Real-time X-ray synchrotron powder diffraction studies of the dehydration processes in scolecite and mesolite

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, Ca₈Al₁₆Si₂₄O₈₀.24H₂O, shows an initial gradual loss of half O(2W). 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 F1d1 to Fd11 when half the Ca ions move by ~1/2c and the remaining O(2W) is expelled. Metascolecite: Ca₈Al₁₆Si₂₄O₈₀.16H₂O, Fd11, 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 ≠ 3n 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(H₂O) varying between 16 and 10.67. Metamesolite: Ca_5.33Na_5.33Al₁₆Si₂₄O₈₀.n(H₂O), Fdd2, Z = 1, n(H₂O) = 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 AlO₄ 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 AlO₄ 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 AlO₄ 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 Ca₈Al₁₆Si₂₄O₈₀.16H₂O; 45-1490 for Ca₈Al₁₆Si₂₄O₈₀.18.9H₂O; 45-1491 for Ca_5.33Na_5.33Al₁₆Si₂₄O₈₀.12.8H₂O; 45-1492 for Ca_5.33Na_5.33Al₁₆Si₂₄O₈₀.16.7H₂O.