Geological background

• 88 elements found in the Earth's crust -- of these, only 8 make up 98%: oxygen, silicon, aluminum, iron, calcium, magnesium, potassium and sodium

• In the whole earth, only 4 elements dominate: iron, oxygen, silicon and magnesium

• These elements go up to make minerals. A mineral is a naturally occurring, inorganic solid with a characteristic chemical composition and a crystalline structure

• Even though there are more than 2500 minerals knows, only nine minerals make up most of the rocks of the Earth's crust -- these are the "rock-forming minerals"

The rock-forming minerals

• Minerals containing silicon and oxygen are called silicates. These make up more than 95% of the crust. The seven most abundant silicates in the crust are feldspar, quartz, pyroxene, amphibole, mica, clay minerals, and olivine. Olivine and pyroxene are the main constituents of the uppermost mantle.

• Most silicates are formed from SiO4 tetrahedra (a silicon atom surrounded by four oxygens) arranged in a variety of ways. Exceptions are quartz and feldspar which are so-called framework silicates. The silicate tetrahedron is exceptionally stable and allows close-packed structures to be formed.

The rock-forming minerals (continued)

• Olivine is made up of individual tetrahedra, pyroxene and amphibole are made up of chains of tetrahedra, mica and clay minerals are made up of sheets of tetrahedra (giving mica its platey character)

• Two other rock-forming minerals are carbonates: calcite (calcium carbonate) and dolomite (calcium-magnesium carbonate)

• As low pressure minerals are squeezed, they may suddenly transform to a denser high-pressure phase.

Rocks

• Under certain conditions, rocks of the upper mantle and lower crust melt, forming a molten or semi-molten material called magma. Igneous rocks form when this magma cools -- sometimes in surface volcanic eruptions (volcanic rocks) though more often on continents, magma cools and solidifies below the surface forming so-called plutonic rocks

• Igneous rock makes up half of the Earth's crust -- the most common igneous rocks are granite and basalt. Rock samples from the mantle are "peridotites" which are dominantly olivine and pyroxene.