COLOR IN MINERALS - FURTHER DESCRIPTION

Without doubt, this is the most challenging module in the course. There is a lot of detail here, so I will try and distill down what you really need to know, as opposed to what I would like you to know

IMPORTANT CONCEPTS:

absorption (removal of certain wavelengths and thus colors) and

transmission (the wavelengths or colors that are allowed to pass out of the stone and which we see).

oxidation and reduction

Be sure you understand that some atoms can exist in more than one oxidation state (they can have a range of total number of electrons, thus a range of charges) and so change of oxidation state involves gain or loss of electron(s).

e.g.,

Fe 2+ -> Fe 3+ plus one electron, thus Fe (iron) is oxidized

Note that iron metal is uncharged (Fe 0)

Ti4+ plus one electron -> Ti3+, thus Ti (titanium) is reduced

Note that titanium metal is uncharged (Ti 0)

COLOR DUE TO IMPURITIES

Impurities are small quantities of atoms in gemstones that are not part of the normal chemical composition. For example, pure corundum is an oxide of aluminum. It is colorless. If corundum contains a small amount of chromium (Cr), it becomes pink or red (ruby). In this case, the impurity is Cr.

The main point of this first section is to emphasise that the color of a mineral may be due to an impurity. The color depends on what the gemstone is, what the impurity is, where it sits in the crystal structure, and how much impurity there is.

Do not memorize the examples. I just want you to understand what they show.

For example,

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i.Different impurities (1-2 %) in the same mineral:

e.g., BERYL

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From the above, I want you to see that you can put one type of atom (iron) into one mineral (beryl) and the resulting color will depend on how many electrons it has lost (it oxidation state: for iron, either 2 or 3).

Secondly, I want you to see that if you add a different impurity (Mn), the color will be different!

So:the message in the section on color due to impurities is:-

Color is determined by what the impurity is, what oxidation state it is in, and by the details of the site in which it sits.

The same impurity (Cr) in the same type of site (octahedral site with Cr surrounded by six O) in different minerals causes the color to be quite different!

Consider: Cr causes:

  • ruby to be red
  • emerald to be green
  • alexandrite to be purple, red, or blue (depending on the illuminating light).

    The alexandrite effect (color change as the result of different illuminating light) is important to understand because it is an excellent example of how transmission and absorption of specific wavelengths determines color.

    ALEXANDRITE EFFECT: Incandescent light (candle light) is rich in red and yellow and fluorescent light is rich in blue. Alexandrite can transmit either blue or red light. If light is rich in both red and blue, alexandrite will transmit both red and blue, and appear purple. If the light is rich in red and poor in blue (candle light), it will look red. If light is rich in blue and poor in red (UV lights), it will look blue.

    CHARGE TRANSFER

    I want you to know that charge transfer involves exchange of an electron between to atoms. Movement of the electron from one atom to another requires a fairly specific amount of energy.

    Light consist of a range of wavelengths and thus a range of energies. If we use up certain wavelengths=energies, then we see those wavelengths or energies that are left. Again, these are the concepts of adsorption and transmission.

    When we get to the lecture on sapphires (and ruby) I will want you to know that sapphire is colored by Fe and Ti. The reason it is blue is that light at the red end of the visible spectrum is removed in order to cause transfer of an electron back and forth between Fe and Ti atoms. The detailed case of sapphire is not necessary just now, but the concept of charge transfer is!

    This looks like:

    Fe 2+ and Ti 4+ <-> Fe3+ and Ti 3+

    Note here that the total charge is the same either side of this equation but that Fe3+ has lost an electron (making it more positive) and this electron has been accepted by Ti3+, making it less positive

    COLOR CENTERS

    You need to know what color centers are and what causes them (in simple terms). For example, you should know that a trapped electron is an example of a color center.

    The electron takes some range of wavelengths of light energy and uses this to "jiggle" in its trap (cage). This removes those wavelengths from the visible spectrum (make sure you know what "visible spectrum" means!).

    Note that the color centers are typically produced by irradiation (radiation damage). In other words, they are very small (atomic-scale) defects in the gemstone. These defects can be "healed" by heating. The heat releases the trapped electron. Consequently, the color center is removed and the gemstone changes color (if the only source of color is the color center, the stone will become colorless).

    We will discuss color centers in more detail in the lecture on topaz and gem irradiation!

    PHYSICAL SUBDIVISION

    Not much required here. Just know that if we divide something up very finely so that the spacing of the subdivisions is close to the wavelength of visible light, we get color.

    More on light waves:-

    You might understand light waves better if you think of something similar, such as waves on a pond. The amplitude is the height of the wave. If the waves cancel (a crest and trough superimpose), no wave remains (flat pond), no light. If the waves add we get a big wave and lots of light. If we cancel all wavelengths (colors) except one, and we amplify this one, we will see the color of the amplified light.

    Examples we will discuss later are opal and labradorite.