title: A near-infrared spectroscopic study of the phosphate mineral pyromorphite Pb5(PO4)3Cl.
abstract: 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 microm). A broad signal observed at approximately 10,000cm(-1) (1.00 microm) 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 microm) 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 nu(3), the antisymmetric 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.
authors.affiliation: Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia.
authors.forename: B Jagannadha
authors.initials: BJ
authors.lastname: Reddy
authors.forename: Ray L
authors.initials: RL
authors.lastname: Frost
authors.forename: Sara J
authors.initials: SJ
authors.lastname: Palmer
mesh_terms: D008903:Minerals / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D013055:Spectrophotometry, Infrared
chemical_list: D008903:Minerals; D010710:Phosphates; C471364:pyromorphite