The crystal structure of a mixed alkali phyllosilicate with composition Na1.55K0.45Si2O5

Abstract

The crystal structure of Na 1.55 K 0.45 Si 2 O 5 has been solved and refined to a final residual R 1 of 0.048 for 1208 independent reflections. The compound is monoclinic with space group P 2 1 /c ( a = 4.845(1) A, b = 8.647(2) A, c = 11.992(3) A, β = 90.31(4)°, V = 503.1(4) A 3 , M r = 189.40 u , Z = 4,λ(Mo K α ) = 0.71073 A, D x = 2.51 g/cm 3 , μ(Mo K α ) = 1.14 mm −1 ). The crystal showed twinning by pseudo-merohedry according to 2 [100] , a feature which was accounted for in the refinements. The compound belongs to the group of single-layer silicates. Individual sheets can be described as being built by the condensation of Zweier single chains of SiO 4 tetrahedra parallel to the a axis or, alternatively, by condensation of Vierer single chains parallel to the b axis. The layers contain six-membered rings in UUDUUD or DDUDDU conformation. The stacking of the layers parallel to the c axis results in a three-dimensional structure in which the alkali cations reside between the layers for charge compensation. The distribution of the alkali atoms among the two crystallographically different M(1) and M(2) positions shows a definite preference by the larger potassium for the M(1) site. The smaller M(2) site is K-free.