Difference between revisions of "Causes of color"

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(The Crystal Field Theory: added "some" to electrons in the d-shell (some may travel in pairs))
(The Molecular Orbital Theory)
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===The Molecular Orbital Theory===
 
===The Molecular Orbital Theory===
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The molecular orbital theory describes the paths (orbitals) electrons travel when multiple atoms (two or more) combine chemically. In order for atoms to combine to molecules, they must share or exchange electrons.<br />
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Although the orbitals themselves are very useful in describing this theory, it is a very complex topic and goes beyond the realm of gemology.
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Instead we will focus on the different types of bonds that can occur between atoms.
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# Ionic bonding
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# Covalent bonding
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While both these bonds have different characteristics, mostly they both play a role in the chemical bonding of gemstones and are directly related to the ''electronegativity'' of the different elements that make up the chemical formula of a gemstone.<br />
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It are only the electrons in the outer shells of atoms that play a role in both ionic or covalent bonding.
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====Electronegativity====
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[[image:electronegativity.jpg|left|thumb|200px|Electronegativity scale]]
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====Charge transfer====
 
====Charge transfer====

Revision as of 08:57, 1 March 2007

Exlamation mark.jpg
This section is currently under construction, do not edit until this message is gone
--Doos 13:01, 3 January 2007 (PST)

Basic

In general color is caused by the absorption of certain wavelengths of light by a substance (as a gemstone) while permitting other wavelengths to pass through the substance unaltered. The net result of the wavelengths that pass through the gem give the final color to the gemstone.

In order to see color we need atleast 3 variables:

  • Light
  • A substance
  • Vision (the eye)

When any of them is absent, we can't see color. For instance, your red sweater will have no color in the dark as there is no light for the sweater to absorb.

Absorption of all wavelenghts execpt red

White light is a mixture of the 7 spectral colors (visible in a rainbow): red, orange, yellow, green, indigo, blue and violet. Each of these colors travel at a certain wavelength (about 700nm for red and 400nm for violet) and carry a specific amount of energy.
When a substance absorbs all the spectral colors (the colors of the rainbow) except red, the residual color is red. Therefore the net result will be red. If all colors except red and blue are absorbed, the residual color (net result) will be a purple gemstone.

In total there are about 16 million combinations that can produce color.

Absorption of light usually results in transformation of light energy to heat. That is why frigidaires are white; white substances don't absorb light so they are very effective at keeping heat away.

Coloring elements
Element Colors
Titanium Blue
Vanadium Green / Color change
Chromium Red - Green
Manganese Pink
Iron Red - Green - Yellow
Cobalt Blue
Nickel Green
Copper Green - Blue

Absorption of wavelengths (colors) happens because certain elements absorb the energy of those wavelengths. The main elements that absorb color are the "transition metal" elements. These transition metals have partially filled d-shells (opposed to fully filled shells) and some of the electrons in the d-shells are unpaired.

When a wavelength carries enough energy to raise an unpaired electron to a higher energy state, the energy of that wavelength is completey absorbed by the electron and the light energy is usually transformed to heat when the electron falls down to its original (ground) state. This means that that particular wavelength (or color) is removed from the spectrum, or better "absorbed".

The energy required to raise an unpaired electron to a higher energy state (and so cause absorption) is far less than the energy required to raise a paired electron. The energy in visible light is not enough to raise a paired electron, but it can raise the unpaired electrons that reside in the d-shells of the transition metal elements. That is why the transition elements are also named the "coloring elements" or "coloring agents". Besides the transition metal elements, some rare earth metals also act as coloring elements.

The coloring agents can either be part of the ideal chemical makeup or occur as impurities in the crystal.
We divide minerals into two groups, depending on this.

  • Idiochromatic - minerals that are colored by a coloring agent that is part of the chemical formula (for instance malachite)
  • Allochromatic - the coloring agents are not part of the ideal chemical composition (such as emerald and ruby)

Although the theory of transition metal elements serves as a good basic understanding for the causes of color, there may be other mechanisms at work. Some elements work together to form color (the molecular orbital theory) while in other cases the absence of an electron or an element at a particular place ("site") in the crystal creates a color center.
What all the theories have in common is that some energy from incoming light (wavelengths) is absorbed and the residual colors (wavelengths) determine the final color of the gemstone (or any other substance).

