Color in Minerals

Minerals may be naturally colored for a variety of reasons. Among these are:

Color is characteristic (idiochromatic) for some minerals, and thus may serve as an aid to identification. Color is often quite variable (allochromatic), and thus may contribute to misidentification.

Color depends on the response of the eye to visible radiation, roughly between 400 and 700 nanometers. Light striking the surface of a mineral may be transmitted, reflected, refracted, absorbed, or scattered. Reflection and scattering contribute to the luster of the mineral, not the color. If no absorption of light occurs, the mineral will be colorless in both reflected and transmitted light. Color results from the absorption of some wavelengths of light, with the remainder being transmitted. Our eye blends the transmitted colors into a single "color." However, spectrometers can resolve the transmitted light into its components. (See figure 4.60, p. 158 of Klein, 22st edition) The absorption spectrum of beryl shows peaks at 400 and 700 nanometers, and maximum transmission near 500 nanometers, corresponding to a bluish-green color.

Selective Absorption

A mineral is colored if certain wavelengths are absorbed as light is transmitted thorugh the mineral. Many minerals absorb several different colors. The wavelenghts of light that are not absorbed combine to give the mineral its color. Absorption occurs because the energy of the absorbed wavelength corresponds exactly to energy differences between allowed energy levels in the atoms or ions in the mineral. Colorless minerals have no transitions whose energy corresponds to visible light. Colored minerals have one of more such transitions. Three causes of such transtions are as follows:

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  • Crystal Field Transitions

  • Partially filled 3d (or, much less common, 4d or 5d) allow transitions between the split d orbitals found in crystals. The electronic configuration for the 3d orbitals is:
  • 1s2 2s2 2p6 3s2 3p6 3d10-n 4s1-2, where n=1-9
  • Many transition elements have partially filled d orbitals, and the presence of these ions in minerals is a major source of color. Iron is the most common example. Compounds lacking transition-row ions, or with the d10 configuration, typically are colorless.
  • An explanation for crystal field transitions comes from the negative anions surrounding the cation. The electrons of the anions create an electronic field, called the crystal field, which surrounds each cation. The symmetry of the crystal field matches the point group symmetry of the cation. The five d orbitals have the same energy in the absence of a crystal field (are "degenerate") but split in various ways in the presence of a non-spherically symmetric external crystal field.

  • Example 1: The mineral olivine ((Mg,Fe)2SiO4) has Fe2+ in two differently distorted octahedra, M1 and M2. Strong absorption occurs in the IR, and limited absorption in the red and blue parts of the visible (see Figure 4.62 of Klein, p. 160). The resulting transmitted light is yellow green in color.

  • Example 2: The mineral chrysoberyl (Al2BeO4) has a structure similar to olivine. Some ferric iron may replace the aluminum ions, which are in octahedral coordination. The resulting spectrum is much different from that of olivine (see Figure 4.62 of Klein, p. 160). Some blue-green absorption occurs in the visible, and the mineral is pale yellow. So the electronic state of the ion greatly affects color.

  • Example 3: In almandine garnet (Fe3Al2Si3O12), ferrous iron is found in an eightfold coordinated site. This changes the visible absorption to strong absorption of the yellow, blue, and green and some absorption of the orange. The mineral thus shows a deep-red color (see Figure 4.62 of Klein, p. 160).

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  • Charge Transfer (Molecular Orbital) Transitions

  • In minerals where electrons are transferred between adjacent ions, the absorption known as charge-transfer (also called molecular orbitals) takes place. Because the electrons are delocalized (shared between ions), crystal field theory is no longer applicable. Molecular orbital theory describes what is happening. Examples include the hopping of electrons between adjacent ferrous and ferric ions.
    • Fe2+ + Fe3+ = Fe3+ + Fe2+
  • This produces a blue color, seen in minerals such as kyanite, glaucophane, crocidolite, and sapphire. The color of sapphire often has an additional component, due to the following transition:
    • Fe2+ + Ti4+ = Fe3+ + Ti3+
  • Sapphire (Al2O3) often contains impurities of both iron and titanium. The iron-titanium charge-transfer produces the deep blue color of gem sapphire.
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  • Color Center Transitions

  • Structural defects in minerals may also cause color. Structural defects may be of several types. The following two are usually called color centers, or F (from German Farbe, color) centers.
  • An excess electron trapped in an interstitial defect (missing ion, or Frankel defect) or on an interstitial impurity

  • Purple fluorite is known to get its color from Frankel defects (missing F anion). The missing anion may result from exposure to high-energy radiation (X-rays or higher energy) or growth in an environment deficient in fluorine. The empty hole was supposed to have a negative charge. Therefore an electron, held in place by the overall crystal field, occupies the site. This electron may occupy either a ground state energy level, or various excited states. Movement of electrons between the ground and excited states may cause color, or optical fluorescence. Heating the crystal anneals such defects, and color fades.

  • A hole - an electron missing from the structure

  • Substitution of a cation with a lower charge for the ion normally present, followed by ejection of an electron from an adjacent anion, such as oxygen, can produce such holes. In smoky quartz, Al3+ may substitute for Si4+. This creates a charge imbalance. High levels of radiation for a short period, or low levels over geologically significant periods, can expel an electron from a lone pair on an adjacent oxygen ion. This creates the "hole." The remaining electron now has several excited states available to it. Transitions to these states may absorb visible light, creating color. If Fe3+ is substituted instead of Al3+, the purple color of amethyst results.

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Dispersion

Mechanical admixtures of impurities may also cause color in minerals. Hematite is a common pigmenting material, imparting a reddish color to minerals like feldspar, calcite, and quartz, variety jasper. Chrysoprase, a green cryptocrystalline variety of color, is usually colored green due to chlorite. The inclusion of water can make quartz milky. Milky quartz is usually formed from hydrothermal solution. Exsolution of minute, submicroscopic, needles of rutile causes the scattering of blue light, and imparts a rose color to quartz. If the needles are larger, red light is scattered, and the result is blue quartz. The presence of fine-grained carbon, either amorphous or as graphite, can cause a gray to black color in colorless minerals.

Minerals like agate, which are quite porous, are susceptible to artificial staining. Soaking the mineral in several successive solutions precipitates minute crystals in the pores of the agate, coloring it.

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PAGE URL: http://www.geosciences.fau.edu/Resources/CourseWebPages/Fall2005/GLY4200/MINCOLOR.htm

© 2005 by David L. Warburton

Quill Pen Questions or comments? mailto:warburto@fau.edu

Last updated: June 29, 2005