WHAT IS IMPORTANT ABOUT THE COLOR OF MINERALS?

ANALCIME	REALGAR	CROCOITE	TYUYAMUNITE
MALACHITE	TURQUOISE	CAVANSITE	AZURITE
FLUORITE	AMETHYST	KAEMMERERITE	BIXBYITE

Color is the first thing someone notices when they view a mineral. Color is also one of the big reasons that attract people to minerals. Generally speaking, color is not a good property to be used in the identification of minerals. It is usually the first property to confuse a novice collector into making an incorrect identification. Many minerals have different colors and some minerals' colors are identical to other minerals' colors. It is important to understand what causes color in minerals in order to understand this mineral property.

Color in minerals is caused by the absorption, or lack of absorption, of various wavelengths of light. The color of light is determined by its wavelength. When pure white light (containing all wavelengths of visible light) enters a crystal, some of the wavelengths might be absorbed while other wavelengths may be emitted. If this happens then the light that leaves the crystal will no longer be white but will have some color.

What absorbs and or emits these wavelengths? Atomic bonds are generally the culprit. Some elements have electrons that absorb certain wavelengths or colors. These wavelengths provide energy to the elements that will often emit another wavelength to get rid of the extra energy. The energy state of the electron is related to the wavelength that it absorbs. The bonding in this element affects the energy state of these electrons. Therefore bonds to different elements produce different colors.

Elements that produce colors through absorption and emission of wavelengths are usually transition metals. They can cause a mineral to always be a certain color if they are part of the chemistry of the mineral. However, if there is just a trace of these elements, they still can strongly influence the color of the mineral. Even tiny amounts of these elements can deeply colored minerals. It is erroneously thought that certain elements cause only certain colors and there is some truth to that. Copper usually produces green and blue colors. Iron is known for the red and yellow colors that it typically produces. However, almost any element can be responsible for any color.

Below is a list of some coloring elements and the color they produce in at least one mineral:

• Cobalt, Co, produces the violet-red color in erythrite, (cobalt arsenic sulfide).
• Chromium, Cr, produces the color orange-red color of crocoite, (lead chromate).
• Copper, Cu, produces the azure blue color of azurite, (copper carbonate hydroxide).
• Iron, Fe, produces the red color of limonite, (hydrated iron oxide hydroxide).
• Manganese, Mn, produces the pink color of rhodochrosite, (manganese carbonate).
• Nickel, Ni, produces the green color of annabergite, (hydrated nickel arsenate).
• Uranium, U, produces the yellow color of zippeite, (hydrated potassium uranyl sulfate hydroxide).
• Vanadium, V, produces the red-orange color of vanadinite, (lead vanadate chloride).

These minerals, although still subject to the effects of trace elements, always have the same basic color. Most minerals, however, are usually white or colorless in a pure state. Many impurities can color these minerals and make their color variable. The property of streak often demonstrates the true or inherent color of a mineral. In addition to coloring elements, other impurities or factors exist that have also been linked to the color of minerals. Such things as elemental fluorine, sulfur, and chlorine; trace amounts of carbonate and other ion groups; chlorine and fluorine ions and even structural defects. Radiation from rare earth minerals can damage a crystal structure and this damage seems linked to coloring as in smoky quartz. Care should always be given when trying to identify a mineral using color. But, even in minerals that have a variable color, a skilled and experienced mineralogist can use the color of these minerals to make positive identification.

Some More Colored Examples:

Wulfenite	Colorless Gypsum	Rhodochrosite	Amber