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Ch 6: Internal Constitution
of the Earth
Mantle composition
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.
Pyrolite composition:
DIRECT SAMPLES (Peridotite)
Common mantle minerals:
• Olivine
• Orthoppyroxene
• Clinopyroxene
• Al-phase
 Spinel
 Garnet
High pressure experiments
Multi-anvil press:
(i)
Guide Blocks
2000 Ton Jack
(ii)
Uniaxial
Press
Upper Guide Block
Split-Sphere
Anvils
Anvils
6
2
4
(iii) Tungsten
Carbide
Anvil
Second-stage
assembly
Presure Medium
Spacer
1
Pre-formed
gasket
Lower Guide Block
32 mm
Large volume multi anvil cells:
3 orders of magnitude
higher than DACs!
Large volume: House probes, synthesize larger specimens, some
experiments require large V (e.g. ultrasonic interferometry)
Hydrostatic Pressure: Closer, since squeezing from 8 directions,
But, not easily used with gas pressure medium
Pressures: Top of lower mantle at best with sintered diamonds and
synchrotron radiation
Diamond anvil cells:
Diamond Anvil Cells:
Why Diamonds?
Can use: Steel, tungsten carbide, boron carbide,
sapphire, cubic zirconia, sintered diamond,
or single-crystal diamond
Single crystal diamond:
1) Strongest material known
2) Transparent (IR, optical, UV, and X-ray)
3) Non-magnetic insulator: , 
Creating Temperature:
3 ways:
1) External heating
2) Internal heating
3) IR Laser Heating
Measuring Material Parameters…
Measurement of Pressure
• Ruby Chips Fluorescence Method
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–
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Freq. shift of ruby with increasing pressure
Linear to 30 GPa
Calibrated to 100 GPa by Raman Spec.
Calibrated to >200 GPa by Gold
Accurate to 15-20% at 200 GPa
Diffuses with temperature (>700K)
Ruby and Diamond Fluorescence overlap
between 120-180 GPa
– KEY: Allows sampling at multiple points in
pressure medium
Laser heating - use black body radiation
T: temperature
I: intensity
: wavelength
Cs: constants
: emissivity
Perfect black body:  = 1
Grey body:  < 1
 is wavelength dependent
But dependence not known for many materials!
(known for Fe)
Measuring Material Parameters…
In-Situ X-Ray Diffraction
• Provides Crystal Structure, Density and melting points
• Synchrotron Radiation provides highly collimated x-ray source
• Braggs Law:
2q = angle of diffraction
l = 2dsin(q)
d = spacing of crystal planes
 = wavelength of X-ray
Phase transformations in the upper
mantle
Structures of minerals
Spinel structure (ringwoodite)
Undistorted (cubic) perovskite structure
Fitting seismic models
Matching seismic observations with the pyrolite model:
Red=density
Blue=Vc
Green=Vs
Xpv=0.65: “pyrolite”
Shock wave data and the core
Melting Temperature: Pure Fe
Boehler(1993)
Yoo et al (1993)
Nguyen & Holmes(2009)
Brown & McQueen (1986)
Belonoshko et al (2000
Alfe et al (2002)
Alfe (2009)
Sola & Alfe (2009)
Anzellini et al (2013)
CMB
ICB
(slide courtesy Chris Davies)