Dehydration dynamics and thermal stability of scolecite CaAl2Si3O10 center dot 3H(2)O, Z = 8 (NAT framework type) has been investigated between 25 and 300 degrees C by in situ single-crystal X-ray diffraction at 30(5) % RH (high P-H2O) and under dry nitrogen conditions (low P-H2O). The dehydration under high P-H2O can be described as a two-step process: scolecite (3H(2)O) metascolecite (2H(2)O) - X-ray amorphous anhydrous phase. At 150 degrees C, below the transformation from scolecite to metascolecite, a new triclinic intermediate structure was analyzed. This transformation represents a transitory stage between scolecite and metascolecite.in terms of the Ca and H2O distribution. For the first time hydrogen positions have been located for metascolecite (220 degrees C) quenched to 100 degrees C, enabling evaluation of the hydrogen bond system. The second set of experiments, carried out under low P-H2O, led to the temperature-dependent phase sequence: scolecite - metascolecite - x2-phase - x1-phase - amorphization. The x2-phase has CaAl2Si3O10 center dot 1/2H(2)O composition (space group Ad11, Z = 8, a = 17.536(4), b = 17.493(5), c = 6.4847(15) angstrom, a = 88.884(17)". The xl-phase is anhydrous CaAl2Si3O10 (space group Fdll, Z = 8, a = 16.327(8), b = 17.433(8), c = 6.521(3) A, a = 85.69(3)). Both structures represent strongly compacted and distorted varieties of the NAT framework type. The volume change from scolecite to metascolecite, x2-phase, and x1-phase amounts to 2, 11, and 7%, respectively. Metascolecite (200 degrees C, high P-H2O) was rehydrated under ambient conditions to scolecite. The resulting rehydrated structure (3H(2)O) is different to original scolecite and shows significant Ca disorder corresponding to the intermediate structure at 150 degrees C. (C) 2015 Elsevier Inc. All rights reserved.
