Raman spectroscopy of the arsenate minerals maxwellite and in comparison with tilasite

Maxwellite NaFe(3+)(AsO4)F is an arsenate mineral containing fluoride and forms a continuous series with tilasite CaMg(AsO4)F. Both maxwellite and tilasite form a continuous series with durangite NaAl(3+)(AsO4)F. We have used the combination of scanning electron microscopy with EDS and vibrational spectroscopy to chemically analyse the mineral maxwellite and make an assessment of the molecular structure. Chemical analysis shows that maxwellite is composed of Fe, Na and Ca with minor amounts of Mn and Al. Raman bands for tilasite at 851 and 831cm(-1) are assigned to the Raman active ν1 symmetric stretching vibration (A1) and the Raman active triply degenerate ν3 antisymmetric stretching vibration (F2). The Raman band of maxwellite at 871cm(-1) is assigned to the ν1 symmetric stretching vibration and the Raman band at 812cm(-1) is assigned to the ν3 antisymmetric stretching vibration. The intense Raman band of tilasite at 467cm(-1) is assigned to the Raman active triply degenerate ν4 bending vibration (F2). Raman band at 331cm(-1) for tilasite is assigned to the Raman active doubly degenerate ν2 symmetric bending vibration (E). Both Raman and infrared spectroscopy do not identify any bands in the hydroxyl stretching region as is expected.