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 972cm−1 assigned to the ν1PO43- symmetric stretching vibrations. The low intensity Raman bands at 1076 and 1173cm−1 are assigned to the ν3PO43- antisymmetric stretching modes. The Raman bands at 1266, 1334 and 1368cm−1 are assigned to AlOH deformation modes. The infrared band at 967cm−1 is ascribed to the PO43-ν1 symmetric stretching vibrational mode. The infrared bands at 1024, 1072, 1089 and 1126cm−1 are attributed to the PO43-ν3 antisymmetric stretching vibrations. Raman bands at 553, 571 and 586cm−1 are assigned to the ν4 out of plane bending modes of the PO43- unit. Raman bands at 432, 457, 479 and 500cm−1 are attributed to the ν2 PO4 and H2PO4 bending modes. In the 2600 to 3800cm−1 spectral range, Raman bands for whiteite are found 3426, 3496 and 3552cm−1 are assigned to AlOH stretching vibrations. Broad infrared bands are also found at 3186cm−1. Raman bands at 2939 and 3220cm−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.