A Raman and infrared spectroscopic characterisation of the phosphate mineral phosphohedyphane Ca(2)Pb(3)(PO(4))(3)Cl from the Roote mine, Nevada, USA

Abstract

Phosphohedyphane Ca(2)Pb(3)(PO(4))(3)Cl is rare Ca and Pb phosphate mineral that belongs to the apatite supergroup. We have analysed phosphohedyphane using SEM with EDX, and Raman and infrared spectroscopy. The chemical analysis shows the presence of Pb, Ca, P and Cl and the chemical formula is expressed as Ca(2)Pb(3)(PO(4))(3)Cl. The very sharp Raman band at 975cm(-)(1) is assigned to the PO4(3-)ν1 symmetric stretching mode. Raman bands noted at 1073, 1188 and 1226cm(-)(1) are to the attributed to the PO4(3-)ν3 antisymmetric stretching modes. The two Raman bands at 835 and 812cm(-)(1) assigned to the AsO4(3-)ν1 symmetric stretching vibration and AsO4(3-)ν3 antisymmetric stretching modes prove the substitution of As for P in the structure of phosphohedyphane. A series of bands at 557, 577 and 595cm(-)(1) are attributed to the ν4 out of plane bending modes of the PO4 units. The multiplicity of bands in the ν2, ν3 and ν4 spectral regions provides evidence for the loss of symmetry of the phosphate anion in the phosphohedyphane structure. Observed bands were assigned to the stretching and bending vibrations of phosphate tetrahedra. Some Raman bands attributable to OH stretching bands were observed, indicating the presence of water and/or OH units in the structure.

Keywords: Arsenate; Hedyphane; Hydroxyl; Phosphate; Phosphohedyphane; Raman spectroscopy.