How do we perceive color?

 
• color perceived depends on the light the object is viewed under: the effect of illumination type can be very important (fluorescent vs. incandescent light)
• human vision
• rods: low intensity light -> one color perceived (gray)
• cones: three pigment types RGB, thus color is seen = %r + %g +%b
• eyes most sensitive to green light.

Electromagnetic spectrum:

We see radiation with wavelengths in the "visible" spectrum

Visible spectrum: Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Why do things look colored?

• thing are colored when some process removes some wavelengths (absorbs specific wavelengths) from the visible spectrum
• Question: blue sapphire viewed in candle looks black, why ?

 
Blue sapphire is blue because this is the only wavelength range of visible light that can be transmitted by the stone. Candle light is rich in red wavelengths and poor in blue wavelengths. Thus, the wavelengths (colors) of visible light available are exactly those that can NOT be transmitted. Result: no light is transmitted, the stone appears black!


If you understand this, then you understand some of the important basic concepts in this module!

Physical processes occurring in the stone:

Let's _define_ adsorbtion here?

• luminescence: electron returns to its ground state (where it started) and releases the amount of energy it absorbed (thus returns light with that wavelength to the spectrum, thus, no color....

• fluorescence: If some of the adsorbed energy is lost but the reminder is returned to the visible spectrum, the light returned has lower energy and thus, different color. [changes in energy = changes in wavelength = changes in color].

Causes of color in minerals

Impurities cause color in gems!

Impurities are elements (e.g., Ti, V, Cr, Mn, Fe, Co, Ni, Cu...) that are not present in the pure compound. Impurities are elements that occur in low concentration in the gemstone.

Example:

 A ruby may contain < 1% Cr and it will look pink or red, but the same material without Cr will be completely colorless. This example contrasts with gems such as turqoise, in which the color-causing impurity is a major ingredient.

 If we take one mineral, beryl, and add different impurities, we get different colors:

Beryl containing iron (Fe):

• Aquamarine = Fe++, beryl is blue
• Heliodor = Fe+++, yellow
• Green beryl :  due to mixtures of Fe2+ and Fe3+

Beryl containing Manganese(Mn):

• Morganite : Mn++ is pink
• Red beryl : Mn+++ is red

Beryl containing Chromium(Cr):

• Emerald = emerald green : Cr+++

From the above examples it is clear that the oxidation state (e.g., Fe2+ vs. Fe3+) also affects the color!

If impurity ions produce color, the color can be changed if the oxidation state can be changed.

Note: heating beryl that is green or yellow reduces ferric iron, and the beryl turns blue.  This greatly increases the stone's value.

The process of color change can simply involve heating the stone in a low oxygen atmosphere. This could be done by wrapping the stone in paper and allowing the paper to burn

 Mn+++ is efficient at absorbing light, (blue end of the spectrum) thus color is strong.

The same impurity colors different gems differently!

Example:

 Chromium (Cr+++) in ruby: red

 Chromium (Cr+++) in beryl: emerald green

 Chromium(Cr+++) in alexandrite: purplish or red (see below!)

 This effect is because the Cr absorbs light differently when it is in beryl, emerald, and alexandrite. This is illustrated here for ruby, alexandrite and emerald

Note the different regions of absorption and transmission in the above diagram.

In the case of ruby, the largest valley (transmission window = low in the absorption graph) occurs at the red end of the spectrum, thus the stone essentially looks red. (However, a smaller transmission window may occur at blue wavelengths (as shown). This gives a purplish cast to the red color of ruby).

In the case of emerald, most tramsmission occurs at green wavelengths and most other wavelengths are absorbed strongly. Thus, emerald looks green.

The "Alexandrite" color change effect:

an example where the details are important!  Color change due to change in the color of incident light! (recall that fluorescent light is bluish (rich in blue wavelengths) and candle light is rich in red and orange wavelenghts).

Alexandrite is the best known example of a gemstone that changes color depending upon the light it is viewed under.

In the case of alexandrite, there are two approximately equal sized tranmission windows - the first at blue and second at red wavelengths. When viewed in light made up of all wavelenghts, the stone tramsmits blue and red and often looks purple or purple-grey.

 Here is a diagram showing the:

case of illumination of alexandrite with regular (white) light

 When viewed in light containing mostly red wavelengths (e.g., candle light) the stone looks red. This is understood because, although the stone could transmit blue light, there is no blue light to transmit.

