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

Abstract

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 m1 symmetric stretching vibration (A1) and the Raman active triply degenerate m3 antisymmetric stretching vibration (F2). The Raman band of maxwellite at 871 cm_1 is assigned to the m1 symmetric stretching vibration and the Raman band at 812 cm_1 is assigned to the m3 antisymmetric stretching vibration. The intense Raman band of tilasite at 467 cm_1 is assigned to the Raman active triply degenerate m4 bending vibration (F2). Raman band at 331 cm_1 for tilasite is assigned to the Raman active doubly degenerate m2 symmetric bending vibration (E). Both Raman and infrared spectroscopy do not identify any bands in the hydroxyl stretching region as is expected.