A near-infrared spectroscopic study of the phosphate mineral pyromorphite Pb-5(PO4)(3)Cl

摘要

Spectral properties as a function composition are analysed for a series of selected pyromorphite minerals of Australian origin. The minerals are characterised by d-d transitions in NIR from 12,000 to 8000 cm(-1) (0.83-1.25 mu m). A broad signal observed at similar to 10,000 cm(-1) (1.00 mu m) 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 mu m) 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, Pb-5(PO4,AsO4)(3)Cl. 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 nu(3), the anti symmetric stretching mode and the third band at 894 cm(-1) is the 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. (c) 2008 Elsevier B.V. All rights reserved.