Abstract
The bright blue minerals cavansite and pentagonite, a calcium vanadium silicate Ca(V4+O)Si4O10·4H2O, 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 789nm for cavansite and pentagonite.The Raman spectrum of cavansite is dominated by an intense band at 981cm−1 and pentagonite by a band at 971cm−1 attributed to the stretching vibrations of (SiO3)n units. Cavansite is characterised by two intense bands at 574 and 672cm−1 whereas pentagonite by a single band at 651cm−1. The Raman spectrum of cavansite in the hydroxyl stretching region shows bands at 3504, 3546, 3577, 3604 and 3654cm−1 whereas pentagonite is a single band at 3532cm−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 ∼531.5eV and for water ∼533.5eV. XPS studies show a strong peak at 531.5eV for cavansite, indicating some OH units in the structure of cavansite.
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