The types of silicates are classified based on the interactive formation of the tetrahedrons. Each group of silicates has a correct ratio of silicon and oxygen, but in some minerals the silicon atoms are replaced by other atoms. This is because an aluminum atom takes the place of a silicon atom, thus leaving only three silicon atoms. In this group of silicates, all four oxygen atoms from each tetrahedron shares itself with an adjacent tetrahedron. The tectosilicates are compact minerals and are light in weight.
None are soft.
The phyllosilicates have each set of tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure. The phyllosilicates have quality basal cleavage, and usually occur in a flaky habit. The tetrahedrons in this group form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains. These minerals are tough, compact, and hard. They form prismatic and fibrous crystals.