Silica Group Minerals

Silicate Minerals

• Quartz SiO2 
• Cristobalite SiO2
• Tridymite SiO2 
• Coesite SiO2 
• Stishovite SiO2

Description of Minerals

The silica group minerals have composition SiO2, and all except stishovite have structures based on SiO4 tetrahedra linked at their vertices by “bridging” oxygen. Quartz, tridymite, and cristobalite have high temperature and low-temperature polymorphs; coesite and stishovite do not. Scientists have synthesized several additional SiO2 polymorphs in the laboratory. Although mineralogists have described many polymorphs, only common quartz, properly called low quartz, exists in substantial amounts; it is the second most abundant mineral in Earth’s crust.

The different polymorphs vary in the way SiO4 tetrahedra join to form a three dimensional framework. Consequently, they vary in symmetry. High quartz is hexagonal, low quartz is trigonal, low tridymite is orthorhombic, low cristobalite is tetragonal, and coesite is monoclinic.

Structural variations among the SiO2 polymorphs reflect the different conditions under which they form. Low quartz is the only stable SiO2 polymorph under normal Earth surface conditions, but some rocks contain metastable stishovite, coesite, cristobalite, or tridymite.

3D Structure

SiO4 tetrahedron in silicates

Silicon-oxygen tetrahedron representations in silicate minerals: (1) Ball & Stick, (2) Space filling and (3) Polyhedral.

The SiO4 tetrahedron (or SiO4 anionic group) is the basic building-block of silicate minerals. It is composed of one silicon ion (Si4+) (blue atom) surrounded by four oxygen ions (O2-) (red atom). The SiO4 tetrahedrons link up in various patterns and bond with metal ions to form specific silicate minerals. This is a fundamental component of most silicates in the Earth’s crust (more than 1200 minerals).

The Si-O bond is very strong and has about 50:50 ionic:covalent character. The average length of the Si-O bond is 1.62 Å. The discrete value of the Si-O bond depends on the nature of the cations present within the crystal structure.

Note: CPK color coding was used for element colors.