Raman microscopy of some natural pseudo‐alums: halotrichite, apjohnite and wupatkiite, at 298 and 77 K

Abstract

The Raman spectra of some common naturally occurring pseudo-alums, wupatkiite [CoSO4·Al2(SO4)3·22H2O], apjohnite [MnSO4·Al2(SO4)3·22H2O] and halotrichite [FeSO4·Al2(SO4)3·22H2O] were obtained at 298 and 77 K. Bands were observed at 996 and 990 cm−1 and assigned to the ν1(Ag) SO4 vibration. A band was observed at 972 cm−1 attributed to the water librational mode. Bands at 1134, 1115, 1085 and 1065 cm−1 were attributed to the antisymmetric stretching vibration ν3(Bg) SO4. A single band at 454 cm−1 attributed to the ν2(Ag) SO4 mode split into three bands at 468, 424 and 390 cm−1 in the 77 K spectrum. The band at 618 cm−1 assigned to the ν4(Bg) SO4 mode resolved into three bands at 623, 603 and 590 cm−1 at 77 K. The splitting of the bands is attributed to the reduction of symmetry of the sulphate ion due to one sulphate oxygen atom being bonded to the M2+ cation as a unidenate ion. Copyright © 2000 John Wiley & Sons, Ltd.