A study of the phosphate mineral kapundaite NaCa(Fe3+)4(PO4)4(OH)3⋅5(H2O) using SEM/EDX and vibrational spectroscopic
Ray L Frost1, Andrés López1, Yunfei Xi1
1School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia.
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 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 ν1PO4(3-) 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 ν1PO4(3-) symmetric stretching mode and ν3PO4(3-) antisymmetric stretching mode. Raman bands in the ν4 out of plane bending modes of the PO4(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.
