The structure of the mineral arthurite (AsO4,PO4,SO4)2(O,OH)2·4H2O – A Raman spectroscopic study

Arthurite minerals [A(Fe3+)2(XO4)2(O,OH)2·4(H2O) where A = Cu(II), Zn(II), Pb(II) or Fe(II) and X = As, P, S] from different origins (Nevada and Cornwall) have been studied using Raman spectroscopy and complimented with infrared spectroscopy. These two arthurite minerals have the following formula Cu(Fe3+)2(AsO4,PO4,SO4)2(O,OH)2·4(H2O). The presence of arsenate, phosphate, sulphate, and hydroxyl anions are readily characterised by Raman spectroscopy. The two arthurite samples observed significant differences in the position of Raman and infrared bands. The differences in band positions have been attributed to the different hydrogen bond strength of water in the two samples. Arthurite (a) has strongly hydrogen bonded water, whilst arthurite (b) has weakly hydrogen bonded water. The strength of the hydrogen bonded water influenced the position of the counter anions arsenate, phosphate and sulphate. For example, bands for (b) generally appeared at lower wavenumbers. A comparison with the Raman spectrum of cobaltarthurite has also been reported.