Vibrational spectroscopic study of the arsenate mineral strashimirite Cu8(AsO4)4(OH)42O-Relationship to other basic copper arsenates

The basic copper arsenate mineral strashimirite Cu8(AsO4)4(OH)4.5H2O from two different localities has been studied by Raman spectroscopy and complemented by infrared spectroscopy. Two strashimirite mineral samples were obtained from the Czech (sample A) and Slovak (sample B) Republics. Two Raman bands for sample A are identified at 839 and 856 cm-1 and for sample B at 843 and 891 cm-1 are assigned to the ν1 (AsO43-) symmetric and the ν3 (AsO43-) antisymmetric stretching modes, respectively. The broad band for sample A centred upon 500 cm-1, resolved into component bands at 467, 497, 526 and 554 cm-1 and for sample B at 507 and 560 cm-1 include bands which are attributable to the ν4 (AsO43-) bending mode. In the Raman spectra, two bands (sample A) at 337 and 393 cm-1 and at 343 and 374 cm-1 for sample B are attributed to the ν2 (AsO43-) bending mode. The Raman spectrum of strashimirite sample A shows three resolved bands at 3450, 3488 and 3585 cm-1. The first two bands are attributed to water stretching vibrations whereas the band at 3585 cm-1 to OH stretching vibrations of the hydroxyl units. Two bands (3497 and 3444 cm-1) are observed in the Raman spectrum of B. A comparison is made of the Raman spectrum of strashimirite with the Raman spectra of other selected basic copper arsenates including olivenite, cornwallite, cornubite and clinoclase.