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

Danisi, Rosa Micaela; Armbruster, Thomas Michael; Lazic, Biljana (2012). In situ dehydration behavior of zeolite-like cavansite: A single-crystal X-ray study. American mineralogist, 97(11-12), pp. 1874-1880. Mineralogical Society of America 10.2138/am.2012.4228

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To track dehydration behavior of cavansite, Ca(VO)(Si4O10)·4H2O space group Pnma, a = 9.6329(2), b = 13.6606(2), c = 9.7949(2) Å, V = 1288.92(4) Å3 single-crystal X-ray diffraction data on a crystal from Wagholi quarry, Poona district (India) were collected up to 400 °C in steps of 25 °C up to 250 °C and in steps of 50 °C between 250 and 400 °C. The structure of cavansite is characterized by layers of silicate tetrahedra connected by V4+O5 square pyramids. This way a porous framework structure is formed with Ca and H2O as extraframework occupants. At room temperature, the hydrogen bond system was analyzed. Ca is eightfold coordinated by four bonds to O of the framework structure and four bonds to H2O molecules. H2O linked to Ca is hydrogen bonded to the framework and also to adjacent H2O molecules. The dehydration in cavansite proceeds in four steps.At 75 °C, H2O at O9 was completely expelled leading to 3 H2O pfu with only minor impact on framework distortion and contraction V = 1282.73(3) Å3. The Ca coordination declined from originally eightfold to sevenfold and H2O at O7 displayed positional disorder.At 175 °C, the split O7 sites approached the former O9 position. In addition, the sum of the three split positions O7, O7a, and O7b decreased to 50% occupancy yielding 2 H2O pfu accompanied by a strong decrease in volume V = 1206.89(8) Å3. The Ca coordination was further reduced from sevenfold to sixfold.At 350 °C, H2O at O8 was released leading to a formula with 1 H2O pfu causing additional structural contraction (V = 1156(11) Å3). At this temperature, Ca adopted fivefold coordination and O7 rearranged to disordered positions closer to the original O9 H2O site.At 400 °C, cavansite lost crystallinity but the VO2+ characteristic blue color was preserved. Stepwise removal of water is discussed on the basis of literature data reporting differential thermal analyses, differential thermo-gravimetry experiments and temperature dependent IR spectra in the range of OH stretching vibrations.

Item Type:	Journal Article (Original Article)
Division/Institute:	08 Faculty of Science > Institute of Geological Sciences > Mineralogical Crystallography [discontinued]
UniBE Contributor:	Danisi, Rosa Micaela, Armbruster, Thomas Michael
Subjects:	500 Science > 540 Chemistry 500 Science > 550 Earth sciences & geology
ISSN:	0003-004X
Publisher:	Mineralogical Society of America
Language:	English
Submitter:	Frank Gfeller
Date Deposited:	20 Jun 2014 11:00
Last Modified:	05 Dec 2022 14:30
Publisher DOI:	10.2138/am.2012.4228
URI:	https://boris.unibe.ch/id/eprint/45497