How do minerals form?

If you have a walk and look at the floor as Kristina did in Montpellier severals months ago, you will see rocks that seem to have the same shape, colour, etc. But they actually are different! You should see inside, as Karin explains with a lemon, and have a look at the minerals that compose the rock!

But what is a mineral? A mineral is a solid substance with chemical composition and structure. A cocktail of atoms linked to one another, and forming a crystalline structure, which is the “DNA” for each mineral. A crystalline structure is formed through a unique arrangement of atoms,ions or molecules in a solid phase. It describes a highly ordered structure forming symmetric patterns. Minerals crystallize from magma, molten rock under the surface of the earth, or fluids that have molecules, atoms and ions waiting to be linked to one another into structures.

So, how do molecules link together to form a crystalline structure? Let’s have a look at this process using water. Water is composed of H2O molecules which are made of two atoms of hydrogen bonded to one atom of oxygen (above image). In nature, we commonly find water in its liquid form. But what happens when we drink a soda and put some ice inside to cool it? What is ice? Well it is water in a solid state. Molecules have become organized thereby forming a solid structure.

Hexagonal structure of ice in 2D.
Hexagonal structure of ice in 2D.

However, we can only test this when water is cold enough. So, this probably means that temperature and pressure will help in the process. Temperature is the amount of kinetic energy that molecules have, or a measure of how much they move. Typically nature tends to put things in configurations which require the least amount of energy.

When temperature is too high, molecules will move, “vibrating” very fast. They will collide each other and won’t be able to link together. The question is how do they actually link up?? Well, when temperature decreases, particles will move more slowly. The less they vibrate the less kinetic energy they have. So, now they can link up forming fixed structures instead of bouncing each other. This is the process that forms a solid. Solids will adopt different structures depending on the geometry of the atoms, and molecules it is formed from. For example, ice has a hexagonal structure.

The geometry of an H2O molecule.
The geometry of an H2O molecule.

So what about pressure? Pressure is the amount of force acting per unit area. Pressure will determine the geometry of the internal structure of the mineral during their formation. If a pressure is applied to molecules they are forced closer together. Going back to the water molecule as an example (image right), the angle between the two hydrogen atoms bonded to the oxygen will change and become smaller. As a consequence, the molecule’s geometry will change. So, the way the molecules will join will be different. With water this doesn’t happen in our planet because the pressure that we need to apply to change it is too high. But in the laboratory scientists can reproduce these conditions and form water with a cubic structure of ice.

Continuing with the water example, snow is made of small crystals of ice. So, now you can guess what the next question is, right?

What is a crystal? A crystal is a piece of material where the molecules are well organized in the same direction. For example, when cooling is slow and pressure conditions are optimal, a crystal can slowly grow as molecules from the fluid in which it forms join the principal structure that was first formed. Minerals develop the same process as the crystallization of ice: they form when the magma or fluid temperature decreases enough to allow the components within it to link together to form a solid structure. For example, the SiO2 molecule, formed of one silicon atom + 2 oxygen atoms will form quartz, CaF2 forms fluorite, and FeS2 forms pyrite. A compound can adopt different crystalline structures depending on the temperature and pressure conditions during its formation. At optimal temperature and pressure conditions compounds can grow into perfect crystals as in the example of ice described above, or in the calcite crystal shown below.

A natural crystal of calcite (Image by Parent Géry).
A natural crystal of calcite (Image by Parent Géry).

One example that illustrates the effect of pressure and temperature is the carbon structure. The CO2 is around all the Earth and we can find it on minerals, like the ones which are made of carbon. Diamonds and graphite have the same chemical composition, but they have different structures. Both have carbon atoms in their structure. The structure is different because diamonds needs high temperature 3000 K and high pressure 125 Kbar to form stable compact structure. In the other hand, the graphite has a laminar structure and needs less temperature and pressure to be stable. So, that’s the reason the graphite is common and easier to find in nature, like in our pencils! You can learn more about the structures of diamond and graphite by watching either video 1 or video 2 which were found on Youtube. If you would like more information about crystals and mineralogy check out the Mineral Society of America’s website.

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