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Structural evolution of natrolite during over-hydration

¹ Institute of Mineralogy and Petrography, Russian Academy of Sciences, Koptyug prosp. 3, 630090 Novosibirsk, Russia
² Institute of Inorganic Chemistry, Russian Academy of Sciences, Lavrentiev prosp. 3, 630090 Novosibirsk, Russia
³ present address: The University of Toronto, Department of Geology, 22 Russell St., Toronto, ON M5S 3B 1, Canada
⁴ Institut für Mineralogie, Johann Wolfgang Goethe-Universität, Senckenberganlage 30, D-60054 Frankfurt am Main, Germany
⁵ ISIS, Rutherford Appleton Lab., Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom

^* Corresponding author, e-mail: yuvs{at}uiggm.nsc.ru

The crystal structure of deuterated natrolite, Na_1.85Mg_0.05Ca_0.03[Al_2.06Si _2.95O₁₀]·nD₂O, compressed in liquid D₂O at 0.9 and 1.0 GPa has been determined from neutron powder diffraction data. At 0.9 GPa, the crystal structure is close to the original natrolite with the same space group Fdd2 and 1 % smaller unit cell volume. New water positions are found in addition to the original ones indicating the early stage of natrolite over-hydration. The unit cell volume of high-pressure phase stable at 1.0 GPa is expanded by 5.4 % with respect to initial natrolite. According to structural investigations, HP phase contains 3.5 water molecules pfu. Higher degree of hydration is accompanied by the drastic rearrangement of extra-framework subsystem, water molecules occupying four independent positions. Three of them belong to Na⁺ coordination sphere and together with three framework O-atoms form a distorted octahedron. Water molecule in the fourth position (occupancy = 0.5) has no contact to the cations. The evolution of natrolite structure with increasing pressure is discussed in terms of framework flexibility and hydrogen bonding rearrangement.

Key-words: zeolite, natrolite, phase transition, high pressure, crystal structure, over-hydration.

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