Framework silicates are formed when all four oxygens of a
silicon tetrahedra are linked to other tetrahedra. A
three-dimensional framework is formed, with every oxygen linked to two silicons and therefore charge-balanced. This formula, SiO
2, is
quartz: the most common of the silica minerals.
Since this arrangement leaves no space for other ions, variety is introduced by replacing the Si ion with either Aluminum(Al), Potassium(K), Sodium(Na) or Calcium(Ca).
The most common framework silicates are the feldspars: these minerals are extremely common, composing roughly 50% of the Earth's crust.
For more general information, see silicon tetrahedra.