Summary
William Lawrence Bragg’s 1930 paper “The Structure of Silicates” establishes that the atomic architecture of silicate minerals is governed by the geometric packing of oxygen ions around silicon, with silicon-oxygen tetrahedra as the fundamental building block. Bragg systematically classifies silicates by how these tetrahedra link—isolated, paired, ring, chain, sheet, or three-dimensional framework—and demonstrates that this linkage determines each mineral’s physical properties, such as cleavage and hardness. The paper synthesizes X-ray diffraction data from the preceding decade to provide the first unified structural scheme for the most abundant mineral group in the Earth’s crust. A reader takes away a clear, rule-based understanding of how silicate structures arise from simple ionic coordination principles, and how this structural classification underpins mineral identification and petrology.
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Key concepts
- Silicon-oxygen tetrahedron — The fundamental structural unit in silicates, consisting of a silicon ion surrounded by four oxygen ions in a tetrahedral arrangement.
- Tetrahedral linkage — The mode of sharing oxygen atoms between adjacent tetrahedra (e.g., corner-sharing, edge-sharing) that defines silicate subclasses.
- Nesosilicate — A silicate subclass with isolated tetrahedra (e.g., olivine), where no oxygen atoms are shared between tetrahedra.
- Phyllosilicate — A silicate subclass with tetrahedra arranged in continuous sheets (e.g., mica), producing perfect basal cleavage.
- Tectosilicate — A silicate subclass with a three-dimensional framework of tetrahedra (e.g., quartz, feldspar), where every oxygen is shared between two tetrahedra.
- Pauling’s rules — Ionic bonding principles (applied by Bragg) that predict stable coordination geometries and charge balance in silicate structures.