Silicate Earth

This refers to the rocky portion of the earth, the Mantle plus the Crust. It represents the earth after the Core separated but before the Crust differentiated.

Elemental & Mineral Abundance

The Elements

The Iron Planet

Four elements would make up 90% of the mass. This reflects the original material of the Solar System minus the gases. The Elements are:

• Iron 32 - 35%
• Oxygen 30%
• Silicon 15%
• Magnesium 14%

The Crustal Composition

Because of differentiation, the crust is chemically distinct. Iron and Magnesium stayed deep down, while lighter element floated up. The main eight elements are:

• Oxygen 46.6%
• Silicon 27.7%
• Aluminum 8.1%
• Iron 5%
• Calcium 3.6%
• Sodium 2.8%
• Potassium 2.6%
• Magnesium 2.1%

The crust is mainly oxygen with some impurities. The weight fraction is 47% but the volume fraction is over 90% since oxygen anions are huge in comparison to tiny silicon and aluminum cations.

The Minerals: What rocks are made of

In the Crust (Felsic Domination)

Because Silicon and Aluminum are so abundant in the crust, the minerals that require them are the most common.

1. Feldspars 60%
   • By far the most abundant group.
   • Includes Plagioclase ($\mathrm{Ca}$-$\mathrm{Na}$) and Alkali Feldspar ($\mathrm{K}$-$\mathrm{Na}$)
   • If you pick up a rock it is probably a Feldspar.
2. Quartz 12%
   • Pure $\mathrm{SiO}{\phantom{A}}_2$. It is abundant because there is usually "leftover" Silica after all the other elements have reacted to form feldspar.
3. Pyroxenes 11%
   • The primary "dark" mineral in the crust.
4. Micas & Clays 10%
   • The result of weathering and hydration.

In the mantle (Ultramafic Domination)

The mantle lacks Aluminum and Potassium, so Feldspar cannot exist there. Instead, the chemistry forces the formation of simpler $\mathrm{Mg}-\mathrm{Fe}$ silicates.

• Upper Mantle: Dominated by Olivine 60 - 80% by volume
• Lower Mantle: Dominated by Bridgmanite 75% by volume

Question and Answers

What are the "Big Four" elements that make up >90% of the Whole Earth mass?::Iron ($\text{Fe}$), Oxygen ($\text{O}$), Silicon ($\text{Si}$), Magnesium ($\text{Mg}$).

What is the single most abundant element in the Earth's Crust by weight?::Oxygen ($\approx 46\mathrm{\%}$).

What is the single most abundant element in the Whole Earth (bulk) by weight?::Iron ($\approx 32-35\mathrm{\%}$).

Which mineral group is the most abundant in the Earth's Crust ($\approx 60\mathrm{\%}$)?::Feldspars.

Why is Oxygen considered to make up >90% of the crust by volume, despite being only 46% by weight?
?
Because the Oxygen anion (${\text{O}}^{2-}$) has a much larger ionic radius (volume) than the cation impurities (Si, Al, Fe) that fit in the spaces between them.

Name the two most abundant elements in the crust, in order.::1. Oxygen, 2. Silicon.

How does the dominant mineralogy change from the Crust to the Upper Mantle?:::Crust = Feldspar dominant; Upper Mantle = Olivine dominant.