Silicate Mineral Structures and Classification

Silicates, the most abundant minerals of Earth's crust and mantle, are classified by how their fundamental SiO4 tetrahedra link together.

Definition

The structural classification of silicate minerals according to the connectivity of their silicon-oxygen tetrahedra and the cations that bind them.

Scope

This topic covers the silicate building block, the SiO4 tetrahedron, and the structural classes defined by its degree of polymerization: nesosilicates (isolated tetrahedra), sorosilicates (paired), cyclosilicates (rings), inosilicates (single and double chains), phyllosilicates (sheets), and tectosilicates (frameworks). It links each class to representative rock-forming minerals such as olivine, pyroxenes, amphiboles, micas, and feldspars.

Core questions

• How does the sharing of oxygen atoms between SiO4 tetrahedra define the six silicate structural classes?
• Why is the framework structure of feldspars and quartz so chemically and mechanically distinct from sheet silicates?
• How does the silicate structural class relate to cleavage, hardness, and habit?
• What cations occupy the non-tetrahedral sites and how do they vary?

Key theories

SiO4 polymerization classification

Silicates are grouped by the number of bridging oxygens shared between tetrahedra, ranging from isolated tetrahedra in nesosilicates to fully linked three-dimensional networks in tectosilicates, a scheme that organizes the rock-forming minerals.

Structure-property correlation in silicates

The dimensionality of tetrahedral linkage controls physical anisotropy: chain silicates cleave along prism faces, sheet silicates have one perfect basal cleavage, and framework silicates lack pronounced cleavage and resist weathering.

Clinical relevance

Because silicates dominate igneous, metamorphic, and many sedimentary rocks, their structural classification is the organizing principle of rock-forming mineralogy and is essential for petrographic identification and the interpretation of rock genesis.

History

Early X-ray structure determinations by W. L. Bragg and collaborators in the 1920s and 1930s revealed the tetrahedral linkages that define silicate classes, and the recognition by Machatschki of silicon-aluminum substitution in tetrahedral sites completed the modern structural classification later codified in Deer, Howie, and Zussman's reference series.

Key figures

• William Lawrence Bragg
• Felix Machatschki
• William Alexander Deer

Seminal works

• klein2007
• deer2013

Frequently asked questions

Why are silicates the most important mineral group?

Silicon and oxygen are the two most abundant elements in Earth's crust, so silicates make up over 90 percent of crustal minerals and most rock-forming species.

What is a tectosilicate?

A framework silicate in which every oxygen of each SiO4 tetrahedron is shared with a neighbor, forming a three-dimensional network; quartz and feldspars are the principal examples.