The tectosilicates (framework silicates) are minerals in which the silica tetrahedra share all four O^2- corners with adjacent tetrahedra. The result is a lattice extending infinitely in three dimensions. As every O atom is bonded to two Si atoms, the Si to O ratio becomes 1 to 2 as in Quartz. The diversity of Tectosilicates is created by the partial substitution of Si⁴⁺ by Al³⁺. In this way a deficit of positive charge is created which is in turn balanced by the introduction of large metal cations such as Potassium, Sodium or Calcium. These ions are accomodated in cavities in the relatively open three dimensional framework. The tectosilicates include many of the main rock forming minerals and are a fairly homogenous group of minerals. They are typically colourless, or only pale coloured, have a vitrouos luster and a relatively low density because of the open structure. The latter also contributes to the relative softness of most member species of between 5 and 7.

Feldspar Silicates:

Feldspar silicates are an important subgroup of the tectosilicates because they are one of the most widespread groups of minerals. Estimates suggest that they alone constitute for more than half of the Earth's crust and are essential minerals in most igneous and metamorphic rocks. They arise because the Si ion is not totally stable in the oxygen tetrahedron and is often replaced by as much as 50% Al³⁺. Because the latter ions are larger than Si⁴⁺ they open out the framework creating gaps amongst the tetrahedra. These gaps are often occupied by opportunistic metal ions such as Potassium, Sodium and Calcium. Such combinations give rise to particular chemical properties and hence to distinguish these from "simple" framework silicates these minerals are known as the feldspar silicates. They are typically rather uniform in physical properties. All have two good cleavage directions perpendicular to each other, hardness of around 6 and relatively low density.

Zeolites:

Zeolites are an important subgroup of the tectosilicates because they have many industrial uses. They feature a very open lattice between the framework tetrahedra. The lattice consists of SiO₄ and AlO₄ tetrahedra that enclose open cavities that give space for H₂O molecules and small metal ions like Sodium and Potassium. The water is loosely bound in the structure and on heating is given off without causing the structure to collapse. It can then be easily reabsorbed by placing the specimen in water. Similarly the metal cations can also be removed and replaced by others. Thus Zeolites are capable of exchanging ions and for this reason they are widely used in industry for filtering out unwanted metal ions from solution in applications such as toxic waste disposal and soil conditioning. Compared with other Framework Silicates, zeolites have a lower hardness and lower density. This is the result of the open structure. They are usually colourless, white or pale coloured, and occur as needle-shaped or fibrous, tabular or cube-like crystals. Many form in cavities in basalts and other extrusive igneous rocks.