THE DEHYDRATION AND REHYDRATION PROCESSES IN THE NATURAL ZEOLITE MESOLITE STUDIED BY CONVENTIONAL AND SYNCHROTRON X-RAY-POWDER DIFFRACTION

The dehydration and rehydration process in the natural zeolite mesolite, Na5.33Ca5.33Al16Si24O80'n H2O [structure type NAT, space group Fdd2, Z = 3, n(H2O) = 21.33, a = 18.4071(4), b = 56.668(1), c = 6.5464(1) Angstrom A] has been studied by powder diffraction using CuK alpha(1), and synchrotron radiation [lambda = 1.1999(1) Angstrom A] and Rietveld analysis. The samples were dehydrated for 1 h at 458, 473, 523, 573, and 598 K, respectively, and sealed in capillaries prior to data collection at room temperature. After partial loss of OW(4), equatorially coordinated to Ca, after dehydration at 458 K, mesolite goes through an order/disorder transition on dehydration at 473 K. The cations become randomly distributed over the Ca and Na sites and OW(4) is expelled completely. Reflections having k not equal 3n are lost and the resulting crystal structure, metamesolite, is very close to that of natrolite, the Na site being equally occupied by Na, Ca, and vacancies, and n(H2O) varying between 14.7(3) (T = 473 K) and 11.1(2) (T = 573 K). T = 523 K: Metamesolite; Fdd2, Z = 1, n(H2O) = 11.3(1), a = 18.11287(8), b = 18.63331(8), c = 6.56618(3) Angstrom A. Dehydration at 598 K destroys the crystalline structure and the material becomes amorphous. Rehydration of metamesolite restores the original water content, but the random Na/Ca distribution is retained and a new, partially occupied OW(4') site coordinated to Ca appears: cation-disordered mesolite; Fdd2, Z = 1, n(H2O) = 23.2(4), a = 18.6180(9), b = 19.0312(9), c = 6.5421(3) Angstrom A.