Advanced

The causes of color can be divided into 4 different theories:

  1. The Crystal Field Theory
    • Transition metal compounds (malachite, almandine) - idiochromatic
    • Transition metal impurities (ruby, emerald, citrine, jade) - allochromatic
    • Color centers (amethyst, maxixe-beryl)
  2. The Molecular Orbital Theory
    • Charge transfer (sapphire, iolite)
    • Organic coloration (amber, coral)
  3. The Band Theory
    • Conductors (copper, silver, iron)
    • Semi-conductors (galena)
    • Doped semi-conductors (diamond)
  4. The Physical Properties Theory
    • Dispersion (Fire in diamond)
    • Scattering (moonstone, cat's eyes, stars)
    • Interference (iridescence, opal)
    • Diffraction (opal)

The Crystal Field Theory

The crystal field theory describes colors originating from the excitation of electrons in transition elements (either idiochromatic or allochromatic) and color centers.

When a transition metal ion has a partially filled d-shell, the electrons in the outer d-shell orbit the nucleus unpaired (or atleast some of the electrons do). The surrounding ions of the crystal lattice create a force (a "crystal field") around such a transition element and the strength of those fields determine which energy levels are available for the unpaired electrons. Such a system of energy levels depends on the strength and nature of the bonding in the lattice aswell as on the valence state of the transition element. These are different in every crystal.
As energy and energy levels are quantized, the electrons need a specific amount of energy to "jump" from its groundstate to a higher energy level. The complex calculations that determine which energy levels are available for the electron to excite also provide a few selection rules which exclude some levels for excitation.

Absorption in ruby

In ruby, Cr3+ substitutes some of the Al3+ ions in the Al2O3 lattice. As the chromium is not part of the ideal make-up, ruby is said to be allochromatic. The crystal field around the chromium impurity makes a few, quantized, energy levels available to the unpaired electrons. These are presented as levels B, C and D. However selection rules determine that level B is not available for excitation in this case.
Levels C and D correspond with energies of respectively 2.23 eV and 3 eV. The energy needed to jump to level C (2.23 eV) corresponds with yellow-green light and level D (3 eV) corresponds with violet light.
This means that when white light enters a ruby, yellow-green and violet light will be absorbed by the unpaired electrons and these electrons now have sufficient energy to be excited to levels C or D. The residual colors that are not absorbed determine the red color of the ruby.
The same selection rules also forbid the excited electron to fall back to their ground states (A), but must instead first fall back to B. When the electrons in level B deexcite to their ground state, emmision of red light occurs (fluorescence) which gives an extra glow to the already red color caused by the absorption of the yellow-green and violet portions of white light.

The data in this section is under evaluation (i.e. numbers are wrong) --Doos 07:12, 21 February 2007 (PST)
Schematic overview of energy levels in ruby and emerald

For emerald, which allochromatic color is also caused by a Cr3+ impurity, the mechanism is similar but the crystal field from the surrounding elements has less strength and causes a shift in absorption bands. The D level is lowered to 2.8 eV and the C level is lowered to 2.05 eV but the B level is almost the same (1.82 instead of 1.79 eV).
The result is that emerald absorbs most violet and red portions of visible light, leaving a dominant blue-green transmission with a red fluorescence.

Alexandrite, a variety of chrysoberyl, is also colored by a Cr3+ impurity. The absorption scheme of alexandrite is between that of ruby and emerald and the intensity of the incident light determines the color of the alexandrite. Natural daylight is richer in blue-green while incandescent light has more red in its spectrum. This causes the alexandrite to be blue-green (emerald like) in daylight and red-violet (amethyst like) in incandescent light.

Vanadium (V3+) causes the same "alexandrite" color change effect in natural and synthetic corundum.

Allochromatic colors caused by transition metal impurities

The mere occurence of a transition metal impurity does not neccessarily cause the color in a gemstone. Apart from the need of a specific valency the ion must have to be responsible for absorption, other mechanisms (such as color centers and charge-transfer) may be more dominant.
Like certain impurities may be responsible for different colors, as red and green for chromium, other impurities may also cause the same color (like vanadium in emerald).

