Molecular Structure of Segnitite: A Raman Spectroscopic Study

ThermoRaman spectroscopy has been used to study the mineral segnitite PbFe3(AsO4)2(OH)6 a member of the jarosite mineral group. The hot stage Raman spectroscopic technique identifies the temperatures for the dehydration and dehydroxylation as 200 and 400 degrees Celsius. No changes are observed in the spectra attributed to the arsenate unit until after dehydroxylation. The results of Raman spectroscopy are in agreement with the results of thermogravimetric analyses coupled to evolved gas mass spectrometry. The Raman spectrum of segnitite is characterised by an intense band at 860 cm-1 and bands at 848 and 811 cm-1 attributed to the arsenate ν1 and ν3 modes. Bands observed at 998 and 931 cm-1 are assigned to ν1 symmetric stretching vibration of the sulphate and phosphate units. Thermal treatment causes the ν1 band to shift to 850 cm-1 at 400 degrees Celsius. Hydroxyl stretching bands are observed at 3216, 2982, 3467 and 3440 cm-1. These bands are attributed to OH units each coordinating the Fe3+ in octahedral FeO2(OH)4 building blocks. The ν2 and ν4 modes are split for segnitite indicating a lowering of the symmetry of the (AsO4)3- unit.