New Mexico Mineral Symposium — Abstracts

The Hansonburg mining district, Bingham, New Mexico, is famous for its vividly colored fluorite crystals. "Bingham blue" with its characteristic sensitivity to sunlight is the most common and most widely recognized color found at Bingham, but crystals of purple, green, and colorless are also found. Color zoning is also common. Zoning of color between concentric zones from the center of the crystal outward is most commonly seen. Examples include crystals with green cores and blue or violet rims and fine alternating zones of blue and violet. The Mex Tex and Royal Flush mines have produced wonderful examples of sectoral zoning of color. In these samples, symmetrically different crystal faces and the zones beneath them (called sectors) exhibit different colors. For example, cubohexoctahedral crystals have been found with purple hexoctahedral faces (and sectors) and colorless cube faces (and sectors). Figure 1 is a sector zoned cubohexoctahedral crystal.

To understand the origin of color in these fluorites, and in minerals in general, we have to look at how visible light interacts with crystals or individual atoms within crystals. Color is imparted to a material that is illuminated with white light (light that contains all wavelengths in the visible region of the electromagnetic spectrum) when the material absorbs certain wavelengths and other wavelengths are transmitted or reflected so that they reach one's eyes. Color is the physiological response of the human eye to those wavelengths that reach it. For example, if white light enters a transparent crystal and all of the visible wavelengths from red through green (wavelengths roughly from 740 to 500 nm) are absorbed, only the remaining wavelengths of light (wavelengths roughly from 500 to 350 nm) will be transmitted through the crystal and eventually reach your eye. What you perceive as the color of the crystal will be the response of your eyes and brain to the combination of visible wavelengths of light from 500 to 350 nm—a blue or violet color.

Thus the question "What causes color in minerals" can be rephrased as "What causes differential absorption of visible wavelengths of light when white light interacts with a crystal?" Most of the mechanisms of the absorption of light by minerals involve the interaction of light and electrons within crystals. Particles of light are known as photons, and photons not only have the property of a wavelength but also have energy associated with them. The relationship between these two properties, wavelength (λ) and energy (E), is given by the equation:

E = hc/λ

where hc is a constant. Electrons in an atom or a crystal also have energy associated with them. Electrons can gain or lose energy by moving from one energy level to another. One of the hallmarks of the nature of matter as described by quantum mechanics is that electrons in an atom or crystal cannot have just any energy, but rather only certain well-defined energy levels are possible. This behavior is ultimately the reason that the absorption of light usually occurs only for certain wavelengths and not others (i.e. differential absorption).

As photons of light move through a crystal, they interact with electrons. If the energy of a given photon is exactly equal to the difference between two possible energy levels of an electron the photon can be absorbed by the electron. The absorbing electron will then undergo a transition to the higher energy level. Thus the specific photon and others with the same energy (or wavelength) will not be transmitted through the crystal and will not reach one's eyes. Photons of other energies that do not match the difference between possible energy levels of electrons in the crystal will not be absorbed but will be transmitted through the crystal and will ultimately reach one's eyes. Again what is perceived as the color of the crystal will be the response of the observers' eyes and brain to the combination of visible wavelengths of light that are transmitted through the crystal.

Electron transitions between energy levels as a result of the absorption of specific energies of visible light can take place on a single atom such as a chromium impurity in beryl (the cause of green in emerald), between atoms such as iron and titanium impurities in corundum (the cause of blue in sapphires), or between energy levels in an electrostatic field where an electron is not associated with a specific atom. The latter type of transitions is associated with defects in crystal. A well-understood type of crystal defect that is commonly found in fluorite is an F-center. An F-center in fluorite is a vacant fluorine site with a trapped electron that is created when a photon of sufficient energy (i.e. and X-ray or gamma ray) knocks a fluorine atom from its original position into an interstitial site in the structure. A free electron can become trapped within the electrostatic field of this vacancy and undergo energy level transitions by the absorption of optical wavelengths of light, thus imparting color to the crystal.

Several studies of the relationship between color, structure, and trace element chemistry of fluorites from Bingham, New Mexico, have been conducted at Miami University (Bosze, 2003; Bosze and Rakovan, 2002; Wright, 2002; Wright and Rakovan, 2001). The results show a complex interplay of color-causing mechanisms in these samples. One mechanism that has been identified is electron transitions on simple F-centers. These are associated with the characteristic "Bingham blue" color. A trait of defects like F-centers is that they can easily be destroyed. If enough energy is imparted to the crystal to move the displaced fluorine atom back to its original site within the crystal structure then the F-center and the color associated with it are lost. This can be done by heating or by exposure to an ultraviolet light source such as the sun. This is exactly the reason that the "Bingham blue" color will fade on exposure to sunlight.

A more complex color center that has been identified is a rare-earth element associated fluorine vacancy. In this case, rare-earth element impurities residing directly adjacent to a fluorine vacancy create a unique electrostatic field by the coupling of the two entities. Two free electrons can become trapped in this field and undergo energy level transitions that absorb light in the visible part of the spectrum, thus imparting color to the crystals.

Color in the sectorally zoned crystals (Fig. 1) is the result of rare-earth element associated fluorine vacancies. During growth of these crystals the rare-earth elements are preferentially incorporated into the (321) crystal faces relative to the (1001 faces. Thus the higher concentration of rare-earth elements in association with defects causes more intense color on the (321) faces and their associated sectors.
 

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