Chemistry · Ch 2 — p-Block Elements-I
Silicates
Silicates
A silicate is any mineral built from [SiO₄]⁴⁻ tetrahedra -- one silicon atom at the centre, four oxygen atoms at the corners -- linked together in one of several characteristic patterns. Nearly 95% of the earth's crust is composed of silicate minerals and silica, and the entire glass and ceramics industries are built on silicate chemistry. Silicates are classified by exactly how their tetrahedra share oxygen atoms with their neighbours:
Ortho silicates (neso silicates) (Figure 2.11) are the simplest class: each [SiO₄]⁴⁻ tetrahedron stands entirely alone, sharing no oxygen with any other. Examples: phenacite, Be₂SiO₄ (Be²⁺ ions tetrahedrally surrounded by O²⁻), and olivine, (Fe/Mg)₂SiO₄ (Fe²⁺ and Mg²⁺ cations octahedrally surrounded by O²⁻).
Pyro silicates (soro silicates) (Figure 2.12) form when two [SiO₄]⁴⁻ tetrahedra join by sharing a single oxygen at one corner, giving the discrete ion [Si₂O₇]⁶⁻. Example: thortveitite, Sc₂Si₂O₇.
Cyclic silicates (ring silicates) (Figure 2.13) form when three or more tetrahedra link up cyclically, each sharing two of its oxygens with its two ring neighbours, giving ions of general formula (SiO₃)ₙ²ⁿ⁻. Example: beryl, Be₃Al₂(SiO₃)₆, an aluminosilicate in which every aluminium is octahedrally surrounded by six oxygen atoms.
Inosilicates are silicates built from 'n' silicate units linked by sharing two or more oxygen atoms, and come in two further sub-types. Chain silicates (pyroxenes) (Figure 2.14) contain [(SiO₃)ₙ]²ⁿ⁻ ions formed by linking 'n' tetrahedra in a straight line, each sharing two oxygens with its neighbours -- example: spodumene, LiAl(SiO₃)₂. Double chain silicates (amphiboles) (Figure 2.15) contain [Si₄O₁₁]ₙ⁶ⁿ⁻ ions, built from two distinct kinds of tetrahedra -- some sharing three vertices, some sharing only two -- example: asbestos, a fibrous, non-combustible silicate used for thermal insulation, brake linings, construction material and filters (though, being carcinogenic, its use is now restricted).
Sheet (phyllo) silicates (Figure 2.16) contain (Si₂O₅)ₙ²ⁿ⁻ ions, in which each [SiO₄]⁴⁻ tetrahedron shares three of its four oxygens with neighbouring tetrahedra, building an extended two-dimensional sheet. These sheets stack over one another, held together only by weak forces, so sheet silicates cleave easily -- exactly like graphite. Examples: talc, mica. …
What this figure shows. A single, discrete [SiO₄]⁴⁻ tetrahedron (silicon at the centre, one oxygen at each of the four corners) shown unshared and unlinked to any neighbouring tetrahedron, the simplest possible silicate uni …
What this figure shows. Two [SiO₄]⁴⁻ tetrahedra joined at a single shared corner oxygen, giving the discrete [Si₂O₇]⁶⁻ ion (one oxygen removed relative to two separate tetrahedra) that defines the pyro/soro silicate …
What this figure shows. Three or more [SiO₄]⁴⁻ tetrahedra linked corner-to-corner into a closed ring, each tetrahedron sharing two of its four oxygens with its two ring neighbours, giving the general (SiO₃)ₙ²ⁿ⁻ cyclic-silicate formul …
What this figure shows. An open-ended, indefinitely extending linear chain of [SiO₄]⁴⁻ tetrahedra, each sharing two of its four oxygens with the tetrahedron on either side, giving the pyroxene (single-chain) inosilicate formula (SiO₃)ₙ …
What this figure shows. Two parallel chain-silicate strands cross-linked to each other, built from two distinct tetrahedron environments -- some sharing three oxygens (with a neighbour in the other strand as well as their own strand) and some sharing only two -- giving the amphibole (double-chain) formula [Si …
What this figure shows. A two-dimensional, honeycomb-like sheet in which every [SiO₄]⁴⁻ tetrahedron shares three of its four oxygens with neighbouring tetrahedra in the same plane, giving the (Si₂O₅)ₙ²ⁿ⁻ sheet-silicate formula; successive sheets stack on top of each other, held together only by weak forces, so the mineral cleaves easily into thin layers jus …