Tetrahedral structures can be joined at a corner or along an edge. The number of tetrahedra and how
they are joined determine the overall structure. An effi cient way to represent silicate structures is illustrated
in Figure 14.1 , and the SiO 4 4 ion is shown as in Figure 14.1a . The structures are interpreted as
if seen from above the tetrahedral unit, so the large open circle represents an oxygen atom projected
upward out of the page. The fi lled circle represents a silicon atom directly below the oxygen atom,
and each vertex in the triangle represents an oxygen atom in the base of the tetrahedron. The various
silicate structures are then made up by sharing one or more corners or edges of the tetrahedra where
oxygen atoms form bridges.