A vibrational spectroscopic study of the phosphate mineral whiteite CaMn++Mg2Al2(PO4)4(OH)2 8(H2O).

Abstract

Vibrational spectroscopy enables subtle details of the molecular structure of whiteite to be determined. Single crystals of a pure phase from a Brazilian pegmatite were used. The infrared and Raman spectroscopy were applied to compare the molecular structure of whiteite with that of other phosphate minerals. The Raman spectrum of whiteite shows an intense band at 972 cm_1 assigned to the m1 PO3_ 4 symmetric stretching vibrations. The low intensity Raman bands at 1076 and 1173 cm_1 are assigned to the m3 PO3_ 4 antisymmetric stretching modes. The Raman bands at 1266, 1334 and 1368 cm_1 are assigned to AlOH deformation modes. The infrared band at 967 cm_1 is ascribed to the PO3_ 4 m1 symmetric stretching vibrational mode. The infrared bands at 1024, 1072, 1089 and 1126 cm_1 are attributed to the PO3_ 4 m3 antisymmetric stretching vibrations. Raman bands at 553, 571 and 586 cm_1 are assigned to the m4 out of plane bending modes of the PO3_ 4 unit. Raman bands at 432, 457, 479 and 500 cm_1 are attributed to the m2 PO4 and H2PO4 bending modes. In the 2600 to 3800 cm_1 spectral range, Raman bands for whiteite are found 3426, 3496 and 3552 cm_1 are assigned to AlOH stretching vibrations. Broad infrared bands are also found at 3186 cm_1. Raman bands at 2939 and 3220 cm_1 are assigned to water stretching vibrations. Raman spectroscopy complimented with infrared spectroscopy has enabled aspects of the structure of whiteite to be ascertained and compared with that of other phosphate minerals.