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

Abstract

Spectral properties as a function of composition are analysed for a series of selected pyromorphite minerals of Australian origin. The minerals are characterised by d-d transitions in NIR from 12000 to 8000 cm-1 (0.83-1.25 µm). A broad signal observed at ~ 10000 cm-1 (1.00 µm) is the result of ferrous ion impurity in pyromorphites and follows a relationship between band intensity in the near-infrared spectra and ferrous ion concentration. The iron impurity causes a change in colour from green-yellow to brown in the pyromorphite samples.

The observation of overtones of the OH-fundamentals, confirms the presence OH- in the mineral structure. The contribution of water-OH overtones in the NIR at 5100 cm-1 (1.96 µm) is an indication of bonded water in the minerals of pyromorphite. Spectra in the mid-IR show that pyromorphite is a known mixed phosphate and arsenate complex, Pb5(PO4,AsO4)3Cl. A series of bands are resolved in the infrared spectrum of pyromorphite at 1017, 961 and 894 cm-1. The first two bands are assigned to ν3, the antisymmetric stretching mode and third band at 894 cm-1 is symmetric mode of the phosphate ion. Similar patterns are shown by other pyromorphite samples with variation in intensity. The cause of multiple bands near 800 cm-1 is the result of isomorphic substitution of (PO4)3- by (AsO4)3- and the spectral pattern relates to the chemical variability in pyromorphite. The presence of (AsO4)3- is significant in certain pyromorphite samples.