MINERALOGY OF THE SILICATES Si, after O is the most abundant element in the Earth’s crust and mantle. 95% of the earth’s crust is made up of silicate minerals In virtually all silicate minerals in the earth crust, Si is tetrahedrally co-ordinated by O, and so the basis of all silicate minerals is the SiO4 tetrahedron.

The SiO4 tetrahedron - the basic building unit of all silicate structures The net charge on an isolated tetrahedron is -4 i.e. [SiO4]4-

SiO4 tetrahedra are always connected by corners Non-bridging oxygens Bridging oxygens are like flexible joints in silicate structures

Calculating the Si:O ratio non-bridging oxygens For a single tetrahedron there is 1 Si and 4 O i.e. [SiO4]4 – A bridging oxygen belongs to two tetrahedra and so contributes 1/2 an O to each Si Therefore for the ‘dimer’ shown above the Si:O ratio is 2:7 i.e. [Si2O7]6-

Building up silicate mineral structures Silicate structures can be formed from single isolated tetrahedra, where the negative charge on the tetrahedra is balanced by positive ions which hold the structure together or more complex structures can be made by connecting the tetrahedra, increasing the number of bridging oxygens from 0 1 2 4 Classification of silicate structures Most classifications are based on the way in which the tetrahedra are connected …….

Neso-, Soro-, Cyclosilicates [SiO4]4- Isolated tetrahedra Nesosilicates Examples: olivine garnet [Si2O7]6- Dimer of tetrahedra Sorosilicates Examples: lawsonite n[SiO3]2- n = 3, 4, 6 Cyclosilicates Examples: benitoite BaTi[Si3O9] axinite Ca3Al2BO3[Si4O12]OH beryl Be3Al2[Si6O18]

Increasing silicate polymerisation II Each tetrahedron shares two corners. The Si:O ratio is therefore 1:3 The formula of the repeating unit (outlined) is [Si2O6]4-

Increasing silicate polymerisation III Double chain of tetrahedra Half of the tetrahedra share two corners, the other half share three corners. The Si:O ratio is therefore 1:3 for one half, and 1:2.5 for the other half, so the overall Si:O ratio is 1:2.75 The formula of the repeating unit (outlined) is [Si4O11]6-

Silicates are classified on the basis of Si-O polymerization Mineral Structures Silicates are classified on the basis of Si-O polymerization [SiO3]2- = [Si2O6]4- single chains Inosilicates [Si4O11]4- Double tetrahedra pryoxenes pyroxenoids amphiboles

Increasing silicate polymerisation IV Part of a continuous sheet of tetrahedra, Each tetrahedron shares 3 corners. The Si:O ratio is therefore 1:2.5. The formula of the repeating unit is [Si2O5]2- or [Si4O10]4-

Silicates are classified on the basis of Si-O polymerization Mineral Structures Silicates are classified on the basis of Si-O polymerization [Si2O5]2- or [Si4O10]2- Sheets of tetrahedra Phyllosilicates Mica, clay, asbestos minerals

Increasing silicate polymerisation V The maximum degree of polymerisation is when each tetrahedron shares all 4 oxygens with neighbouring tetrahedra. These are the framework silicates.

Increasing silicate polymerisation V The maximum degree of polymerisation is when each tetrahedron shares all 4 oxygens with neighbouring tetrahedra. These are the framework silicates. The Si:O ratio is 1:2 and therefore the formula is SiO2, with no net charge on the framework. The mineral quartz, SiO2 is a framework silicate.

Increasing silicate polymerisation V The maximum degree of polymerisation is when each tetrahedron shares all 4 oxygens with neighbouring tetrahedra. These are the framework silicates. The Si:O ratio is 1:2 and therefore the formula is SiO2, with no net charge on the framework. The mineral quartz, SiO2 is a framework silicate. There are many ways in which frameworks can be built up.

Increasing silicate polymerisation V The maximum degree of polymerisation is when each tetrahedron shares all 4 oxygens with neighbouring tetrahedra. These are the framework silicates. The Si:O ratio is 1:2 and therefore the formula is SiO2, with no net charge on the framework. The mineral quartz, SiO2 is a framework silicate. There are many ways in which frameworks can be built up. Also, Al3+ can substitute for Si4+ in the tetrahedra, which gives the framework a net negative charge which requires additional cations for charge balance.

Silicates are classified on the basis of Si-O polymerisation Mineral Structures Silicates are classified on the basis of Si-O polymerisation low-quartz [SiO2] 3-D frameworks of tetrahedra: fully polymerized Tectosilicates quartz and the silica minerals feldspars feldspathoids zeolites

FRAMEWORK SILICATES More than three quarters of the Earth’s crust is composed of framework silicates. By far the most common are quartz and feldspars. The structure of all framework silicates is based on a network of TO4 tetrahedra, in which T is Si4+ or Al3+, and all four O atoms are shared with other tetrahedra.

Conventionally, there are seven silicate subclasses: Nesosilicates (isolated silicates) Sorosilicates (double silicates) Cyclosilicates (ring silicates) Inosilicates (single-chain silicates) “ “ (double-chain silicates) Phyllosilicates (sheet silicates) Tectosilicates (framework silicates)

However, there are many exceptions in this system However, there are many exceptions in this system. A more rational system is given below: isolated silicates double silicates 0 - dimensional ring silicates single-chain silicates 1 - dimensional double-chain silicates sheet silicates 2 - dimensional framework silicates 3 - dimensional