Assessment of the molecular structure of natrodufrénite – , 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( Fe 0.72 2 + Mn0.11Mg0.08Ca0.04Zr0.01Cu0.01)∑0.97( Fe 4.89 3 + 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 PO 4 3 - ν1 symmetric stretching mode. Raman bands are observed at 1059 and 1118 cm−1 and are attributed to the PO 4 3 - 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 ν4 PO 4 3 - bending modes and Raman bands at 425, 444, 477 and 507 cm−1 are due to the ν2 PO 4 3 - 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.