In the silicate anion, the oxygen ions pack into the smallest space possible for four large spheres. The smallest space is taken if the oxygens sit at the corners of a tetrahedron and the small silicon cation sits in the space between the oxygens at the center of the tetrahedron.
Most silicate minerals contain a large number of silicate anions.
Silicate Structure - The silicate structure is formed by the linking of silicate tetrahedrons in a regular pattern. This regular pattern makes the silicate a crystal. There are five basic structures for the silicate minerals as listed below. Isolated tetrahedra - distinguished by the fact that none of the oxygens are shared by tetrahedrons so that individual silica tetrahedrons are bonded together by positively charged ions. Minerals with this structure include olivine .