The crystal structure of wairakite

The crystal structure of wairakite has been determined and refined to R = 3.6%, using a crystal from Onikobe, Miyagi Prefecture, Japan: I2/a, a = 13.692(3), b = 13.643(3), c = 13.560(3)A, β = 90.5(1)°, Z = Ca_7.19Na_1.2K_0.01(Si_32.59 Al_15.38) O₉₆ · 16H₂O. The location of Ca atoms is fixed to a set of eightfold positions among four distinct sets of positions available for cations, the eightfold positions being occupied by 0.899 Ca and 0.059 Na. The Na atoms are nearly evenly distributed over the available cation positions with occupancies of 0.05-0.07. The Al atoms are preferentially concentrated in two sets of tetrahedral positions associated to Ca; the Al fraction in the tetrahedral positions are the same. The ratio of Ca fraction in the Ca site to Al fraction in the tetrahedra is almost unity. These structural features characterize the monoclinic symmetry of wairakite. The occurrence of twinning on (110) characteristic of wairakite can be explained in terms of the structure. However, to elucidate structurally the nature of the wairakite-analcime solid solution, further structural study is required, particularly of crystals with intermediate compositions.