Silicates

An Introduction to
Silicate Structure Types
Because the chemical elements Si (silicon) and O (oxygen) make up 95% by volume (75% by weight) of the earth's crust, the most common minerals found there are silicates. When one atom of Si and four atoms of O combine in nature, the resulting configuration forms a tetrahedron. Within the mineral, these silicate tetrahedra link in various degrees to form a variety of silicate structure types. These include so called nesosilicates, sorosilicates, inosilicates, cyclosilicates, phyllosilicates and framework silicates.

Each of the silicate structure types is illustrated below and is presented first by showing the traditional "ball and stick" model of the structure, and second by showing only the silicate tetrahedra drawn as polygons.


Figure 1a. The crystal structure of olivine ((Fe,Mg)₂SiO₄).	Figure 1b. Looking only at a polygon view of the silicate tetrahedra, we can see each is isolated from the other. Because the structure possesses isolated silicate tetrahedra, olivine is called an nesosilicate. The term is derived from the Greek word (nesogaean) that means "island".

Figure 2a. The crystal structure of Ilvaite (CaFe₃Si₂O₈(OH)).	Figure 2b. Looking only at a polygon view of the silicate tetrahedra, we can see each tetrahedra is linked at a corner to form pairs. Because the structure possesses double island silicate tetrahedra, ilvaite is called an sorosilicate. The term is derived from a Greek word that means "group".

Figure 3a. The crystal structure of Pectolite (Ca₂NaH(SiO₃)₃).	Figure 3b. Looking only at a polygon view of the silicate tetrahedra, we can see each tetrahedra is linked to two others at the corners to form single chains. Because the structure possesses parallel single chains of silicate tetrahedra, pectolite is called an inosilicate (single chain). This type of structure is represented by the pyroxenes. The term is derived from a Greek word that means "chain".

Figure 4a. The crystal structure of Tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂).	Figure 4b. Looking only at a polygon view of the silicate tetrahedra, we can see each tetrahedra is linked at the corners to form double chains. Because the structure possesses parallel double chains of silicate tetrahedra, tremolite is called an inosilicate (double chain). This type of structure is represented by the amphiboles.

Figure 5a. The crystal structure of Beryl (Be₃Al₂(Si₆O₁₈)).	Figure 5b. Looking only at a polygon view of the silicate tetrahedra, we can see each tetrahedra is linked at the corners to form rings. Because the structure possesses isolated rings of silicate tetrahedra, beryl is called an cyclosilicate. The term is derived from a Greek word that means "ring".

Figure 6a. The crystal structure of Biotite (K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂).	Figure 6b. Looking only at a polygon view of the silicate tetrahedra, we can see each tetrahedra is linked at three corners to form a sheet. Because the structure possesses parallel sheets of silicate tetrahedra, biotite is called an phyllosilicate. This type of structure is represented by the micas. The term is derived from a Greek word that means "sheet".

Figure 7a. The crystal structure of Quartz (SiO₂).	Figure 7b. Looking only at a polygon view of the silicate tetrahedra, we can see that every tetrahedra is linked at each corner to form a framework. Because the structure possesses a three-dimensional framework of silicate tetrahedra, quartz is called an framework silicate.

These images were generated using the program Xtaldraw. They are meant to serve as an introduction to the basic silicate structure types as well as to illustrate some of the program features. Note that figure xb is obtained from figure xa with very few keystrokes. This method can be applied to identify the structure type of any silicate mineral.