Here is a diagram showing illumination of alexandrite by reddish light

The reverse is also true. In light rich in blue wavelengths (e.g., fluorescent light), the stone looks blue because, although it could also transmit red, there is little red in the light to transmit.

Here is a diagram showing illumination of alexandrite by light rich in blue wavelenghts. Different specimens of the same gem will be characterized by slightly different adsorption/transmission characteristics (different adsorption spectra shapes) and so their colors will vary!

Note: this color change effect in response to change in illumination type (e.g., incandescent vs. fluorescent) is not restricted to alexandrite! Many gems have color change varieties, e.g., sapphire, garnet.

color change garnet viewed in fluorescent light

color change garnet viewed in incandescent light!

In all cases the explanation for color change is the same, involving the range of wavelengths in the light and the ability of the stone to transmit two different ranges of wavelengths of light (e.g., red and green). Other examples.

Visit a spectroscopy site with many additional examples of color caused by impurities!

Charge Transfer causes color in gems.

Charge transfer can only occur in compounds that have at least two elements in different and variable oxidation states. Charge transfer can produce very intense colors in gems and minerals.

The term charge transfer refers to the process where electrons are swapped between elements.  Examples of elements that can participate in charge transfer are:

 Fe2+ and Fe3+

Ti3+ and Ti4+

Mn2+ and Mn3+ and Mn4+ etc.

Furthermore, a crystal can contain mixtures of these elements (e.g,. Mn and Fe) and these can participate in charge transfer.

Energy is absorbed from visible light to transfer electrons from one atom to another.

For example:

A crystal contains metals (M) in two oxidation states: M2+ and M4+
 
• M2+ can loose an electron and become M3+

M4+ can accept the electron (from above) and become M3+.

• Thus, the crystal can exist with

 M3+ plus M3+ or M2+ plus M4+.

As you can see, these pairs are interchangeable by movement of an electron.
This is described more fully as intervalence charge transfer!

More examples:

• cation - cation
• sapphire: Fe++ <-> Ti4+ , requires red light therefore...Deep blue of sapphire

 
• in beryl, Fe++ and Fe+++ exchange of electron (charge transfer): requires energy = red light, therefore you'll see...aquamarine ; with more Fe+++ -> greener color due to absorption

 
• In tourmaline, Mn++ <-> Ti 4+, and the result is a yellow-green color
• anion - anion
• Lazurite (in lapis lazuli) involves charge transfer between a triangle of sulfur atoms

 
• cation - anion :
• golden beryl, yellow sapphire

 Visit a spectroscopy site with additional information about color caused by charge transfer.

Color centers

Color centers are imperfections in crystals that cause color (defects that cause color by absorption of light).

They are most often due to radiation damage: e.g., damage due to exposure to gamma rays. This irradiation may be from both natural (U, Th, K in minerals) or artificial sources. In rare cases, UV light can produce color centers.

If damaged by radioactive decay, electrons can be removed from their normal sites, bounce around, loose energy, and eventually come to rest in a vacant site in the structure (a trap).

One crystal may have many different types of electron traps

Electrons in specific traps absorb only a certain range of wavelengths, color that is seen is the color not absorbed by these trapped electrons.

Examples:

• e.g,. green diamond: due to missing carbon atom which absorbs red light
• smoky quartz: Al+++ <=> Si4+
• Pure zircon is a colorless mineral, whereas zircon containing uranium (U) is blue and zircon damaged by radioactive decay of U is brownish-red!

Because they are a form of damage, color centers can be removed by addition of energy. This may involve heating the stone to a few 100 C.

Example: Heat treat brown zircon, it may turn blue!! (this is a common gem treatment!)

• In some cases, exposure to sunlight (especially UV) provides sufficient energy to remove the color center! - amethyst is an example.

 

Review: when electrons escape their traps, color centers are removed, so color is removed.

• Because irradiated minerals may have several color centers (several traps with different energies required to allow electrons to escape, color can be manipulated by selective removal of unwanted color centers (controlled heating).

We will revisit this topic when we discuss topaz, for example!

Visit a spectroscopy site with additional examples of color caused by radiation damage.

Other causes of color in minerals

Important for EPS2 students:  Further explanation of basic concepts.