High-temperature crystal structure of wairakite

Single crystal X-ray structure data have been obtained for wairakite (Wairakei, New Zealand), Ca0.95Na0.06[Al1.96Si4.04O12].2H(2)O, at temperatures of 20degreesC, 170degreesC, 210degreesC, 400degreesC, and 600degreesC. Heating of wairakite up to 200degreesC is accompanied by a significant increase in the unit cell volume. At 145degreesC, the initial monoclinic phase (I2/a) transforms into a tetragonal one (I4(1)/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 200degreesC the dehydration of wairakite begins and is accompanied by continuous contraction with no fundamental changes in the structure while retaining symmetry I4(1)/acd. The Ca2+ cations remain in the vicinity of the original positions, but their coordination changes from octahedral [6o] = O-4(H2O)(2), to semi-octahedral [5y] = O-4(H2O), square-pyramidal [4n] = O-4, and square-planar [4s] = O-4.