOI:
10.1016/j.saa.2013.07.015
[Indexed for MEDLINE]
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