WHAT CAUSES COLOR IN MINERALS?
by Eric Greene and Helene Grogan
When I look at a mineral, the first thing I usually notice is its color. How can you help but be attracted to the riveting red of a Tasmanian crocoite, the vibrant violet of Uruguayan amethyst, and the glowing green of an emerald? But what really produces these colors? Why do most minerals have their own specific colors (for example, green emeralds)? And why is it that sometimes the same mineral is found in many colors (think fluorite: blue, green, purple, yellow, orange, red, pink, etc.)? Unfortunately, the answers to these seemingly simple questions are remarkably complex. In fact, physicists have identified at least 14 different causes of color!
First it helps to understand something about the physics of color. Basically, visible light is a form of electromagnetic radiation, just like radio waves, microwaves, X-rays, and gamma rays. If we picture these as waves, the wavelength is the distance from the peak of one wave to the next. Gamma rays and X-rays have very short wavelengths - far too short for us to see. Slightly longer waves give us ultraviolet (or UV) light, and past UV light we come into the realm of visible light, starting with violet, which has the shortest wavelength we can see. As the wavelengths get longer, the color shifts from violet through indigo to blue, into green, etc. Red has the longest wavelength we can see, above which comes infrared, then microwaves and radio waves.
White light, such as sunlight, contains all the colors of the visible spectrum (and in the case of sunlight, also some infrared and UV, which is why we wear sunblock). The accepted theory of color is that some of the wavelengths of incoming light are absorbed by the colored object, and that the remaining wavelengths determine its final color. We'll take a closer look at the mechanism for this in just a minute.
Minerals are divided into two classes, based on what causes their color:
Some minerals always have the same color, because of the presence of certain elements in the mineral's normal molecular structure.
An atom can be visualized as a nucleus surrounded by electron shells. These shells represent the different energy states available to that atom's electrons. Incoming energy, including visible light, can be used to raise an electron from one shell up to a higher one, but only if its wavelength is exactly right. When it is, the energy of that wavelength is completely absorbed. Other wavelengths either pass right through or are reflected back (or scattered). The wavelength of light that is mostly strongly reflected or scattered is what gives an object its visible color, because this is the one that most clearly reaches our eyes. To take the example of an emerald, it absorbs primarily the violet and red wavelengths, reflecting the green, and so the emerald looks green.
The primary elements that absorb visible-spectrum color are the transition metals on the Periodic Table. The most familiar of the 38 transition metals are iron (red, green, yellow), cobalt (blue), and nickel (green). Others include vanadium (red, orange), manganese (red), chromium (red, green), gold (yellow), titanium (blue), and copper (green, blue). Some rare earth elements also create color through their absorbtion patterns, including scandium, yttrium, lanthanum, and cerium.
Other minerals get their colors from elements that are not ordinarily part of their chemical makeup. To greatly simplify this exceptionally complex subject, here are the 4 most widely agreed upon causes of color in allochromatic minerals:
Tiny amounts - as little as one tenth of 1% - of an impurity in the molecular structure of a mineral can determine that mineral's color. The amount and type of impurities affects the color of the mineral.
Inclusions of a second mineral (or even air!) within a host mineral can also alter a mineral's color.
3. Charge Transfer
When two or more elements in a mineral exchange electrons, this is called charge transfer. The movement of electrons results in selective absorption of light.
4. Color Centers
A color center is a defect in the molecular structure of a mineral. The defect is often due to damage from heating or natural radiation. UV light (from prolonged exposure to sunlight, for example) can also cause color change. Color centers can be removed by adding energy, as is sometimes done by heating gemstones to change their color. There are two types of color center defects:
In both cases, the flaws result in the absorption of certain wavelengths of light, and the colors not absorbed are seen as the mineral's color.