Vibrational spectroscopic characterization of the phosphate mineral hureaulite - (Mn, Fe)5(PO4)2(HPO4)2.4(H2O).

Abstract

This research was done on hureaulite samples from the Cigana claim, a lithium bearing pegmatite with triphylite and spodumene. The mine is located in Conselheiro Pena, east of Minas Gerais. Chemical analysis was carried out by Electron Microprobe analysis and indicated a manganese rich phase with partial substitution of iron. The calculated chemical formula of the studied sample is: (Mn3.23, Fe1.04, Ca0.19, Mg0.13)(PO4)2.7(HPO4)2.6(OH)4.78. The Raman spectrum of hureaulite is dominated by an intense Sharp band at 959 cm−1 assigned to PO stretching vibrations of HPO4 2− units. The Raman band at 989 cm−1 is assigned to the PO4 3− stretching vibration. Raman bands at 1007, 1024, 1047, and 1083 cm−1 are attributed to both the HOP and PO antisymmetric stretching vibrations of HPO4 2− and PO4 3− units. A set of Raman bands at 531, 543, 564 and 582 cm−1 are assigned to the _4 bending modes of the HPO4 2− and PO4 3− units. Raman bands observed at 414, and 455 cm−1 are attributed to the _2 HPO4 2− and PO4 3− units. The intense A series of Raman and infrared bands in the OH stretching region are assigned to water stretching vibrations. Based upon the position of these bands hydrogen bond distances are calculated. Hydrogen bond distances are short indicating very strong hydrogen bonding in the hureaulite structure. A combination of Raman and infrared spectroscopy enabled aspects of the molecular structure of the mineral hureaulite to be understood.