Molecular structure of the phosphate mineral brazilianite NaAl3(PO4)2(OH)4 – A semi precious jewel

The phosphate mineral brazilianite NaAl3(PO4)2(OH)4 is a semi precious jewel. There are almost no minerals apart from brazilianite which are used in jewellery. Vibrational spectroscopy was used to characterise the molecular structure of brazilianite. Brazilianite is composed of chains of edge-sharing Al–O octahedra linked by P–O tetrahedra, with Na located in cavities of the framework. An intense sharp Raman band at 1019 cm−1 is attributed to the PO 4 3 - symmetric stretching mode. Raman bands at 973 and 988 cm−1 are assigned to the stretching vibrations of the HOPO 3 3 - units. The infrared spectra compliment the Raman spectra but show greater complexity. Multiple Raman bands are observed in the PO 4 3 - and HOPO 3 3 - bending region. This observation implies that both phosphate and hydrogen phosphate units are involved in the structure. Raman OH stretching vibrations are found at 3249, 3417 and 3472 cm−1. These peaks show that the OH units are not equivalent in the brazilianite structure. Vibrational spectroscopy is useful for increasing the knowledge of the molecular structure of brazilianite.