A study of the phosphate mineral kapundaite NaCa(Fe~(3+))_4(PO_4)_4 (OH)_3·5(H_2O) using SEM/EDX and vibrational spectroscopic methods

摘要

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 Fe~(3+) 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 v_1 PO_3~(-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 v_1 PO_4~(3-) symmetric stretching mode and m3 PO_4~(3-) antisymmetric stretching mode. Raman bands in the v_4 out of plane bending modes of the PO_4~(3-) 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.