A study of the phosphate mineral kapundaite NaCa(Fe3+)4(PO4)4(OH)3 5(H2O) using SEM/EDX and vibrational spectroscopic methods.

Abstract

Vibrational spectroscopy enables subtle details of the molecular structure of kapundaite to be determined. Single crystals of a pure phase from a Brazilian pegmatite were used. Kapundaite is the Fe3+ member of the wardite group. The infrared and Raman spectroscopy were applied to compare the structure of kapundaite with wardite. The Raman spectrum of kapundaite in the 800–1400 cm_1 spectral range shows two intense bands at 1089 and 1114 cm_1 assigned to the m1 PO3_ 4 symmetric stretching vibrations. The observation of two bands provides evidence for the non-equivalence of the phosphate units in the kapundaite structure. The infrared spectrum of kapundaite in the 500–1300 cm_1 shows much greater complexity than the Raman spectrum. Strong infrared bands are found at 966, 1003 and 1036 cm_1 and are attributed to the m1 PO3_ 4 symmetric stretching mode and m3 PO3_ 4 antisymmetric stretching mode. Raman bands in the m4 out of plane bending modes of the PO3_ 4 unit support the concept of non-equivalent phosphate units in the kapundaite structure. In the 2600–3800 cm_1 spectral range, Raman bands for kapundaite are found at 2905, 3151, 3311, 3449 and 3530 cm_1. These bands are broad and are assigned to OH stretching vibrations. Broad infrared bands are also found at 2904, 3105, 3307, 3453 and 3523 cm_1 and are attributed to water. Raman spectroscopy complimented with infrared spectroscopy has enabled aspects of the structure of kapundaite to be ascertained and compared with that of other phosphate minerals.