What is the structural difference between …

• olivine
• garnet
• aluminosilicates
  • kyanite
  • andalusite
  • sillimanite
• topaz
• staurolite
• zircon
• titanite or sphene
• chloritoid
• humite group

Pyrosilicates are silicates which contain Si₂O₇^6-. They are formed by joining two tetrahedral Si₂O₄^4-. When two Si₂O₄^4- are joined, there is removal of one oxygen atom and the two units join at the corner oxygen atom. Example is thortveitite, Sc₂Si₂O₇. The structure of pyrosilicate is:

The reds are oxygen atoms and greens are silicon.

Cyclic silicates contains (SiO₃)_n^2n- which are formed by linking three or more tetrahedral SiO₄^4- units cyclically. Each SiO₄^4- shares two oxygen atoms with other unit. Example is Benitoite, BaTi(SiO₃)₃. Here three SiO₄^4- units are arranged cyclically.

<i>Orthosilicates: </i>In simple Orthosilicates the SiO₄ ^4- tetrahedron units do not share any of its oxygen atoms with any of the other tetrahedra units. E.g. Willmenite ZnSiO₄, Zircon ZrSiO₄.

<i> </i><i>Pyrosilicates or Island silicates</i>

In Pyrosilicates one oxygen atom is shared by two silicate units General formula is (Si₂O₇)^6-

E.g. Thortevitite Sc₂[Si₂O₇] and Hemimorphite Zn(OH)₂(Si₂O₇)H₂O