A Raman spectroscopic study of the phosphate mineral pyromorphite Pb5(PO4)3Cl

A series of selected pyromorphite minerals Pb-5(PO4)(3)Cl from different Australian localities has been studied by Raman spectroscopy complemented with selected infrared spectroscopy. The Raman spectrum of unsubstituted pyromorphite shows a single band at around 920 cm(-1) but for the natural minerals two bands at 919 and similar to 932 cm(-1) attributed to the nu(1) (PO4)(3-) stretching vibration. The observation of multiple bands is attributed to the non-equivalence of phosphate units in the pyromorphite structure and the reduction in symmetry of the (PO4)(3-) units. This symmetry reduction is confirmed by the observation of multiple bands in both the nu(4) bending region (500595 cm(-1)) and the nu(2) bending region (350-500 cm(-1)). The presence of isomorphic substitution of (PO4)(3-) by (AsO4)(3-) units is identified by the nu(1) symmetric stretching bands at around 824 and 851 cm(-1) and the nu(2) bending region around 331 and 354 cm(-1). Contrary to expectation Raman bands in the 3320-3700 cm(-1) region are observed and assigned to OH stretching bands of OH units resulting from the substitution of chloride anions in the pyromorphite structure. This study brings in to question the actual formula of natural pyromorphite as it is better represented as Pb-5(PO4,AsO4)(3)(Cl,OH)center dot xH(2)O. (c) 2007 Elsevier Ltd. All rights reserved.