Synthesis and spectroscopic characterization of apjohnite and pickingerite

Raman spectroscopy complimented with infrared ATR spectroscopy has been used to characterise two synthetic halotrichites: apjohnite MnSO4 · Al2(SO4)3 · 22H2O and pickingerite MgSO4 · Al2(SO4)3 · 22H2O. A comparison is made with the spectra of natural pseudo-alums halotrichite, pickingerite and apjohnite. Two Raman bands observed at 995 and 990 cm−1 for apjohnite and a single band at 984 cm−1 for pickingerite are assigned to the ν1(Ag) SO4 symmetric stretching vibrations. Raman bands observed at 1141 and 1080 cm−1 for apjohnite and 1096 and 1058 cm−1 for pickingerite are assigned to the antisymmetric stretching vibration ν3(Bg) SO4. Raman bands at 474, 460 and 423 cm−1 for apjohnite and at 459 and 445 cm−1 for pickingerite are attributed to the ν2(Ag) SO4 mode. The band at 618 cm−1 is assigned to the ν4(Bg) SO4 mode. The splitting of the ν2, ν3 and ν4 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.

Raman and infrared spectroscopy at 298 K has been used to characterise synthetic and natural halotrichites.Figure optionsDownload as PowerPoint slide