Silicon–oxygen tetrahedron

The SiO4 unit of one silicon atom bonded to four oxygen atoms at the corners of a tetrahedron; the basic building block of all silicate minerals.

Silicate minerals: Rock-forming minerals built from silicon–oxygen tetrahedra; they make up over 90 percent of Earth's crust. The tetrahedron is the single unit from which every silicate mineral is assembled.
Ionic radius: The effective radius of an ion in a crystal; relative sizes of cations and anions determine coordination. The silicon cation is small enough to fit four oxygens around it, but not six, hence the tetrahedron.
Olivine: A nesosilicate with isolated tetrahedra bonded to magnesium or iron cations; a major mantle mineral. The extreme case: tetrahedra share no corners with each other at all.
Lawrence Bragg: Physicist who shared the 1915 Nobel Prize for X-ray crystallography and led structural analysis of silicates. His Manchester school determined the first silicate structures, establishing the tetrahedral building block.
Mohs hardness: A ten-point scratch scale ordering minerals from talc (1) to diamond (10). Quartz's hardness of 7 follows directly from its fully linked tetrahedral framework.
Bridging oxygen: An oxygen atom shared between two tetrahedra, linking them into larger structures. Whether an oxygen bridges one or two tetrahedra determines how units connect.
Coordination number: The number of nearest neighbor atoms surrounding a central atom in a crystal structure. Fourfold coordination of silicon by oxygen defines the tetrahedral unit itself.
Pyroxene: Chain silicates in which tetrahedra share two corners each, forming single chains with formula Si2O6. Shows two-corner sharing producing one-dimensional chains.
X-ray diffraction: A technique inferring atomic positions from how crystals scatter X-rays, based on Bragg's law. The method that revealed silicon's fourfold coordination with oxygen in real crystals.
Cleavage (mineralogy): The tendency of a mineral to break along planar surfaces where bonds are weakest. Mica cleaves because bonds between tetrahedral sheets are far weaker than bonds within them.

Linked from 13 pages