Rocks and minerals
The Earth’s lithosphere is the rigid outer layer, which contains many useful chemical compounds.
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 Earth is almost a sphere. These are its main layers, starting with the outermost:
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 and silicon are the most common elements.
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 has a giant covalent structure. Part of this structure is shown in the diagram - oxygen atoms are shown as red, silicon atoms shown as brown:
Each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is covalently bonded to two silicon atoms. This means that, overall, the ratio is two oxygen atoms to each silicon atom, giving the formula SiO2.
Silicon dioxide is very hard. It has a very high melting point (1,610 °C) and boiling point (2,230 °C), is insoluble in water, and does not conduct electricity. These properties result from the very strong covalent bonds that hold the silicon and oxygen atoms in the giant covalent structure.
Silicon dioxide is found as quartz in granite, and is the major compound in sandstone. The sand on a beach is made mostly of silicon dioxide.
Pure quartz crystals are transparent, but the mineral is often found with traces of other minerals present. These minerals are very hard.
Because of their attractive appearance when polished, and their resistance to wear and corrosion, they are used as gemstones in jewellery.
Amethyst is a form of quartz that contains manganese and iron oxides. These give the gemstone its violet colour.
Many other giant covalent compounds are used in jewellery. The most famous is diamond.
The rarity of these minerals in the crust makes them very expensive.
Read on if you are taking the Higher paper.
Many other compounds have giant covalent structures. They generally have the similar properties of hardness, high melting and boiling points, and low electrical conductivity.
A good example is diamond. This is made of carbon atoms, with each joined to four others by strong covalent bonds.
Diamond is the hardest naturally occurring substance. It is used as a gemstone, but also on the cutting edges of drills and saws.
Graphite is another giant covalent structure made of carbon atoms. In graphite, each carbon atom is joined to three others, forming layers:
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 also conduct electricity, between the layers of carbon atoms.
Graphite is used as pencil ‘lead’. As the pencil moves across the paper, layers of graphite rub off. Graphite is also used as a lubricant, and as an electrode in electrolysis - for example, in the manufacture of aluminium.