The structure of mimetite, arsenian pyromorphite and hedyphane – A Raman spectroscopic study

The minerals mimetite Pb5(AsO4)3Cl, arsenian pyromorphite Pb5(PO4,AsO4)3Cl and hedyphane Pb3Ca2(AsO4)3Cl have been studied by Raman spectroscopy complimented with infrared spectroscopy. Mimetite is characterised by a band at 812–3 cm−1 attributed to the Ag mode. For the arsenian pyromorphite this band is observed at 818 cm−1 and for hedyphane at 819 cm−1. For mimetite and hedyphane bands at 788 and 765 cm−1 are attributed to Au and E1u vibrational modes and are both Raman and infrared active. For the arsenian pyromorphite, Raman bands at 917–1014 cm−1 are attributed to phosphate stretching vibrations. Raman spectroscopy clearly identifies bands attributable to isomorphous substitution of arsenate by phosphate. The observation of low intensity bands in the 3200–3550 cm−1 region are assigned to adsorbed water and OH units, thus indicating some replacement of chloride ions with hydroxyl ions.

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