Crystal structure of cavansite dehydrated at 220°C

Abstract

A single crystal of the new mineral cavansite, Ca(VO) (Si4010).4H20, from the type locality (Malheur County, Oregon) was dehydrated at 220_+ 10°C in a quartz capillary at ~ 10 -5 atm for 24 h. Intensity data from a four-circle diffractometer were refined anisotropically by least-squares calculations and Fourier techniques to R=0.036, Rw=0.022 in space group Pcmn. The cell dimensions change upon dehydration from a=9.792 (2), b= 13.644 (3), c=9.629 (2) A, V= 1286.5 (3) A3 to a=9.368 [2), b= 12.808 (3), e= 9.550 (2) ,~, V= 1145-8 (3) ,3. The 11% reduction of volume results mainly from removal of water between the silicate layers which lie perpendicular to b. The layers consist of four-and eightmembered rings of SiO4 tetrahedra linked so that the sequence of tetrahedra around the eight-rings is UUUUDDDD (U up, D down). The layers are connected vertically (b direction) by V 4+ cations in a square pyramidal coordination to give an infinite silicate-vanadyl complex. Replacement of each VOs group by two bridging oxygens gives the framework of the zeolite gismondine which has the same layers of four-and eight-membered rings but displays a fully expanded configuration. The Ca cations and the water molecules reside in the channels formed by the eight-rings and between the SiOz layers. At 220°C only one water molecule per Ca ion occurs in the structure. Removal of this last water molecule probably corresponds to the breakdown of the structure observed at ~400~'C. The vanadyl groups were found to have two orientations with the unshared oxygens pointing in opposite directions with a ratio of 10: 1. Therefore there are two ideal structural models of cavansite with an infinite number of disordered intermediates.

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

“…1b). Therefore, the last H 2 O group of cavansite is not zeolitic, as predicted by Rinaldi et al (1975).…”

“…However, it was evident from high -temperature crystal structure refinement (220 °C) that the H 2 O molecules released upon heating were not zeolitic (Rinaldi et al, 1975). The H 2 O content in cavansite (Phadke and Apte, 1994;Powar and Byrappa, 2001) was not in agreement with the ideal value.…”

Hydrothermal Synthesis and Characterisation of Two Novel Large‐Pore Framework Vanadium Silicates

“…The acid-base and redox properties of AM- 17 [17] Cavansite and pentagonite, dimorphs of the mineral Ca(VO)(Si 4 O 10 )·4H 2 O, are small-pore materials containing V IV in a square-pyramidal coordination. [18,19] The framework of cavansite is formed by silicate layers of fourand eight-membered rings of tetrahedra connected vertically by V IV cations. Calcium cations and water molecules reside in the channels formed by the eight-membered rings and between the SiO 2 layers.…”

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

“…At first glance, one should assume that, upon heating, H 2 O III with the weakest bond is expelled first. However, the structural study of partly dehydrated cavansite by Rinaldi et al (1975) indicates that H 2 O (their W1) close to H 2 O III (room temperature) is the most stable one before complete dehydration occurs accompanied with structural breakdown.…”

“…Previous structural studies of the dehydration behavior of cavansite are limited to a room-temperature single-cystal X-ray diffraction experiment of a crystal previously partly dehydrated under vacuum at 220 °C (Rinaldi et al 1975). 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.…”