Single crystal X-ray structure data have been obtained for wairakite (Wairakei, New Zealand), Ca0.95Na0.06[Al1.96Si4.04O12]·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.

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