Hydrogen-bond system and dehydration behavior of the natural zeolite partheite

The crystal structure, including H positions, of the monoclinic zeolite mineral partheite (space group C2/c) of framework type -PAR from Denezhkin Kamen (Urals, Russia) was refined, from single-crystal X-ray data to R-I = 3.46%. In addition, in situ single-crystal X-ray data have been measured in steps of 25 degrees C up to 375 degrees C to analyze dehydration behavior. In situ Raman spectra of the natural (room temperature) and partly dehydrated varieties of partheite have been recorded at 100, 150, and 275 degrees C. The structure of partheite, Ca2Al4Si4O15(OH)(2)center dot 4H(2)O, is characterized by a tetrahedral framework interrupted by an OH-group forming the apex of one AlO4 tetrahedron. In addition, this OH-group coordinates extraframework Ca together with two H2O molecules and four framework oxygen sites. The structure has four strong hydrogen bonds with H center dot center dot center dot O interactions below 2 angstrom. The fifth hydrogen associated with a H2O molecule, exhibits two potential acceptors of weak hydrogen bonds with H center dot center dot center dot O distances of ca. 2.5 angstrom. This softly bound H2O molecule is released at 150 degrees C without severe impact on framework distortions but with decrease of Ca coordination from seven to six. Concurrently with loss of the second H2O molecule at 250 degrees C, the structure further compacts and becomes severely distorted. The space group C2/c and the tetrahedral connectivity are preserved but beta changes from 91 to 79 degrees and the volume drops from 1730 to 1600 angstrom(3). Ca is still six-coordinated by five framework O atoms and OH.