Vibrational spectra of the rare‐occurring complex hydrogen arsenate minerals pharmacolite, picropharmacolite, and vladimirite: Dominance of R aman over IR spectroscopy to discriminate arsenate and hydrogen arsenate units

This work represents the first complete experimental and theoretical study of the infrared (IR) spectra of pharmacolite, Ca(AsO 3 OH)·2H 2 O; vladimirite, Ca 4 (AsO 3 OH)(AsO 4 ) 2 ·4H 2 O; and picropharmacolite, Ca 4 Mg(AsO 3 OH) 2 (AsO 4 ) 2 ·11H 2 O. IR spectra, collected at both room and liquid nitrogen temperature, have shown similar spectral view among minerals in the region of ν 1 to ν 4 modes, surprisingly not involving complexity in vladimirite and picropharmacolite spectral view due to the presence of both AsO 3 OH and AsO 4 units compared to pharmacolite where only AsO 3 OH units appear in the structure. On the other hand, vladimirite and picropharmacolite Raman spectra in the corresponding ν 1 to ν 4 region (955–350 cm −1 ) exhibit existence of 3 sets of 2 bands (6 in total) completely reflecting the existence of 3 symmetrically nonequivalent AsO 4 groups in the structures of vladimirite and picropharmacolite (of which one is protonated) and only one set of ν 1 to ν 4 bands from the structurally equivalent AsO 3 OH units in the pharmacolite structure. The liquid nitrogen temperature IR spectra enabled to infer important information regarding the OH vibrational bands related to the AsO 3 OH groups and to the water molecules. Essentially, all bands in the vibrational spectra were assigned and correlated with the findings for a plethora of structurally similar complex hydrogen arsenate minerals. To support the tentative assignment of bands in the vibrational spectra, quantum theoretical calculations were performed within the framework of density functional theory. On the basis of theoretical calculations, a few reassignments of bands appearing in the OH stretching region are proposed. X‐ray powder diffraction was used to confirm authenticity of the title systems.