EVOLUTION OF CALCIUM-COORDINATION IN WAIRAKITE DURING HIGH TEMPERATURE TRANSFORMATIONS. Seryotkin Yu.V.1, Joswig W.2, Bakakin V.V.3, Fursenko B.A.1, Belitsky I.A.1. 1Institute of Mineralogy and Petrography, Novosibirsk, 630090, Russia; 2Institute of Kristallography, Frankfurt University, Germany; 3Institute of Inorganic Chemistry, Novosibirsk, 630090, Russia.
Single crystal X-ray structure data are obtained for wairakite (Wairakei, New Zealand) - Ca0.95Na0.06[Al1.96Si4.04O12]*2H2O at temperatures 20deg.C (I), 170deg.C (II), 210deg.C (III), 400deg.C (IV), 600deg.C (V), and again at 20deg.C (Idh - dehydrated at 600deg.C and quenched). I - monoclinic, 13.666, 13.623, 13.531Å, ( = 90.50deg., I2/a. Al-fractions in 6 various Si,Al-tetrahedra correlate with Ca(Na) positions and are (%): 2, 3, 8, 13, 84, 89. Above 140deg.C I transforms to tetragonal phase II (I41/acd) - 13.712, 13.681 Å, [[Delta]]V(+2%. No water loss was observed below 200deg.C. III - 13.713, 13.687 Å. Na sites were localized in II and III. IV - is partially dehydrated (to 0.62 H2O per formula unit) - 13.662, 13.555 Å. V is fully dehydrated - 13.645, 13.515Å. Phase Idh has monoclinic superstructure - 27.257, 27.253, 13.432Å, [[beta]] =90.20deg.. Structure data for Idh are presented in a subcell with 13.629, 13.627, 13.435Å, [[beta]]=90.21deg., I2/a. Ca coordination changes as follows (Fig.): in I, II, III - octahedral [6o] ( O4(H2O) 2, (avr Ca-O = 2.39 Å); in IV - semioctahedral [5y] ( O4(H2O) (2.36 Å) and pyramidal [4n] ( O4 (2.40 Å) - statistically with the ratio 0.48/0.47); in V - [4n] (2.37 Å) and square-coplanar [4s] (2.31 Å) with the ratio 0.65/0.31; in Idh - five-fold [5] ( O5 (2.40 Å), [4n] (2.38 Å) and [4s] (2.33Å) with the ratio 0.52/0.27/0.19.