The Earth's Crust




Regions of the Earth

First, let's review the structure of the Earth. The planet is made up of three main shells: the very thin, brittle crust, the mantle and the core. The core forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent. The composition of the crust is quite different from that of the Earth as a whole. Heavy elements are segregated towards the center and lighter elements towards the surface.


Composition

The most common minerals are those that have a chemical composition made of the common elements found in their environment.

The Earth's crust is made up of about 95% igneous and metamorphic rocks, 4% shale, 0.75% sandstone, and 0.25% limestone. The continental crust has an average composition that approximates granodiorite (a medium to siliceous igneous rock), whereas the oceanic crust has an average composition that is basaltic (a low silica igneous rock).

Element Wt% Atomic% Volume%
O 46.60 62.55 ~94
Si 27.72 21.22 ~6
Al 8.13 6.47  
Fe 5.00 1.92  
Ca 3.63 1.94  
Na 2.83 2.34  
K 2.59 1.42  
Mg 2.09 1.84  
Total 98.59 100.00 100


As you can see in the table above, oxygen is the most abundant mineral in the crust and the most common minerals are silicates. (We'll talk about those next time.)

Carbonates are also very important at the surface of the Earth because these minerals are formed directly or indirectly from carbon dioxide in the atmosphere.


Crust

The rocks that make up the crust can be classified into three types.
  • Sedimentary Rocks - rocks that are formed from the lithification of sediments, chemical precipitation or by direct biogenic deposition. Some common types are sandstone, shale, coal, limestone and coral.

  • Igneous Rocks - rocks that cool from a magma. The two most common types are granite and basalt.

  • Metamorphic Rocks - rocks that have been altered by high pressures, temperatures and/or chemical reaction while still in the solid state. Two common types are marble, which comes from limestone, and slate that comes from shale.


Mantle

The mantle is composed mainly of iron and magnesium silicates. The temperature increases with depth from 870 deg to 2200 deg C.

Core

The core is mainly composed of hot (greater than 5000 deg C!) metallic nickel and iron. The outer core is liquid but the inner core is solid due to the higher pressure.


Mineral structure: Close Packing

In many minerals, the ions have a close-packed structure. That is, the larger ions, usually the anions, pack as closely as possible to minimize empty space. The smaller ions, usually the cations, occupy holes in the structure. Close packed structures begin with a hexagonally packed layer. Imagine that each anion is a sphere. It has 6 other anions packed around it.



In any close packed array of ions, there are both octahedral and tetrahedral holes where smaller ions could reside.

In this figure, there are three layers of ions. Look at the first layer. There are holes surrounded by 3 of these anions.

We add a second layer (red) so that each ion fits into a depression in the layer below it. Some of the holes in the first layer are capped by another ion in the second layer. These are tetrahedral holes. Other holes are not capped in this way. A bigger cation could fit into these octahedral holes that are surrounded by 3 anions from one layer and 3 from another layer.

A third layer covers these octahedral holes in the ABC layering of the cubic close packed structure (ccp). If the third layer were a position identical to the first layer, the structure would have hexagonal close packing (hcp).


    The smallest unit that when repeated gives the structure of the material is the unit cell. Three unit cells of hexagonal close packed (hcc) structure are below. The unit cell is indicated by the rectangle. You can see that the bottom and the top layers are the same for this ABA... packing.


    The layers in the unit cell of a cubic close packed structure are more difficult to see because they are on the diagonal of the cell. At one vertex there is one atom from layer A and at the opposite vertex is another atom of the next layer A so the unit cell contains parts of ABCA layers.





Other Structures

Some minerals pack in structures that are less compact than the two above. Cubic structures have one layer directly on top of another. Body-centered cubic packing of anions is like cubic packing with an additional anion in each cubic hole. The smallest unit that when repeated gives the structure of the material is the unit cell. The unit cell for cubic, body-centered cubic, and face-centered cubic are shown below. Face-centered cubic is another name for cubic close packed.



The unit cells can be classified according to their dimensions (height, width, length; a, b, c) and angles.



Professor Patricia Shapley, University of Illinois, 2010