A vibrational spectroscopic study of the phosphate mineral minyulite KAl2(OH,F)(PO4)2 4(H2O) and in comparison with wardite.

Abstract

Vibrational spectroscopy enables subtle details of the molecular structure of minyulite KAl2(OH,F)(PO4)2_ 4(H2O). Single crystals of a pure phase from a Brazilian pegmatite were used. Minyulite belongs to the orthorhombic crystal system. This indicates that it has three axes of unequal length, yet all are perpendicular to each other. The infrared and Raman spectroscopy were applied to compare the structure of minyulite with wardite. The reason for the comparison is that both are Al containing phosphate minerals. The Raman spectrum of minyulite shows an intense band at 1012 cm_1 assigned to the m1 PO3_ 4 symmetric stretching vibrations. A series of low intensity Raman bands at 1047, 1077, 1091 and 1105 cm_1 are assigned to the m3 PO3_ 4 antisymmetric stretching modes. The Raman bands at 1136, 1155, 1176 and 1190 cm_1 are assigned to AlOH deformation modes. The infrared band at 1014 cm_1 is ascribed to the PO3_ 4 m1 symmetric stretching vibrational mode. The infrared bands at 1049, 1071, 1091 and 1123 cm_1 are attributed to the PO3_ 4 m3 antisymmetric stretching vibrations. The infrared bands at 1123, 1146 and 1157 cm_1 are attributed to AlOH deformation modes. Raman bands at 575, 592, 606 and 628 cm_1 are assigned to the m4 out of plane bending modes of the PO3_ 4 unit. In the 2600– 3800 cm_1 spectral range, Raman bands for minyulite are found at 3661, 3669 and 3692 cm_1 are assigned to AlOH/AlF stretching vibrations. Broad infrared bands are also found at 2904, 3105, 3307, 3453 and 3523 cm_1. Raman bands at 3225, 3324 cm_1 are assigned to water stretching vibrations. A comparison is made with the vibrational spectra of wardite. Raman spectroscopy complimented with infrared spectroscopy has enabled aspects of the structure of minyulite to be ascertained and compared with that of other phosphate minerals.