Download Overheads for background on mantle minerals

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 socalled 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.