Crystal Structure of Na2Mg2Si6O15

THE compound Na2Mg2Si6O15 has been known since 1937 (ref. 1), but crystal data for it were only determined recently2. It is orthorhombic with a=10.205 Å, b=17.59 Å and c= 14.165 Å; the space group is Acam or Aba2 and Z=8. We have now found the crystal structure, using three-dimensional methods, to be Acam. The structure was solved using symbolic addition, and has so far been refined to R=0.15 on 383 observed, independent reflexions. It is interesting because it contains a new type of silicate anion (Fig. 1). This can be described as a corrugated ribbon, two tetrahedra wide, which is made from rings of four tetrahedra fused together, and which repeats at intervals of six pairs of tetrahedra. Each tetrahedron shares three corners with neighbouring ones, thus giving the empirical formula Si2O52−. The anions are held together by Mg2+ and Na+ ions. Some of the Mg2+ ions are tetrahedrally coordinated and others are octahedrally coordinated, and some of the Na+ ions have nine oxygen neighbours within 3.1 Å whereas others have ten.