High-temperature crystal structure of wairakite

Abstract

Single crystal X-ray structure data have been obtained for wairakite (Wairakei, New Zealand), Ca 0.95 Na 0.06 [Al 1.96 Si 4.04 O 12 ]·2H 2 O, 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 ( I4 1 /acd ). The main features of this reversible phase transformation are the rearrangement and the length changes in various H 2 O-O contacts. Below the transition point the shortened H 2 O-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 I4 1 /acd. The Ca 2+ cations remain in the vicinity of the original positions, but their coordination changes from octahedral [60] = O 4 (H 2 O) 2 , to semi-octahedral [5y] = O 4 (H 2 O), square-pyramidal [4n] = O 4 , and square-planar [4s] = O 4 .