Allochromatism - How imperfections provide some of the world’s most beautiful minerals

A few years ago when I was beginning my education towards a degree in geology, I had the opportunity to take a class on minerals. One of the objectives of the class was to develop an ability to identify minerals using a number of characteristics such as hardness and color. With color being the most striking characteristic of a mineral, and considering that there are so many different colors that minerals can be found in, it wouldn’t be hard to believe that color would be a very useful characteristic to study when trying to identify a mineral. Surprisingly, though, color is just about the most useless identifier of them all.

Why is color a useless identifier of minerals? As it turns out, there are a lot of minerals that can be found in a variety of different colors that are not the minerals’ natural color.

Why do some minerals come in different colors? The answer to that question, dear reader, is a bit more complicated.

 Quartz varieties from the Bruce Museum's mineral collection. Starting at the top, moving counterclockwise: Harlequin quartz, quartz, amethyst, herkimer diamond (quartz), rose quartz, smoky quartz, milky quartz, and citrine. Photo by Haley Royer.

Quartz varieties from the Bruce Museum's mineral collection. Starting at the top, moving counterclockwise: Harlequin quartz, quartz, amethyst, herkimer diamond (quartz), rose quartz, smoky quartz, milky quartz, and citrine. Photo by Haley Royer.

In order to understand how minerals can develop different colors, it’s really important to understand some defining characteristics of minerals. There are five requirements that a material must meet in order to be considered a mineral, but for the purpose of this article it’s only necessary to know two of them which are that all minerals must have a consistent chemical composition and an organized internal structure. When a mineral has a consistent chemical composition it means that the mineral in question is always made up of the same elements on the periodic table. Diamonds, for example, are composed of carbon atoms. Quartz, as another example, is made of silicon and oxygen atoms. When a material has an organized internal structure it means that the atoms that comprise it are always arranged in the same exact pattern. Essentially, a mineral is a molecule oriented in a specific way and repeated trillions of times. In a mineral’s purest form, this will be true. However, when the mineral’s composition is altered, it can sometimes result in a change or appearance of color.

Not all minerals are susceptible to changes in color, but those that are susceptible are called allochromatic minerals. Allochromatic minerals are naturally colorless and obtain color through a variety of different imperfections that affect the chemical composition or the internal structure of the mineral. One of the most popular of these minerals is quartz. If you’ve ever been to a mineral exhibit, you’ve probably seen quartz and its many varieties like amethyst, harlequin quartz, rose quartz, citrine, smokey quartz, milky quartz, and many others. For every allochromatic mineral there are potentially dozens of different colors that it can take on. Considering there are so many different colors that an allochromatic mineral can adopt, you might imagine that there are just as many ways in which they can be imperfected. It’s true that these minerals can be colored for a number of different reasons, but in most cases the color in allochromatic minerals can be attributed to one or a combination of three factors: elemental inclusions, radiation, or heat.


 These minerals are idiochromatic. Their color is constant and predictable. Moving clockwise from the top left: azurite, malachite, and cinnabar. Minerals are from the Bruce Museum's collection. Photo by  Haley Royer.

These minerals are idiochromatic. Their color is constant and predictable. Moving clockwise from the top left: azurite, malachite, and cinnabar. Minerals are from the Bruce Museum's collection. Photo by  Haley Royer.

Elemental Inclusions

In a mineral's purest form only the elements that make up the mineral will comprise it. But more often than not, a mineral will be riddled with foreign elements called elemental inclusions. These elemental inclusions can go visually undetected if the element has no coloring effect or if too few atoms are replaced in the mineral. But if the element does have a coloring effect and enough atoms are replaced, the affected mineral can exhibit color as a result. Imagine a bucket of white paint as a mineral. If you poured a tablespoon of water into the bucket, there would be no noticeable change. If, however, you poured a tablespoon of blue dye, the paint would take on a bluish hue. The same concept applies to minerals. If the elemental inclusions in a mineral have no coloring effect, the mineral will appear unchanged. However, if the inclusions do have a coloring effect, they can give the mineral color. The greater the number of inclusions, the more saturated the color in the mineral.

This type of impurity is responsible for a majority of colors in allochromatic minerals, including some very well-known minerals like yellow diamonds (diamonds with nitrogen inclusions), blue sapphire (corundum with titanium inclusions), and green emeralds (beryl with chromium substitutions).  


Radiation

 Calcite can turn blue when exposed to radiation. Photo by Rob Lavinsky.

Calcite can turn blue when exposed to radiation. Photo by Rob Lavinsky.

Radiation can also give a naturally colorless mineral color. Natural sources of radiation can come from the Earth itself in the form of trace radioactive elements like uranium, thorium, and radium that are present in soil and rocks. If the area in which a mineral forms is concentrated with these radioactive elements, and the mineral is exposed to the radiation for long enough, the radiation can alter the mineral in a way that gives the mineral color.

Instead of staining the mineral like elemental inclusions do, radiation actually alters the atoms that are already present inside of the mineral. As mentioned, some elements have a coloring effect while others do not. In some cases, a single element can have a coloring effect or no coloring effect at all depending on the charge on the atom. Radiation is basically a form of energy that has the ability to change the charge on the atom, and as a result can transform the colorless form of an atom into a colored version.

The color that radiation can bring out in a mineral completely depends on the elements found in the mineral, just like colors resulting from elemental inclusions. For example, if quartz with aluminum inclusions is exposed to radiation over a long period of time, the charge of aluminum atoms will change and give the crystal the brown hue that characterizes smokey quartz. If, however, the crystal includes iron instead of aluminum inclusions, the crystal will take on a purple color that characterizes amethyst.


Heat

 Heat can be used to enhance the color of topaz. Photo by Rob Lavinsky.

Heat can be used to enhance the color of topaz. Photo by Rob Lavinsky.

Heat can play a role in coloring minerals, but only if the mineral already contains impurities like elemental inclusions or has effects from radiation. The effect of heat on a mineral is similar to the effect of radiation: if a mineral contains elemental inclusions, heating the mineral can alter the charge of the foreign atoms and either eliminate or bring out color in the mineral.

There are only a few cases in which heat naturally results in a color change within a mineral. Instead, minerals are often treated with heat after they are dug up from the ground to make them more valuable. For example, golden beryl is naturally colored by iron inclusions and radiation but can exhibit a blue color when heated.

There’s no doubt that the processes responsible for coloring minerals can be quite complex. Rarely is a mineral ever completely pure. Rarely do the minerals we find fit into the rules we set up to try and understand them. Although minerals would be so much easier to study if they were never altered by the additions and alterations mentioned here, they would also be significantly less interesting.

- Haley Royer, Guest Writer


Haley Royer is one of our amazing Bruce Museum volunteers. She graduated from the University of Southern California with a B.S. in Geologic Sciences in 2016. Thank you Haley for your contribution to the Storage Two blog!