Polysilicates

Compounds formed by the reaction of the acidic oxide silica (SiO₂) with various basic metal oxides are of great importance in the geochemistry of the earth's crust.

These compounds contain silicon oxo anions which possess covalent Si-O bonds. Although the simple silicate ion SiO₄^4- is sometimes found, many of these compounds have 2-coordinate oxygen atoms that link silicon atoms together into oligomeric or one-, two-, or three-dimensional polymers.

These complex silicates are the most common examples of polynuclear oxo anions. Follow the links on this page to learn more about the chemistry and geology of silicates.

Background Information:

Some Mineral Basics

The Chemistry of Silicic Acid

Minerals by Chemical Composition

Silicates

Structures of Silicates

Silicate minerals are divided into subclasses according to their structures.

The basic structural types are:

The orthosilicates (single tetrahedrons)
Oligomeric Silicates
Cyclic Polysilicates
Chain Polysilicates
Sheet Polysilicates
Framework Polysilicates

The Orthosilicates

The orthosilicate ions SiO₄^4- is a very strong base that cannot persist in aqueous solution. It tends to occur in nature as the insoluble salt of acidic cations. The orthosilicates are also referred to as the Nesosilicates.

Oligomeric Silicates

The disilicate ion Si₂O₇ contains a 2-coordinate oxygen atom. When the oxygen from one unit becomes a bridge to the other unit, a terminal oxygen is removed. In this ion the charge density per silicon is lowered from the -4 in the orthosilicate to -3. The disilicate ion is an example of an dimer of the [SiO₃]^2- ion. The disilicate ion is less basic than the orthosilicate ion. Minerals containing the disilicate ion are fairly uncommon in nature and those containing larger oligomers are even more rare. These mineral are classified as sorosilicates.

Cyclic Polysilicates

Cyclic silicates form when the ends of chain structures join. The ring formation removes an additional terminal oxygen. In these structures each silicon has two bridging and two terminal oxygen atoms, and the charge density is -2 per silicon atom. Cyclic trimers [SiO₃] ₃^6- and hexamers [SiO₃] ₆^12-. These structures are sometimes called metasilicates and can be classified as cyclosilicates.

Chain Polysilicates

Single chain polysilicates form when a number of [SiO₃]^2- units link together into long chains. In these structures each silicon has 2 terminal oxygens and 2 bridging oxygens. The charge density is -2 per silicon atom. Single chain polysilicates comprise part of the subclass inosilicates.

Linear chains may be linked side-by-side into a double chain structures when a terminal oxide on silicon is replaced by terminal oxygen on another chain. When two chains are joined by bridging alternate SiO₃ groups in each chain, a double chain structure [Si₄O₁₁^6n- results. Each silicon used to bind together the chains will have a charge density of -1 while those not linked will have a charge density of -2. These double chain structures are also classified as inosilicates.

Sheet Polysilicates

When side-to-side linking of chains is continued indefinitely, more oxides are eliminated yielding 2-dimensional polymers called sheet polysilicates of the formula [Si₄O₁₀]_n^4n-. A polysilicate sheet looks a bit like a sheet of chicken wire composed of a hexagonal network. These sheet polysilicates are classified as phyllosilicates. In these sheets each silicon possesses 1 terminal oxygen and 3 bridging oxygens. The charge density per silicon is -1.

Framework Polysilicates

Framework polysilicates form when each silicon is joined to 4 other silicons via bridging oxygens giving a a 3-dimensional polymeric structure. The monomeric structural unit of these polymers is SiO₂. This is an uncharged oxide silica which is no longer basic, but rather acidic. These framework polysilicates are classified as tectosilicates.

Chemical Weathering

Weathering is a combination of physical and chemical changes which break down rocks and is an important process in the formation of soils.

The basicity of a polysilicate species determines how susceptible that species will be to chemical weathering. The more highly polymerized the polysilicate ions the lower the basicity of those ions. The more basic the polysilicate anion of a mineral, the more readily it will react with weakly acidic solutions causing weathering. Even pure rainwater is slightly acidic due to dissolved CO₂. Overtime, rainwater will react with the less polymerized silicate anions, replacing terminal oxygens (lost as water molecules) with bridging oxygens to yield a more highly polymerized silicate. In these processes, the negative charge on the resulting silicate is reduced (negative charge density on each silicon decreased by one for each terminal oxygen lost) allowing cation nutrients in the soil to be washed away. The age and often the fertility of a soil can be determined by the types of polysilicates present. Soils which contain large amounts of orthosilicates such as olivine are either youthful or present in desert regions. Soils with large amounts of layer silicates such as clay are considered intermediate in age and are often found in terperate regions under a cover of vegitation. These soils are less fertile than the youthful types of soils. Soils with large amounts of acidic silica minerals are often quite infertile because they are incapable of holding onto essential non acidic and feebly acidic nutrient metal ions. In the tropics, soils weather more quickly when exposed due to the hot and rainy climate. Thus, when tropical rain forests are cleared, the soils can maintain nutrient cations to sustain agriculture for a few years.