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

The crystal structure of Na-1.55 K0.45Si2O5 has been solved and refined to a final residual R1 of 0.048 for 1208 independent reflections. The compound is monoclinic with space group P2(1)/c (a = 4.845(1) Angstrom, b = 8.647(2) Angstrom, c = 11.992(3) Angstrom, beta = 90.31(4)degrees, V= 503.1(4) Angstrom (3), M-r = 189.40 u, Z = 4, lambda (MoKalpha) = 0.71073 Angstrom, D-x = 2.51 g/cm(3), mu (MoKalpha) = 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 SiO4 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(l) and M(2) positions shows a definite preference by the larger potassium for the M(l) site. The smaller M(2) site is K-free.