Thermal behaviour of zeolites: single crystal X-ray study of dehydration and rehydration mechanism in yugawaralite
Alberti, Alberto; Quartieri, Simona; Vezzalini, Giovanna
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1273 - 1282
published: Jan 8, 1997
manuscript accepted: Jun 27, 1996
manuscript received: Nov 20, 1995
ArtNo. ESP147050806006, Price: 29.00 €
Abstract Dehydration and rehydration processes and phase transformations of yugawaralite have been studied by single crystal X-ray diffraction. Yugawaralite kept in vacuum at room temperature does not undergo any noticeable structural modifications. When yugawaralite is heated at 100°C in vacuum for 24 hours, only the water molecule not coordinated with the Ca atom is lost (about 6% of water loss), while the two water molecules occupying two alternative positions in the hydrated sample move into only one fully occupied site placed in an intermediate position: the Ca coordination remains virtually unchanged. Yugawaralite heated at l50°C undergoes a water loss of 53%, and the residual water molecules are localized in only two fully occupied sites. The Ca atoms are 6-coordinated by four framework oxygens and the two residual water molecules. The most relevant modifications of the framework are: 1) the increased difference between the Al-OCa-bonded and Al-OCa-nonbonded distances; and 2) a lengthening (by about 0.04 Å) and a narrowing (by about 0.04 A) of the structural units of yugawaralite - formed by a pair of 5-membered rings sharing an edge - when compared with hydrated samples. When yugawaralite is heated at 180°C, a sudden decrease of the unit cell volume occurs (about 12%), the cell parameter a is doubled, and the space group changes from Pc to Pn. The structure refinement of a crystal heated at 200 °C shows a further 50% loss of the residual water present at 150°C, a collapsing of the channel system, and strong squashing of the framework. Owing to the doubling of parameter a there are two symmetrically independent Ca sites, both 6-coordinated with five framework oxygens and one water molecule. Heating of yugawaralite at 250°C does not cause any significant modifications with respect to the structure at 200°C. When yugawaralite heated at 150°C is brought back to room conditions for two weeks, complete rehydration occurs; the water molecules are in the same positions and have the same occupancies as in the hydrated sample.