Vibrational spectroscopic characterization of the sulphate mineral khademite Al(SO4)F⋅5(H2O)

Abstract

Vibrational spectroscopy has been used to characterize the sulphate mineral khademite Al(SO4)F⋅5(H2O). Raman band at 991cm−1 with a shoulder at 975cm−1 is assigned to the ν1 (SO4)2− symmetric stretching mode. The observation of two symmetric stretching modes suggests that the sulphate units are not equivalent. Two low intensity Raman bands at 1104 and 1132cm−1 are assigned to the ν3 (SO4)2− antisymmetric stretching mode. The broad Raman band at 618cm−1 is assigned to the ν4 (SO4)2− bending modes. Raman bands at 455, 505 and 534cm−1 are attributable to the doubly degenerate ν2 (SO4)2− bending modes. Raman bands at 2991, 3146 and 3380cm−1 are assigned to the OH stretching bands of water. Five infrared bands are noted at 2458, 2896, 3203, 3348 and 3489cm−1 are also due to water stretching bands. The observation of multiple water stretching vibrations gives credence to the non-equivalence of water units in the khademite structure. Vibrational spectroscopy enables an assessment of the structure of khademite.