Vibrational spectroscopic study of the minerals cavansite and pentagonite Ca(V⁴+O)Si₄O10·4H₂O.

The bright blue minerals cavansite and pentagonite, a calcium vanadium silicate Ca(V(4+)O)Si(4)O(10)·4H(2)O, have been studied by UV-Visible, Raman and infrared spectroscopy. Cavansite shows an open porous structure with very small micron sized holes. Strong UV-Visible absorption bands are observed at around 403, 614 and 789 nm for cavansite and pentagonite. The Raman spectrum of cavansite is dominated by an intense band at 981cm(-1) and pentagonite by a band at 971 cm(-1) attributed to the stretching vibrations of (SiO(3))(n) units. Cavansite is characterised by two intense bands at 574 and 672 cm(-1) whereas pentagonite by a single band at 651 cm(-1). The Raman spectrum of cavansite in the hydroxyl stretching region shows bands at 3504, 3546, 3577, 3604 and 3654 cm(-1) whereas pentagonite is a single band at 3532 cm(-1). These bands are attributed to water coordinated to calcium and vanadium. XPS studies show that bond energy of oxygen in oxides is 530eV, and in hydroxides ∼531.5 eV and for water ∼533.5 eV. XPS studies show a strong peak at 531.5eV for cavansite, indicating some OH units in the structure of cavansite.