Assessment of the molecular structure of natrodufrénite - NaFe2+Fe53+(PO4)4(OH)6·2(H2O), a secondary pegmatite phosphate mineral from Minas Gerais, Brazil

The mineral natrodufrénite a secondary pegmatite phosphate mineral from Minas Gerais, Brazil, has been studied by a combination of scanning electron microscopy and vibrational spectroscopic techniques. Electron probe analysis shows the formula of the studied mineral as (Na0.88Ca0.12)∑1.00(Fe0.722+Mn0.11Mg0.08Ca0.04Zr0.01Cu0.01)∑0.97(Fe4.893+Al0.02)∑4.91(PO4)3.96(OH6.15F0.07)6.22⋅2.05(H2O). Raman spectroscopy identifies an intense peak at 1003 cm−1 assigned to the PO43- ν1 symmetric stretching mode. Raman bands are observed at 1059 and 1118 cm−1 and are attributed to the PO43- ν3 antisymmetric stretching vibrations. A comparison is made with the spectral data of other hydrate hydroxy phosphate minerals including cyrilovite and wardite. Raman bands at 560, 582, 619 and 668 cm−1 are assigned to the ν4PO43- bending modes and Raman bands at 425, 444, 477 and 507 cm−1 are due to the ν2PO43- bending modes. Raman bands in the 2600-3800 cm−1 spectral range are attributed to water and OH stretching vibrations. Vibrational spectroscopy enables aspects of the molecular structure of natrodufrénite to be assessed.