Vibrational spectroscopy of the phosphate mineral lazulite - (Mg, Fe)Al2(PO4)2 (OH)2 found in the Minas Gerais, Brazil.

Abstract

This research was done on lazulite samples from the Gentil mine, a lithium bearing pegmatite located in the municipality of Mendes Pimentel, Minas Gerais, Brazil. Chemical analysis was carried out by electron microprobe analysis and indicated a magnesium rich phase with partial substitution of iron. Traces of Ca and Mn, (which partially replaced Mg) were found. The calculated chemical formula of the studied sample is: (Mg0.88, Fe0.11)Al1.87(PO4)2.08(OH)2.02. The Raman spectrum of lazulite is dominated by an intense Sharp band at 1060 cm_1 assigned to PO stretching vibrations of of tetrahedral [PO4] clusters presents into the HPO2_ 4 units. Two Raman bands at 1102 and 1137 cm_1 are attributed to both the HOP and PO antisymmetric stretching vibrations. The two infrared bands at 997 and 1007 cm_1 are attributed to the m1 PO3_ 4 symmetric stretching modes. The intense bands at 1035, 1054, 1081, 1118 and 1154 cm_1 are assigned to the m3 PO3_ 4 antisymmetric stretching modes from both the HOP and tetrahedral [PO4] clusters. A set of Raman bands at 605, 613, 633 and 648 cm_1 are assigned to the m4 out of plane bending modes of the PO4, HPO4 and H2PO4 units. Raman bands observed at 414, 425, 460, and 479 cm_1 are attributed to the m2 tetrahedral PO4 clusters, HPO4 and H2PO4 bending modes. The intense Raman band at 3402 and the infrared band at 3403 cm_1 are assigned to the stretching vibration of the OH units. A combination of Raman and infrared spectroscopy enabled aspects of the molecular structure of the mineral lazulite to be understood.