C5.3 What types of chemicals make up the Earth’s lithosphere?
The Earth’s lithosphere is the rigid outer layer that is made up of the crust and the part of the mantle just below it – the lithosphere is made up of a mixture of minerals
The chart shows the relative abundance of some of the elements in the lithosphere:
Oxygen, silicon and aluminium are the most abundant (plentiful) elements in the lithosphere.
The rocks in the lithosphere are made up mainly of silicates. These are compounds that contain silicon and oxygen, together with smaller amounts of other elements.
Much of the silicon and oxygen in the Earth’s crust is present as the compound silicon dioxide also known as silica.
Silicon dioxide forms a giant covalent structure, where each silicon atom is covalently bond to four oxygen atoms and each oxygen atom is bonded to two silicon atoms – the result is a very strong, rigid, 3D structure which is very difficult to break down.
Silicon dioxide does not conduct electricity because there are no ions or free electrons in the structure.
It does not dissolve in water because there are no charges to attract the water molecules
It has a very high melting and boiling point
Silicon dioxide is found as quartz in granite, and is the major compound in sandstone.
Carbon is another example of a mineral that forms a giant covalent structure – two forms of carbon are:
Diamond has a large number of covalent bonds, which means it has a very high melting and boiling points. Each carbon atom is covalently bonded to four other carbon atoms, resulting in a very strong, rigid, 3D structure that is difficult to break down.
Diamond is insoluble (does not dissolve) because there are no charges to attract water molecules
It does not conduct electricity because there are no ions or free electrons in the structure.
Diamond is the hardest naturally occurring substance.
Graphite is a form of carbon that has a giant covalent structure – each carbon atom is covalently bonded to three other carbon atoms in a layered structure.
The bonds between the layers are much weaker than covalent bonds – this enables the layers to slide across each other, making graphite soft
Graphite can conduct electricity because the electrons forming the weak bonds between the layers are free to move throughout the whole structure.
Graphite is used as pencil ‘lead’ – as the pencil moves across the paper, layers of graphite rub off