
%0 Journal Article
%A Moore, PB 
%T Betpakdalite Unmasked, and a Comment on Bond Valences
%J Australian Journal of Chemistry
%D 1992/09/01
%V 45
%N 9
%P 1335-1354
%K 
%R https://doi.org/10.1071/CH9921335
%U https://www.publish.csiro.au/paper/CH9921335 
%X A new representation of the crystal structure of complex betpakdalite reported in 1984 (R &#8776; 0.08) reveals a remarkable case of . hccchhccch 10-layer anion (O2-)/void ( &#8298;) close-packing. The transformation matrix from earlier C12/m 1 cell to new F 2/m 11 cell is [010/100/102], new a = 11.10, b = 19.44, c = 22. 93 Å, &#945;)/10 = 2.29 &#506;. The packing efficiency is Ve = 15.45 Å &#945;3 for 10 layers of 32(O2-+ &#8298;) per layer. For Pauling r(O2-) = 1.40 Å, Ve = 15.52 Å3. Oxide anions populate only 46.25% of the close-packed slots; the rest are voids. The difference, &#916;, between real structure and ideal (perfect) structure is (0.13 Å) for O2-, (0.20 Å) for cations and (0.15 A) for all atoms in the close-packed frame, range 0.00-0.43 Å. The cell formula for x = 2 is H8 [K(H2O)6]4 [Ca(H20)6]8 [MO6+32 As5+8O148]28-.8H2O. It is suggested that H8 bonds with 8O(10) in the cell, the only anion bonded to only one Mo6+. The cell formula gives D(calc) = 2.90 g cm-3, close to reported D( obs ) = 2.98, 3.05; and 2.92 g cm-3. Bond strength/bond valence calculations for assorted Mp+6 ø q-  (M = octahedral cation linked to generalized anion ø) often show extreme deviations. This is shown to be a consequence of Pauling's third rule of cation-cation repulsions across shared polyhedral edges and faces. 
