Systematic Raman Spectroscopic Study of the Isomorphy Between the Arsenate Minerals Roselite, Wendwilsonite,...

In nature, numerous minerals are known with the general formula X₂M(TO₄)₂·2(H₂O) and an important group is formed by minerals with T = As. Most of these occur as minor or trace minerals in environments such as hydrothermal alterations of primary sulfides and arsenides. X-ray photoelectron spectroscopy and Raman spectroscopy have been utilized to study the chemistry and crystal structure of the roselite subgroup minerals, Ca₂M(AsO₄)₂·2H₂O (with M = Co, Mg, Mn, Zn, and Cu). The AsO₄ stretching region exhibited minor differences between the roselite subgroup minerals, which can be explained by the ionic radius of the cation substituting on the M position in the structure. Multiple AsO₄ antisymmetric stretching and bending modes were found, pointing to a tetrahedral symmetry reduction. Bands around 450 cm^-1 were attributed to ν₄ bending modes. Several bands in the 300-350 cm^-1 region attributed to ν₂ bending modes also provide evidence of symmetry reduction of the AsO₄ anion. Two broad bands for roselite were found around 3330 and 3120 cm^-1 and were attributed to the OH stretching bands of crystal water. These bands are accompanied by two bands around 1700 and 1610 cm^-1 attributed to the corresponding OH-bending modes. In conclusion, both XPS and Raman spectroscopy are valuable nondestructive analytical tools to characterize these secondary arsenate minerals. X-ray photoelectron spectroscopy and Raman microspectroscopy allow the chemistry and molecular structure of the roselite group minerals to be studied in a nondestructive way. The minerals in the roselite subgroup are easily distinguished based on their chemical composition as determined by XPS. As expected for minerals with the same crystal structure, similarities exist in the Raman spectra, sufficient differences exist to be able to identify these minerals.