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The Richness of Color in Minerals

By Chuck DeFlorin, Minnesota Mineral Club

Color is the most obvious and attractive feature of any mineral. The intense red of cinnabar, the rich green hues of malachite and the vivid blue of lapis always deserve a second look. Generally speaking though, color is not a very good property when identifying a mineral. Many minerals have several different colors and it is important to understand what causes color in minerals in order to understand minerals themselves.

Agents in minerals responsible for their color are called chromophores or transition elements. These elements contain atoms with electrons that produce the color. Some minerals are always a certain color because these elements are part of a the mineral's chemical makeup.

When a mineral's own structure and composition are the sole agents of its colors, it is said to be idiochromatic or self colored. Examples of this are peridot, which is always green, and gold, which is always a golden color. Minerals of variable color caused by trace amounts of transition elements are referred to as allochromatic or other colored. It is the infusion of different impurities that allows fluorite to come in every hue of the rainbow. Even tiny amounts of these elements can deeply color minerals. Some people think that certain elements or impurities cause only certain colors, but a single transition metal can be responsible for a variety of colors based on its oxidation state.

Minerals can also get their color from minute air bubbles which cause different colors to be seen. Some minerals are referred to as pseudochromatic or false colored when neither their mineral nor atomic properties are responsible for their colors. Instead they contain layers or films which create colors by light interference. Moonstone and opal are good examples of this effect.

Another color effect is pleochroism which occurs among minerals that demonstrate refraction. This effect causes a light beam to split in half and directs the two halves into different directions. As a result, each part is subjected to different amounts or types of color absorption depending on the path and vibration of the light. When viewing from different angles against a light, you'll notice a change in either the color or the depth of color. When there is a difference between the colors, the phenomenon is referred to as dichroism. Other minerals, such as smoky quartz and blue halite, owe their color to radioactivity which distorts the crystal the crystal lattice and permits the absorption of light that would otherwise be transmitted.

The deep Prussian blue of azurite, bornite with its spectacular iridescent purple-blue, the shining green translucency of chrysacolla, blue-green of malachite; all get their colors from the transition metal, copper.

Cobalt gives erythrite its violet-red color, calcite a pretty pink color and roselite a delicate pink luster. The hyacinth orange-red of crocoite and the emerald green of uvarovite are both produced by chromium. Reddish-brown hematite, yellow ochre of limonite and even the violet color in amethyst are due to iron.

Manganese gives rhodochrosite its brilliant deep pink color and is also responsible for the pastel pink of rose quartz and the purplish-red color of richterite. The apple green color of annabergerite and emerald green of garnierite come from the element nickel. Uranium gives us the yellow-green color of autunite, the canary yellow of carnotite and the pearly emerald green of tobernite. Vanadium produces the fine red color of vanadinite and the butterscotch color of wulfenite.

There are many more elements that could cause color in different minerals. The next time you pick up a pretty mineral, try to determine what element contributed to its color. It is easier to find related minerals if you know what caused the coloring.

The San Francisco Gem & Mineral Society, Inc.

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San Francisco, CA 94122


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