What are silicates in mineralogy ?

Silicates : definition

Silicates constitute the essential mineralogical class, comprising minerals made up of a sequence of elementary tetrahedra (SiO4)4-, the fundamental building block of the crystalline structure. The very strong Si-O bond can be considered as the cement of the earth's crust.

These tetrahedra can be free, assembled by different cations (Mg2+, Fe2+, Ca2+, Mn2+, K+, Na+...), or linked together by their vertices. There are 6 ways to achieve these sequences depending on the number of oxygen ions involved, giving 6 structural configurations which are the basis of the subdivision of silicates into six subclasses :

- nesosilicates with isolated tetrahedra (olivines, garnets, etc...)
- sorosilicates with tetrahedra linked in pairs (epidote, vesuvianite, etc...)
- cyclosilicates with ring structures involving at least 3 tetrahedra (tourmalinesberyls, etc...)
- inosilicates, made up of single or double chains (ribbons), (pyroxenes, amphiboles, etc...)
- phyllosilicates with tetrahedra assembled in layers (micas, clays, talc, chlorite, etc...).
- tectosilicates with tetrahedra bound by their four vertices in the three directions of space (quartz, feldspars, feldspathoids, etc...).