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 R1 = 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, Ca2Al4Si4O15(OH)2·4H2O, 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⋯O interactions below 2 Å. The fifth hydrogen associated with a H2O molecule, exhibits two potential acceptors of weak hydrogen bonds with H⋯O distances of ca. 2.5 Å. This softly bound H2O 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 H2O 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 Å3. Ca is still six-coordinated by five framework O atoms and OH.