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 1012cm−1 assigned to the ν1PO43- symmetric stretching vibrations. A series of low intensity Raman bands at 1047, 1077, 1091 and 1105cm−1 are assigned to the ν3PO43- antisymmetric stretching modes. The Raman bands at 1136, 1155, 1176 and 1190cm−1 are assigned to AlOH deformation modes. The infrared band at 1014cm−1 is ascribed to the PO43- ν1 symmetric stretching vibrational mode. The infrared bands at 1049, 1071, 1091 and 1123cm−1 are attributed to the PO43- ν3 antisymmetric stretching vibrations. The infrared bands at 1123, 1146 and 1157cm−1 are attributed to AlOH deformation modes. Raman bands at 575, 592, 606 and 628cm−1 are assigned to the ν4 out of plane bending modes of the PO43- unit. In the 2600–3800cm−1 spectral range, Raman bands for minyulite are found at 3661, 3669 and 3692cm−1 are assigned to AlOH/AlF stretching vibrations. Broad infrared bands are also found at 2904, 3105, 3307, 3453 and 3523cm−1. Raman bands at 3225, 3324cm−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.