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

By B. Jagannadha Reddy, Ray L. Frost and Sara J. Palmer

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.\ud \ud 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

Topics: 030606 Structural Chemistry and Spectroscopy, 030600 PHYSICAL CHEMISTRY (INCL. STRUCTURAL), 030606 Structural Chemistry and Spectroscopy, 030603 Colloid and Surface Chemistry, Pyromorphite, Mimetite, Vanadinite, Phosphate, Arsenate, Near, IR and Mid, infrared spectroscopy, OH, overtones, Cation/anion substitution effects
Publisher: Elsevier
Year: 2008
DOI identifier: 10.1016/j.saa.2007.12.030
OAI identifier: oai:eprints.qut.edu.au:15386

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Citations

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