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High-temperature crystal structure of wairakite

¹ Institute of Mineralogy and Petrography, Russian Academy of Sciences, Koptyug prosp. 3, 630090 Novosibirsk, Russia
² Institut für Mineralogie, Johann Wolfgang Goethe-Universität, Senckenberganlage 30, D-60054 Frankfurt am Main, Germany
³ Institute of Inorganic Chemistry, Russian Academy of Sciences, Lavrentiev prosp 3, 630090 Novosibirsk, Russia

^* e-mail adress: yuvs{at}uiggm.nsc.ru

Single crystal X-ray structure data have been obtained for wairakite (Wairakei, New Zealand), Ca_0.95Na_0.06[Al_1.96Si_4.04O₁₂]·2H₂O, at temperatures of 20°C, 170°C, 210°C, 400°C, and 600°C. Heating of wairakite up to 200°C is accompanied by a significant increase in the unit cell volume. At 145°C, the initial monoclinic phase (I2/a) transforms into a tetragonal one (I4₁/acd). The main features of this reversible phase transformation are the rearrangement and the length changes in various H₂O-O contacts. Below the transition point the shortened H₂O-O contacts exist and provide structure stabilization possibility through the formation of weak H bonds. Upon heating above 200°C the dehydration of wairakite begins and is accompanied by continuous contraction with no fundamental changes in the structure while retaining symmetry I4₁/acd. The Ca²⁺ cations remain in the vicinity of the original positions, but their coordination changes from octahedral [60] = O₄(H₂O)₂, to semi-octahedral [5y] = O₄(H₂O), square-pyramidal [4n] = O₄, and square-planar [4s] = O₄.

Key-words: zeolite, wairakite, crystal structure, high temperature, phase transformation, dehydration.

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