The picture shows a ring of three SiO4 tetrahedra seen from above, each sharing two corners with its neighbours. The anion has the formula

Fig. 1: Three triangles joined at their corners in a ring, drawn as one large triangle with a hexagonal hole in the middle. Each small triangle is an SiO4 tetrahedron seen from above, with a silicon dot at its centre. Each triangle shares two of its corners with its two neighbours. The fourth oxygen of each tetrahedron lies above its silicon and is not drawn. Six of the corners are shared in pairs and three corners are free.