Natural halotrichites – an EDX and Raman spectroscopic study
Raman spectroscopy has been used to characterise four natural halotrichites: halotrichite FeSO4.Al2(SO4)3. 22H2O, apjohnite MnSO4.Al2(SO4)3.22H2O, pickingerite MgSO4.Al2(SO4)3.22H2O and wupatkiite CoSO4.Al2(SO4)3.22H2O. A comparison of the Raman spectra is made with the spectra of the equivalent synthetic pseudo-alums. Energy dispersive X-ray analysis (EDX) was used to determine the exact composition of the minerals. The Raman spectrum of apjohnite and halotrichite display intense symmetric bands at 985 cm-1 assigned to the v1(SO4)2- symmetric stretching mode. For pickingerite and wupatkiite, an intense band at 995 cm-1 is observed. A second band is observed for these minerals at 976 cm-1 attributed to a water librational mode The series of bands for apjohnite at 1104, 1078 and 1054 cm-1, for halotrichite at 1106, 1072 and 1049 cm-1, for pickingerite at 1106, 1070 and 1049 cm-1 and for wupatkiite at 1106, 1075 and 1049 cm-1 are attributed to the v3(SO4)2- antisymmetric stretching modes of v3(Bg) SO4. Raman bands at around 474, 460 and 423 cm-1 are attributed to the v2(Ag) SO4 mode. The band at 618 cm-1 is assigned to the 4(Bg) SO4 mode. The splitting of the v2, v3 and v4 modes is attributed to the reduction of symmetry of the SO4 and it is proposed that the sulphate coordinates to water in the hydrated aluminium in bidentate chelation.
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