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Composition of the Earth

GLY 4200

Fall, 2012

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Interior of the Earth

• Earth’s interior is divided into zones, with differing properties and compositions

• Since we live on the crust, it is the most studied

• The core and mantle are very important in understanding the behavior of the earth

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Composition of the Crust – Major Elements

• Earth’s crust is composed predominantly of eight elements

• Figure for Si here is correct – figure 5.2 in text has a misprint

• Numbers are in weight percent

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Abundances Measurements

• We can specify abundances using differ methods

• The most common are: Weight per cent Atom per cent Volume percent

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Comparison of MethodsElement Weight % Atom %

O 46.60 62.55

Si 27.72 21.22

Al 8.13 6.47

Fe 5.00 1.92

Ca 3.63 1.94

Na 2.83 2.64

K 2.59 1.42

Mg 2.09 1.82

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Minor and Trace Element Definition

• Minor elements have abundances between 0.1 to 1.0 weight percent

• Elements with abundances less than 0.1% are called trace elements

• Their abundance is usually given in parts per million (ppm) or parts per billion (ppb)

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Minor and Trace Elements in Crust

• Only 17 elements occur with abundances of at least 200 parts per million (ppm) – in addition to those on the major element slide, these include:

Element Weight % Element Weight ppm

Ti 0.44% F 625

H 0.14% Sr 375

P 0.10% S 260

Mn 0.09% C 200

Ba 0.04%

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Ores

• Many valuable elements are in the trace element range, including the gold group (Au, Ag, and Cu) and the platinum group (Pt, Pd, Ir, Os), mercury, lead, and others

• Useage does not always reflect abundance – copper (55 ppm) is used more than zirconium (165 ppm) or cerium (60 ppm)

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Effect of Pressure

• As pressure increases, minerals transform to denser structures, with atoms packed more closely together

• This is seen in the mantle

• The upper mantle is dominated by the mineral olivine, Mg2SiO4

• Magnesium is in VI, and Si in IV

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Transition Zone

• In the transition zone, from about 400 to 660 kilometers below the surface, olivine transforms to denser structures olivine (ρ = 3.22 gm/cm3) →

wadsleyite (ρ = 3.47 gm/cm3) → ringwoodite (ρ = 3.55 gm/cm3)

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Lower Mantle

• Pressures are so great that silicon becomes six coordinated (CN = VI), and some magnesium becomes eight-coordinated (perovskite structure) Ringwoodite (ρ = 3.55 gm/cm3) → MgSiO3

(perovskite structure) and (Mg, Fe)O (magnesiowűstite - halite structure)

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Core

• The core is divided into two regions, the liquid outer core and the solid inner core

• There is a definite chemical discontinuity between the lower mantle and the outer core

• The main elements in the core are an iron and nickel alloy

• Increasing temperature first melts the alloy to make the outer core

• Increasing pressure freezes the alloy to produce the inner core

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Outer Core

• Ranges from 2900 to 5100 kilometers below the earth

• Composition is iron with about 2% nickel

• Density of 9.9 gm/cm3 is too low to be pure metal

• Best estimates are that silica makes up 9-12% of the outer core

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Inner Core

• From 5100 to 6371 kilometers below surface

• 80% iron, 20% nickel alloy

• Pressures reach about 3 megabars, or 300,000 megapascals

• Temperature at the center is about 7600ºC