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

Abstract

The chemical compositions of cavansite and pentagonite, in which H 2 O 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 H 2 O, H 3 O + , 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 (H 3 O + ) and hydroxyl ions (OH − ); the IR absorption bands of cavansite observed at 3186 and 3653 cm −1 were assigned to H 3 O + 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 H 3 O + and OH − . Thus, the structural formula of cavansite should be revised to Ca(VO)(Si 4 O 10 )·(H 2 O) 4−2x (H 3 O) x (OH) x , in contrast to that of pentagonite, Ca(VO)(Si 4 O 10 )·4H 2 O. 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.

In situ dehydration behavior of zeolite-like cavansite: A single-crystal X-ray study

“…4) observed at different temperatures correlate with the thermo-gravimetric data reported by Phadke and Apte (1994), Powar and Byrappa (2001), Prasad and Prasad (2007), and Ishida et al (2009) (Table 8). The four steps of weight loss in succession correspond to the expulsion of four water molecules, one at a time.…”

“…and the bvs of acceptor oxygen may be increased by 0.2 v.u., indicating that O7, O8, and O9 represent H 2 O molecules. This finding is in contrast to the interpretation of Ishida et al (2009) suggesting, by applying the "valence-matching principle," the existence of H 3 O + and OH -.…”

“…In particular, the angle between donor-hydrogen-acceptor (DHA) is probably too narrow (less than 120°) for the assumed hydrogen bonds (H 2 O)1-H2 . Moreover, Ishida et al (2009) reported two hydrogen bonds (O9-H4 ... O7 and O9-H5 ... O7) with the distance between acceptor and hydrogen greater than 3.1 Å and two hydrogen bonds (O8-H2 ... O2 and O8-H2 … O1) with the DHA angle less than 120°. We did not consider the above interactions because they are not consistent with the geometric criterion for hydrogen bonds.…”

“…The O9 ... H7a interaction, not reported in the study of Solov'ev et al (1993) and Hughes et al (2011), connects two H 2 O molecules located inside the elliptical channels parallel to b formed by eight-membered rings of tetrahedra. Solov'ev et al (1993) and Ishida et al (2009) assumed other interactions (Table 7). In particular, the angle between donor-hydrogen-acceptor (DHA) is probably too narrow (less than 120°) for the assumed hydrogen bonds (H 2 O)1-H2 .…”

“…In addition, differential thermal analysis (DTA) and differential thermo-gravimetry (DTG) (Phadke and Apte 1994;Powar and Byrappa 2001;Ishida et al 2009), in situ FTIR and Raman spectroscopy (Prasad and Prasad 2007) were done.…”