Geological background

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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"

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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.

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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.

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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.

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