A vibrational spectroscopic study of the phosphate mineral cyrilovite Na(Fe3+)3(PO4)2(OH)4·2(H2O) and in comparison with wardite

Abstract

Vibrational spectroscopy enables subtle details of the molecular structure of cyrilovite to be determined. Single crystals of a pure phase from a Brazilian pegmatite were used. Cyrilovite is the Fe3+ member of the wardite group. The infrared and Raman spectroscopy were applied to compare the structure of cyrilovite with that of wardite. The Raman spectrum of cyrilovite in the 800–1400cm−1 spectral range shows two intense bands at 992 and 1055cm−1 assigned to the ν1PO43- symmetric stretching vibrations. A series of low intensity bands at 1105, 1136, 1177 and 1184cm−1 are assigned to the ν3PO43- antisymmetric stretching modes. The infrared spectrum of cyrilovite in the 500–1300cm−1 shows much greater complexity than the Raman spectrum. Strong infrared bands are found at 970 and 1007cm−1 and are attributed to the ν1PO43- symmetric stretching mode. Raman bands are observed at 612 and 631cm−1 and are assigned to the ν4 out of plane bending modes of the PO43- unit. In the 2600–3800cm−1 spectral range, intense Raman bands for cyrilovite are found at 3328 and 3452cm−1 with a broad shoulder at 3194cm−1 and are assigned to OH stretching vibrations. Sharp infrared bands are observed at 3485 and 3538cm−1. Raman spectroscopy complimented with infrared spectroscopy has enabled the structure of cyrilovite to be ascertained and compared with that of wardite.