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) Ang strom (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 fo r in the refinements. The compound belongs to the group of single-layer sil icates. Individual sheets can be described as being built by the condensati on of Zweier single chains of SiO4 tetrahedra parallel to the a axis or, al ternatively, by condensation of Vierer single chains parallel to the b axis . The layers contain six-membered rings in UUDUUD or DDUDDU conformation. T he stacking of the layers parallel to the c axis results in a three-dimensi onal structure in which the alkali cations reside between the layers for ch arge compensation. The distribution of the alkali atoms among the two cryst allographically different M(l) and M(2) positions shows a definite preferen ce by the larger potassium for the M(l) site. The smaller M(2) site is K-fr ee.