Monday, August 31, 2020

Classification of the silicates (6 classes)

 Have you ever seen a glamorous purple crystal known as amethyst? Do you know from what is amethyst made up of? The answer is simple. It is SiO2 or silica.

Classification of the silicates (6 classes)

Silicate minerals are abundantly available in nature and are also of greater importance than any other mineral group.

According to Dana, about 25% of the known minerals and 40% of the common ones are silicates. They constitute 90% of the Earth's crust. With few exceptions, all igneous rock forming minerals are silicates.

There are various soil types in nature but silicate minerals form a large part of the soils. The building material like bricks, stones, concrete and glass are made up of silicates or are derived from silicate minerals.

Also the importance of silicate minerals can be realized from the fact that the moon and the terrestrial planets have a rocky crust made up of silica and oxygen.

The building unit for all the silicates is a tetrahedron structure having Si at the center and O2- at the apices of tetrahedron.

The bonding between the silicon and oxygen is 50% ionic and 50% covalent as per the Pauling electronegativity concept. Thus the bond is by the attraction of oppositely charged ions and also by the sharing of valence electrons.

Every oxygen ion at the apices of tetrahedron has the capacity to bond with silica of another tetrahedra. A bridge-like structure gets formed. The process is known as "Polymerization".

Diverse configurations are obtained by the sharing of oxygens that may involve sharing of some or all of the oxygen of tetrahedra.

Based on the various configurations, there are six classes of silicate minerals:

  • Nesosilicates or orthosilicates
  • Sorosilicates or bisilicates
  • Cyclosilicates or ring silicates
  • Inosilicates (Single chain)
  • Inosilicates (Double chain)
  • Phyllosilicates
  • Tectosilicates

Nesosilicates: The term "Neso" denotes an island. Nesosilicates have an isolated silica tetrahedron. The oxygens are not bonded with other silica. The various tetrahedra are bonded to each other by ionic bonds due to interstitial cations.

The minerals belonging to the nesosilicate class have high specific gravity because of the dense atomic packing. Also the minerals are hard.

Classification of the silicates (6 classes)

Sometimes Al3+ substitutes Si in the nesosilicate class. The minerals belonging to this class are equidimensional.

The most common minerals belonging to this class is the olivine (Mg,Fe)2SiO4 series. Garnets, kyanite, sillimanite, andalusite, phenacite, willemite, zircon, topaz and sphene are also the minerals of this group.

Sorosilicates: are also known as disilicate minerals. The meaning of the term "Soro" is heap. 

Two SiO4 are linked together giving rise to Si2O7. About 70 minerals belong to this class. However most minerals are rare in nature.

Epidote group, hemimorphite and lawsonite belong to this class.

Cyclosilicates: are also referred to as "ring silicates". The cyclosilicates contain a ring of linkes SiO4 tetrahedra. The ratio of Si:O is 1:3. 

Three configurations are possible for the cyclosilicates. The simplest ring is Si3O9 represented by a rare mineral known as titanosilicate bentonite.Another ring structure is Si4O12 represented by papagonite. 

Classification of the silicates (6 classes)

The Si6O18 is the basic framework of the structures of important minerals of this group i.e. beryl and tourmaline.

The common minerals are beryl, tourmaline, cordierite and axinite.

Inosilicates: are known as chain silicates. It can be single chain or double chain. SiO4 tetrahedra are linked in the form of chains by sharing of the oxygen.

Simple chains may join side by side by sharing oxygen in alternate tetrahedra to form bands or double chain structure.

In the single chain structures, two oxygen out of four are involved in forming covalent bonds thereby giving the 1:3 silicon: oxygen ratio.

Double chain inosilicates have a quite complicated structure. Half of the tetrahedra of double chains share three oxygen whereas the remaining half share two oxygen. The ratio of Si:O is 4:11.

We have two important rock forming groups of minerals in inosilicates i.e. pyroxene as single chain members and amphiboles as double chain members.

Classification of the silicates (6 classes)
Thin section of basalt 

There are many similarities between both the groups. Similarities exist in crystallographic, physical and chemical parameters. 

Most of the pyroxenes and amphiboles are monoclinic  but orthorhombic members are also present.

Classification of the silicates (6 classes)

Both pyroxenes and amphiboles have the same cations but the difference is the presence of OH groups in the amphiboles.

Because of the hydroxyl group, amphiboles are characterized by lower specific gravity than pyroxenes. 

Both the groups can be differentiated on the basis of crystal habits. Pyroxenes occur as prisms whereas amphiboles have acicular habit. Acicular means fibrous nature and the crystals replicate needle structure.

Based on the hydroxyl group, one more interpretation can be done. Interpretation about the temperature of crystallization can be done. As the hydroxyl group is absent in pyroxenes, the temperature of crystallization must be higher compared to amphiboles.

Thus we can say that pyroxenes form earlier than amphiboles during the cooling of magma.

Bonus tip

 We can have some idea about the type of metamorphism occurring in an area by looking at the mineralogy.

During prograde metamorphism, amphiboles get converted to pyroxenes by the loss of OH group.

During retrograde metamorphism, due to presence of metamorphic fluids pyroxenes may transform to amphiboles. 

Phyllosilicates:  the greek term "phyllon" is used for leaf. Thus the crystal habit of phyllosilicates is flaky and leaf like.

Classification of the silicates (6 classes)
Mica flake

The minerals belonging to this group are generally soft, flaky and have low specific gravity.

Tetrahedra having four oxygen share three oxygen with the neighbouring tetrahedra. The ratio of Si:O is 2:5.

The common mineral of this group is mica. Mica is known for its flexibility  and elasticity. 

The members of the group bear hydroxyl group and that is located in the center of 6 fold rings of tetrahedra.

Tectosilicate: all the four oxygen are shared with the neighbouring tetrahedra. The ratio of Si:O is 1:2. 

The bond between the silicon and oxygen is stable and strong.

The common minerals belonging to this class are SiO2 group, feldspar, feldspathoid and zeolite group.

Classification of the silicates (6 classes)
Rose quartz