Vibrational spectroscopy of the phosphate mineral lazulite – (Mg, Fe)Al2(PO4)2·(OH)2 found in the Minas Gerais, Brazil

Abstract

This research was done on lazulite samples from the Gentil mine, a lithium bearing pegmatite located in the municipality of Mendes Pimentel, Minas Gerais, Brazil. Chemical analysis was carried out by electron microprobe analysis and indicated a magnesium rich phase with partial substitution of iron. Traces of Ca and Mn, (which partially replaced Mg) were found. The calculated chemical formula of the studied sample is: (Mg0.88, Fe0.11)Al1.87(PO4)2.08(OH)2.02. The Raman spectrum of lazulite is dominated by an intense sharp band at 1060 cm(-1) assigned to PO stretching vibrations of of tetrahedral [PO4] clusters presents into the HPO4(2-) units. Two Raman bands at 1102 and 1137 cm(-1) are attributed to both the HOP and PO antisymmetric stretching vibrations. The two infrared bands at 997 and 1007 cm(-1) are attributed to the ν1PO4(3-) symmetric stretching modes. The intense bands at 1035, 1054, 1081, 1118 and 1154 cm(-1) are assigned to the ν3PO4(3-) antisymmetric stretching modes from both the HOP and tetrahedral [PO4] clusters. A set of Raman bands at 605, 613, 633 and 648 cm(-1) are assigned to the ν4 out of plane bending modes of the PO4, HPO4 and H2PO4 units. Raman bands observed at 414, 425, 460, and 479 cm(-1) are attributed to the ν2 tetrahedral PO4 clusters, HPO4 and H2PO4 bending modes. The intense Raman band at 3402 and the infrared band at 3403 cm(-1) are assigned to the stretching vibration of the OH units. A combination of Raman and infrared spectroscopy enabled aspects of the molecular structure of the mineral lazulite to be understood.

H-bonding in lazulite: a single-crystal neutron diffraction study at 298 and 3 K

“…The H-bonding scheme in the structure of lazulite here described is compatible with the findings based on infrared and Raman spectroscopies (e.g., Frost et al 2013, RRUFF database: http://rruff.info/Lazulite), as IR and Raman spectra, in the region of the OH stretching active modes, show evidence of more than one unique H-bond. The description of the IR and Raman mode in lazulite provided by Frost et al (2013) is likely affected by misinterpretation: the structure does not contain HPO 4 or H 2 PO 4 units, and, in addition, the conclusion "The proton on the hydroxyl units is apparently very mobile and enables the formation of the monohydrogen and dihydrogen phosphate units" (Frost et al 2013) is inconsistent with our experimental findings and, in general, not plausible. We expect that a more robust description of the active IR and Raman vibrational modes will be delivered on the basis of the structure models of this study.…”

“…1). However, in a more recent paper, based on infrared and Raman investigations of lazulite, Frost et al (2013) reported evidence of bending modes ascribed to tetrahedral PO 4 , HPO 4 and H 2 PO 4 units. These experimental findings are not consistent with the structural model reported by Lindberg and Christ (1959) and Giuseppetti and Tadini (1983), in which HPO 4 and H 2 PO 4 groups are supposed not to occur.…”

“…These experimental findings are not consistent with the structural model reported by Lindberg and Christ (1959) and Giuseppetti and Tadini (1983), in which HPO 4 and H 2 PO 4 groups are supposed not to occur. In addition, on the basis of their data, Frost et al (2013) concluded that the proton on the hydroxyl units is apparently very mobile, promoting the formation of the monohydrogen and dihydrogen phosphate units.…”

“…In order to answer to the open questions about the structure of lazulite, and in the framework of a long-term project on the crystal-chemistry of hydrous phosphates (e.g., Gatta et al 2013aGatta et al , 2013bGatta et al , 2014aGatta et al , 2014bGatta et al , 2015Rotiroti et al 2016), we have reinvestigated the crystal chemistry of lazulite by electron microprobe analysis in wavelength-dispersive mode (EMPA) and single-crystal neutron diffraction, in order to provide: i) the reliable location of the proton site(s) and the real configuration of the OH-group(s), for a full description of the H-bonds; ii) the anisotropic displacement parameters of all the atomic sites, H-site(s) included. In order to reduce the thermal displacement of the H-sites, and to confirm or deny the assumption on the proton mobility reported by Frost et al (2013), single-crystal neutron diffraction data were collected at room temperature (298 K) and at low temperature (3 K).…”

Design, synthesis, structural characterization and cell cytotoxicity of a new derivative poly(maleic anhydride-co-vinyl acetate)/miltefosine polymer/drug conjugate

“…1394.31 cm -1 could be associated with MF because the same peak was also observed on pure MF spectrum. Peak 1066.28 cm -1 belongs to PO4 3-antisymmetric stretching frequency [37][38][39][40][41][42][43], 117267 cm -1 and 1052.22 cm -1 were also assigned to PO4 3antisymmetric stretching movement [44]. According to literature, Breitinger and co-workers emphasized that the deceptively simple strong vibrations centered at 1059 cm -1 including minimum four components of inorganic phosphate (PO4), were produced by removal of the threefold degeneracy of PO4 and activation of the general mode of phosphate (PO4) [44].…”