Why do minerals have colours?

Because some of you asked during ‘La Fête de la Science’ in Paris why minerals have different colors I decided to write this post about it.

First of all, I need to define the electromagnetic spectrum, and the energetic distribution of electromagnetic waves. But what is a wave? Let’s make that clear by looking at waves in the ocean (figure 1).

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Figure 1: Ocean waves (source: http://www.livescience.com/38361-how-do-ocean-waves-form.html)

Have you ever swum in the sea? And did you realise that sometimes the waves were more frequent and shake you more? This has to do with energy in the waves: in theory they do not transport matter but energy (of course this is different in the sea, where water and sediments are moved, but the principle of energy stored in them is the same). A wave propagates along a direction with a certain velocity. In the ocean, this velocity is quite complex because it depends on different parameters such as the presence of a sand bank, rocks, or a reef, but for light waves, the velocity is constant. 

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Figure 2 : Waves physical parameters (source: https://en.wikipedia.org/wiki/Wave)

So to explain the electromagnetic spectrum, that relates to light, with the analogy of ocean waves let use two parameters that define a wave: the constant wave velocity and a wavelength λ (the distance between two consecutive wave crest) traveled in a given period of time. So, how is energy transport defined? Some waves have smaller wavelengths than others, meaning that over a second they travel a smaller distance, therefore their frequency is higher (figure 2), and the amount of energy that they transport is bigger. Electromagnetic waves have the same behaviour. The lower the energy, the higher the wavelength. Hence, infrared waves are less energetic than ultraviolet ones (figure 3).

 

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Figure 3: Electromagnetic spectrum (source: https://audioboom.com/rowlandphysics)

So what does this have to do with our question? Well, minerals absorb or emit light so they handle electromagnetic energy, which is the energy that the light waves transport.

Light can be described as waves of various wavelengths, so the colours composing the electromagnetic spectrum will correspond to different energy levels. The human eye can only see electromagnetic waves of the visible part of the spectrum, with wavelengths ranging between 400 nm and 750 nm (i.e., above the ultraviolet up to the beginning of the infrared; figure 3). The colours in the visible spectrum are red (618-780 nm), orange (581-618 nm), yellow (570-581 nm), green (497-570 nm), cyan (476-497 nm), blue (427-476 nm) and purple (380-427 nm). Other colours are a combination of these waves, so we could have an infinite number of colours.

Now we know how electromagnetic waves work. Let’s see what happens when they are travelling through a mineral!

As I explained in my last post, minerals are solids that have a chemical structure formed by atoms. These atoms have electrons and a nucleus that is made of positively-charged protons and neutrons. The electrons, with negatives charges, move around the nucleus in orbitals which define where they can spatially be, spending as little energy as possible. Orbitals have their own energetic levels and electrons have the capacity to move from one orbital to another when looking for energy stability (like a quiet place to stay:  remember, when you are walking in the overcrowded corridors of the metro at rush hours, how happy you are to find a free place to sit). But stability does not last long… The interaction between energetic waves and a solid results in the absorption of the energy carried by the wave, by the mineral-forming atoms. The atoms will then have more energy and their electrons will “jump” to different orbitals. By traveling from one orbital to the other, energy in excess can then be released through the emission of a new electromagnetic wave, hence restoring stability. See more about orbitals here.

The wavelength of the emitted electromagnetic waves will depend on the amount of excess energy released. Our eyes can see this as colour if they are in the visible spectrum.

But what happens in minerals that can have multiple colours? In minerals, a colour change is defined by the set of crystallographic defects in the structure that alters the behavior of the electrons in a mineral. Defects can be atoms that are missing compared to the normal mineral structure (figure 4a and b) or unexpected atoms originally not here. Those can enter the structure to fill a space and stabilize the structure (figure 4).

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Figure 4: Fluorite structure. a. complete structure with Ca (black) and F2 ,(blue) b. structure with a missing Ca atom. c. Ca is substituted by Sm (green)

 The electrons of these new atoms absorb and release the light energy in a different way and as a reply we can see a different colour compared to a “normal” structure. The more colour influencing-defaults  a mineral structure has, the more intense the colour will be.

This is a common process in fluorite which originally has no colour (figure 5.a). Its formula, CaF2, tells you that it should normally be made of calcium (Ca) and fluorine (F) atoms. But when Ca is substituted by another atom, let’s say samarium for example, the colour changes (with samarium it is green, figure 5.b).

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Figure 5: a.Fluorite with no colour (source: http://www.rocksandminerals4u.com/fluorite.html). b. Green Fluorite (source: http://desertscope.com/collItem.php?itemId=24)

So when you look at rocks and see all the differents colours of their minerals, think about the electrons that are moving around to give you these amazing colours!

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