The molecular structure of the phosphate mineral senegalite Al2(PO4)(OH)3-3H2O - a vibrational spectroscopic study.

Abstract

We have studied the mineral senagalite, a hydrated hydroxy phosphate of aluminium with formula Al2(-PO4)(OH)3_3H2O using a combination of electron microscopy and vibrational spectroscopy. Senegalite crystal aggregates shows tabular to prismatic habitus and orthorhombic form. The Raman spectrum is dominated by an intense band at 1029 cm_1 assigned to the PO3_ 4 m1 symmetric stretching mode. Intense Raman bands are found at 1071 and 1154 cm_1 with bands of lesser intensity at 1110, 1179 and 1206 cm_1 and are attributed to the PO3_ 4 m3 antisymmetric stretching vibrations. The infrared spectrum shows complexity with a series overlapping bands. A comparison is made with spectra of other aluminium containing phosphate minerals such as augelite and turquoise. Multiple bands are observed for the phosphate bending modes giving support for the reduction of symmetry of the phosphate anion. Vibrational spectroscopy offers a means for the assessment of the structure of senagalite.