Heating-induced phase transformation in zeolite brewsterite: new 4- and 5-coordinated (Si,Al) sites |
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Authors: | A Alberti M Sacerdoti S Quartieri G Vezzalini |
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Institution: | (1) Istituto di Mineralogia, Universita’ di Ferrara, C.so Ercole I° d’Este 32, I-44100 Ferrara, Italy Fax: +39-532-293760 e-mail: Alberti@dns.unife.it e-mail: M.Sacerdoti@dns.unife.it, IT;(2) Dipartimento di Scienze della Terra, Universita’ di Modena, via S.Eufemia 19, I- 41100 Modena, Italy Fax: +39-59-417211 e-mail: quartieri@unimo.it e-mail: vezzalini@imomn1.unimo.it, IT |
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Abstract: | The dehydration-rehydration processes and phase transformations of brewsterite (space group P21/m, ideal formula (Sr,Ba)Al2Si6O16·5H2O) were studied by single crystal X-ray diffraction on two samples heated in an evacuated capillary for 24 h at 280°C and
330°C, respectively, and then cooled to room temperature. After the data collection, the capillary of the crystal heated at
280°C was broken, the sample was kept at room conditions for one month, and diffraction data were recollected. Brewsterite
heated in vacuum at 280°C shows a strong decrease of the unit cell volume (about 10%) and the statistical breaking of the
T1-O7-T2 bridge of the 4-ring of the 4254 PBU. T1 cation, in 31% of cases, migrates to a new tetrahedral site, which shares three vertices with the previously occupied
one; the fourth vertex is on the mirror plane, and joins two adjacent layers. T2 cation, in 20% of cases, migrates to a new
site, which is characterized by a 5-fold coordination. This coordination polyhedron shares three vertices with the previously
occupied tetrahedron; the fourth vertex is on the mirror plane, and joins two adjacent layers; the fifth one is unshared.
The formation of these new T-O-T bridges prevents the re-expansion and rehydration of the heated brewsterite; as a consequence,
the crystal brought back to room conditions does not show any noticeable structural difference with respect to that heated
at 280°C. The exchangeable cations, which occupy only one extraframework site in the untreated brewsterite, spread over several
sites. The sample heated at 330°C shows a further decrease in the unit cell volume of about 2% with respect to the sample
heated at 280°C, and an increase in the migration percentage of T1 and T2 cations to the new sites.
Received: 11 November 1997 / Revised, accepted: 26 June 1998 |
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Keywords: | Zeolite Brewsterite Dehydration Phase transformation 5-Coordinated (Si Al) |
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