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

Abstract

The crystal structure, including H positions, of the monoclinic zeolite mineral parthéite (space group C2/c) of framework type -PAR from Denezhkin Kamen (Urals, Russia) was refined, from single-crystal X-ray data to R 1 = 3.46%. In addition, in situ single-crystal X-ray data have been measured in steps of 25 °C up to 375 °C to analyze dehydration behavior. In situ Raman spectra of the natural (room temperature) and partly dehydrated varieties of parthéite have been recorded at 100, 150, and 275 °C. The structure of parthéite, Ca 2 Al 4 Si 4 O 15 (OH) 2 ⋅4H 2 O, is characterized by a tetrahedral framework interrupted by an OH-group forming the apex of one AlO 4 tetrahedron. In addition, this OH-group coordinates extraframework Ca together with two H 2 O molecules and four framework oxygen sites. The structure has four strong hydrogen bonds with H···O interactions below 2 Å. The fifth hydrogen associated with a H 2 O molecule, exhibits two potential acceptors of weak hydrogen bonds with H···O distances of ca. 2.5 Å. This softly bound H 2 O molecule is released at 150 °C without severe impact on framework distortions but with decrease of Ca coordination from seven to six. Concurrently with loss of the second H 2 O molecule at 250 °C, the structure further compacts and becomes severely distorted. The space group C2/c and the tetrahedral connectivity are preserved but β changes from 91 to 79° and the volume drops from 1730 to 1600 Å 3 . Ca is still six-coordinated by five framework O atoms and OH.