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

•We have studied the arsenate mineral maxwellite NaFe3+(AsO4)F.•A comparison is made with tilasite CaMg(AsO4)F.•Chemical analysis shows that maxwellite is composed of Fe, Na and Ca with minor amounts of Mn and Al.•Raman and infrared bands are assigned.•The molecular structure of maxwellite is assessed.

Maxwellite NaFe3+(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 NaAl3+(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 831 cm−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 871 cm−1 is assigned to the ν1 symmetric stretching vibration and the Raman band at 812 cm−1 is assigned to the ν3 antisymmetric stretching vibration. The intense Raman band of tilasite at 467 cm−1 is assigned to the Raman active triply degenerate ν4 bending vibration (F2). Raman band at 331 cm−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.

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