Polymorphic relation between cavansite and pentagonite: Genetic implications of oxonium ion in cavansite

Abstract

The chemical compositions of cavansite and pentagonite, in which H₂O contents and vanadium (in an unknown oxidation state) are present, were determined by thermogravimetry-differential thermal analysis (TG-DTA), electron spin resonance (ESR), and electron microprobe analysis (EMPA). Furthermore, the mechanism of dehydration of the minerals and presence of the hydrous species such as H₂O, H₃O⁺, and OH^- in the aforementioned minerals have been investigated by TG-DTA, high-temperature X-ray diffraction (HT-XRPD) analysis, Fourier-transform infrared (FTIR) spectroscopy, and single-crystal XRD analysis. The results of TG-DTA and HT-XRD revealed that no reversible transitions occur between cavansite and pentagonite when they are heated in air and that no intermediate amorphous phase exists in these two minerals. Gradual dehydration of cavansite in the temperature range of 225-550 °C was attributed to the removal of both oxonium (H₃O⁺) and hydroxyl ions (OH^-); the IR absorption bands of cavansite observed at 3186 and 3653 cm^-1 were assigned to H₃O⁺ and OH^- stretching vibrations, respectively. Moreover, the exact distribution of hydrogens in the crystal structure of the cavansite refined in this study was determined by applying the valence-matching principle; the results showed the existence of H₃O⁺ and OH^-. Thus, the structural formula of cavansite should be revised to Ca(VO)(Si₄O₁₀)·(H₂O)_4-2x(H₃O)_x(OH)_x , in contrast to that of pentagonite, Ca(VO)(Si₄O₁₀)·4H₂O. The changes in the ion product constant of water with temperature and pressure suggest that pentagonite is formed when the hydrothermal fluid is in supercritical condition (>300 °C), while cavansite is formed when the hydrothermal fluid is not in supercritical condition. Thus, cavansite is identified as a low-temperature form and pentagonite as a high-temperature one.