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

A series of selected pyromorphite minerals Pb5(PO4)3Cl 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 ∼932 cm−1 attributed to the ν1 (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 ν4 bending region (500–595 cm−1) and the ν2 bending region (350–500 cm−1). The presence of isomorphic substitution of (PO4)3− by (AsO4)3− units is identified by the ν1 symmetric stretching bands at around 824 and 851 cm−1 and the ν2 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 Pb5(PO4,AsO4)3(Cl,OH) · xH2O.

A comparison of Raman spectra of pyromorphites with published infrared and Raman data of similar minerals has been made. Crystals of the pyromorphite–mimetite series are chemically zoned, resulting in changes in the crystal structure with the isomorphic replacement of phosphate by arsenate with the consequential change in crystal structure from monoclinic to hexagonal. Such dimorphic relationships are exemplified by the shift in the Raman spectral band positions of phosphate with increasing arsenate substitution.Figure optionsDownload as PowerPoint slide