Silicates – a simple guide to the Earth’s rock forming minerals

While high school students are taught about chemistry using simple examples of inorganic and organic substances, very few are exposed to the chemistry of the most abundant group of chemicals on Earth- the silicate group of minerals.  Silicate minerals can be used to study bonding, crystal structure, differentiation of chemistry due to temperature, physical properties and so much more.So what’s so special about the silicates?

While they don’t have systematic naming conventions like carbon compounds, silicate minerals follow a systematic structural system – from simple single units through to complex frameworks which can make the amazing diversity of minerals easier for students to understand.

But let’s go back to the basic building blocks of the silicates.

Elements in the Earth’s Crust

The average composition of the Earth’s crust, calculated in grams per ton, gives the following elements in order of abundance:

Oxygen 46.6%
Silicon 27.7%
Aluminum 8.1%
Iron 5.0%
Calcium 3.6%
Sodium 2.8%
Potassium 2.6%
Magnesium 2.1%

This means that the majority of the minerals must be made up of a combination of oxygen and silicon with the other elements acting as bonding cations.

Silicon

• Silicon is a non-metallic element
• It has four electrons in its outer shell i.e. valence 4
• It is a semi-conductor
• It does not occur free in nature
• It is a small atom compared to oxygen

Its structure can be modeled like this:

Silicates – the building blocks

Taking the atoms silicon and combining it with the other abundant element oxygen gives us the basic building block known as the silicon tetrahedron. This structure involves four oxygen atoms bonded to one silicon atom.  The size of the silicon ion (radius= 0.39 Å) and the size of the oxygen ion (radius= 1.40 Å) is such that the silicon atom sits surrounded by the oxygen atoms in a triangular pyramid form and why the name ‘tetrahedron’ if given for the structure.

As each oxygen in the tetrahedron has a free bonding site, a tetrahedron can either bond to another tetrahedron or to a metallic ion. It is this ability that allows the multitude of silicate minerals to be formed.

For example, if a single tetrahedron is surrounded by iron and magnesium atoms, then you form a mineral with the simplest silicate structure called a Nesosilicate.  An example of nesosilicates are the Olivines.  The amount of iron to magnesium can change from 100% Mg through to 100% Fe. This means that Olivine represents a group of minerals (the Olivine Group) with Forsterite (Mg2SiO4) and Fayalite (Fe2SiO4) being end members.

At the other end of the silicate spectrum, each tetrahedron joins to another forming a complete framework of SiO2 atoms – which is the mineral quartz.

In between, we have a progression –

Single Tetrahedron  – e.g. Olivine
Double Tetrahedrons – e.g. Epidote
Rings of Tetrahedrons – e.g. Tourmaline and Emerald
Single Chains of Tetrahedrons – e.g. Pyroxenes
Double Chains of Tetrahedrons – e.g. Amphiboles
Sheets – e.g. Micas
Frameworks – e.g. Quartz

Enter Aluminum!

This simple classification of silicates is further complicated by the substitute of some of the silicon atoms by aluminum atoms. This can occur because of the similarity in size of the two atoms. Aluminum is able to squeeze between the oxygen to replace silicon but causes two changes to the mineral structure:

1. The slight difference in size changes the alignment of atoms in the structure, and

2. The lower valence of Al3+ to Si4+ leaves an overall negative charge on the tetrahedron.

The first change is manifested in the zones of weakness in the mineral, called cleavage, along which a mineral will split. The second allows further cations to be introduced into the structure.

For example, the formula for the framework silicates (i.e.the mineral Quartz) is SiO2.  If an Aluminum atom replaces a silicaon atom then the building block becomes AlSi3O8-. That negative charge allows a metallic cation to fit into the structure and, in the case of the feldspars, the cations are either Potassium (K) or Sodium (Na) or Calcium (Ca) or a combination of these cations.  The end members of the group are Orthoclase (KaAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8). The slight change in chemistry gives the feldspars their unique cleavage.

A little more on silicate minerals….