Idiochromatic colors caused by transition metal constituents

The crystal field theory described above also applies to minerals that have a transition metal ion in its ideal chemical composition.

Color centers

Unpaired electrons on non-transition metal ions may also produce colors in certain circumstances. This can either happen due to misplacement of an ion (and an unpaired electron taking its vacancy), or due to the displacement of an electron from (natural or artificial) radiation.
In both cases a "color center" is created and the unpaired electron can be raised to higher energy levels through the absorption of incident light as with unpaired electrons of the transition metal ions. In the first case (an electron substituting a misplaced ion), the color center is an "electron hole center" and in the latter case it creates a "hole color center".

Electron color centers
Electron color center in fluorite


Fluorite is most often used to describe the mechanism of an electron color center. The purple color of fluorite is caused by the absense of a fluorine (F-) ion and an electron is trapped in the vacancy it leaves behind.
There are various reasons why the fluorine ion is missing from a particular site in the crystal lattice. Among those are an excess of calcium and radiation. either during or after growth of the crystal. This creates a so called "F-center" (or "Farbe center" - Farbe is the German word for color) and a free electron from the pool of unpaired electrons in the crystal (see Band Theory) is trapped in the vacancy. This unpaired electron can then be raised to ,the now available, higher energy levels by the absorption of energy in photons and similar crystal field rules of absorption and fluorescence, as described above, are in effect.

The fluorine ion is usually displaced and creates an interstitual in the lattice, a so called "Frenkel defect", meaning that there is an ion at a particular site in the lattice where it normally would not be. This displaced fluorine ion does not play a role in the development of color (only the vacancy it leaves behind does).

The term "electron color center" refers to the fact that there is a "free" electron where it normally wouldn't be.
Situation A in the image shows the ideal configuration of fluorite, while situation B shows the electron that is trapped in the vacancy left behind by the displaced fluorine ion.

Hole color centers
Hole color center in smokey quartz


Hole color centers are usually illustrated by smokey quartz as in the image on the right.
In quartz (SiO2) some silicon ions with a valence state of 4+ are substituted by aluminium ions with a valency of 3+. In order to keep electroneutrality, a hydrogen atom (or Na+) will be present nearby. This causes the forces on the electrons of the oxygen atoms to be weakened and radiation (X-ray, gamma rays etc.) can remove one of the weaker bonded electrons of the oxygen atoms. This leaves a hole (one electron is missing) and different energy levels become available to the now unpaired electron on the oxygen ion.
The crystal field theory now applies for the remaining, unpaired, oxygen electron and the resulting color is a smokey brown to which smokey quartz owes its name. The displaced electron will be trapped at other sites in the crystal lattice and it doesn't contribute to the color making scheme.
The substitutional aluminum ion acts as a "precursor" and is vital to the mechanism.

In amethyst the workings are similar, but the precursor is ferric iron (Fe3+) and produces the typical purple color.

The term "hole color center" refers to the missing electron, leaving behind a hole.


If the crystal is heated (around 400° C. for smokey quartz and ± 450° C. for amethyst), the displaced electron is freed from it's trap and returns to its original site, traveling as paired electrons again. The color of the crystal will then return to its original color (usually yellow ("citrine") or green ("prasiolite") for amethyst and colorless for smokey quartz). After re-irradiation, the hole color center can be reproduced and aslong as the crystal is not overheated, this process can be repeated infinitly.
The process of heating the crystal and destroying the hole color center is named "bleaching".

The Molecular Orbital Theory

The molecular orbital theory describes the paths (orbitals) electrons travel when multiple atoms (two or more) combine chemically. In order for atoms to combine to molecules, they must share or exchange electrons.
Although the orbitals themselves are very useful in describing this theory, it is a very complex topic and goes beyond the realm of gemology.

Instead we will focus on the different types of bonds that can occur between atoms.

  1. Ionic bonding
  2. Covalent bonding

While both these bonds have different characteristics, mostly they both play a role in the chemical bonding of gemstones and are directly related to the electronegativity of the different elements that make up the chemical formula of a gemstone.
It are only the electrons in the outer shells of atoms that play a role in both ionic or covalent bonding.

Electronegativity

Electronegativity scale


Charge transfer

Charge transfer in Iolite