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Real‐time X‐ray synchrotron powder diffraction studies of the dehydration processes in scolecite and mesolite
Author(s) -
Ståhl K.,
Hanson J.
Publication year - 1994
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s002188989301235x
Subject(s) - dehydration , synchrotron , crystallography , powder diffraction , ion , chemistry , phase (matter) , x ray crystallography , diffraction , x ray , phase transition , crystal structure , analytical chemistry (journal) , physics , optics , biochemistry , organic chemistry , chromatography , quantum mechanics
The high intensity of an X‐ray synchrotron source in combination with a curved position‐sensitive detector covering 120° (CPS120 by INEL) has been used to collect complete powder diffraction patterns, suitable for Rietveld analyses, within minutes. From consecutive patterns ( ΔT ≃ 5 K), detailed information was obtained on water expulsion and phase transitions induced by dehydration of the natural zeolites scolecite and mesolite. Scolecite, Ca 8 Al 16 Si 24 O 80 .24H 2 O, shows an initial gradual loss of half O(2 W ). At ~480 K, scolecite goes through a phase transition already described by Rinne [ Neues Jahrb. Mineral. Beil. (1923). 48 , 240–249]. The crystal symmetry changes from F 1 d 1 to Fd 11 when half the Ca ions move by ~1/2 c and the remaining O(2 W ) is expelled. Metascolecite: Ca 8 Al 16 Si 24 O 80 .16H 2 O, Fd 11, Z = 1, T = 489 K, a = 18.1465 (7), b = 18.8604 (7), c = 6.5396 (3) Å, α = 88.986 (2)°, R p = 5.42%, R B = 2.51% from 3620 observations, 802 Bragg reflections and 79 refined parameters. Continued heating to 615 K did not further reduce the water content. The dehydration process in mesolite starts with a loss of half the same Ca‐coordinated water as in scolecite. The initial water loss is followed by an order/disorder transition, where the Na and Ca ions become randomly distributed over the cation sites. Consequently, the b axis of metamesolite is reduced to 1/3 of b mes and the k ≠ 3 n reflections disappear. Metamesolite is very similar to natrolite, with equal numbers of Na atoms, Ca atoms and vacancies in the natrolite Na‐atom site and with n (H 2 O) varying between 16 and 10.67. Metamesolite: Ca 5.33 Na 5.33 Al 16 Si 24 O 80 . n (H 2 O), Fdd 2, Z = 1, n (H 2 O) = 10.6 (2), T = 582 K, a = 18.1039 (6), b = 18.5763 (6), c = 6.5589 (2) Å, R p = 4.85%, R B = 2.01% from 3620 observations, 424 Bragg reflections and 54 refined parameters. Both scolecite and mesolite have one AlO 4 tetrahedron with only one O atom coordinated by a cation. The initially expelled waters are in both cases hydrogen bonded to an O atom in those AlO 4 tetrahedra. The initial water expulsions thus worsen the underbonding and trigger the cation rearrangements. The resulting cation distributions in metascolecite and metamesolite are the most even with respect to the AlO 4 tetrahedra and are considered the driving force of both phase transitions. The cation rearrangement in mesolite requires cross‐channel diffusion and is considerably slower than the in‐channel rearrangement taking place in scolecite. In both zeolites, the lower limit in the calcium coordination number was found to be six. The ICDD Powder Diffraction File Nos. are: 45‐1489 for Ca 8 Al 16 Si 24 O 80 .16H 2 O; 45‐1490 for Ca 8 Al 16 Si 24 O 80 .18.9H 2 O; 45‐1491 for Ca 5.33 Na 5.33 Al 16 Si 24 O 80 .12.8H 2 O; 45‐1492 for Ca 5.33 Na 5.33 Al 16 Si 24 O 80 .16.7H 2 O.