The Wonder of Color.

Fluorite/Calcite.

The basis of color is pretty universally understood and serves as the theme of this Website. Although the human eye can discern more than a million different colors, the fundamentals involve the three primary colors of light-red, green, and blue. These colors are plucked from the color spectrum, that portion of the electromagnetic spectrum with wave lengths between about 400 nanometers and 700 nanometers (1nm=10 Angstroms). The shorter wavelengths possess greater energy, with energy coefficients decreasing sequentially from violet (400nm) through blue, green, and yellow to red (700nm). This division is classically visualized by filtration of white light through a prism (such as a prismatic face of a quartz crystal) or atmospheric water (such as a rainbow). Wavelengths longer than red are referred to as "infrared" and shorter than violet termed "ultraviolet"-all part of a much greater electromagnetic spectrum which includes microwaves, X-rays and radio waves, distinguished by their invisibility to the human eye.

The primary colors can be blended to form "complementary colors." Red and blue merge to produce magenta, the complementary color of green. Likewise, blue and green yield blue-green or cyan, the complement of red, and green and red equals yellow, the complement of blue. Combinations of these and other color variations are particularly pleasing to the eye, producing color harmonies such as the red and green of Christmas or the purple and gold of the Minnesota Vikings or the LA Lakers.

Notably absent from the spectrum are the "colors" black and white. These are not true colors but instead combinations of all colors (white) or the complete absence of color (black). These extremes come into play in the production of "shades" (color + black), "tints" (color+ white), and "tones" (color + black and white).

All of these factors play a role in mineral aesthetics particularly in regard to in what we refer to as contrast. Think of the combination of malachite and azurite or the rubellite/verdelite bicolors of the tourmaline group. Now try to picture the aesthetic appeal of sulfur on rhodochrosite. It's all in the eye of the beholder, of course, but yellow on pink has a distinct lack of appeal to me.

The whole concept of color takes on a much more fascinating and controversial aspect when the reason for a given color of a mineral species is considered. No one contests that color is one of the physical properties that is most often, sometimes automatically, applied in mineral identification, especially in the field. Does the discovery of an olive green crystal suggest cinnabar or epidote?

But just how is that characteristic color produced? Obviously, we determine color by the wavelength(s) of light which reach our eye from the mineral, either by reflection, refraction or transmission of the incident light. The details of the genesis of that wavelength are as complex as they are fascinating. The minutiae of factors that determine the color of a given mineral are nothing short of amazing. Entire monographs are devoted to the molecular reasons for the range of colors displayed by just a single species and it serves no purpose to delve into these details in this short essay. However, let's look at a synopsis of the five most important principal factors about which there is general consensus.

When light strikes a mineral of specific chemical composition, certain wavelengths (or more specifically, the energy coefficient of certain wavelengths) are preferentially absorbed and we perceive the remainder as the color of the mineral. The factors that determine which wavelengths are absorbed are fascinating. Briefly, the electronic processes responsible for determining absorption, and therefore resultant color, are:

1. crystal field transitions
2. molecular orbital transitions
3. color centers
4. impurities
5. surface or internal interference

Crystal field transitions refer specifically to the behavior of partially filled 3d-electron orbitals of certain transition elements. These include Fe, Cr, V, Mn, Co, Ni, and Cu. Of these, Fe is the most common and therefore, iron is often the most critical factor in the color of many of our collectible minerals. All of these elements feature an incomplete 3d orbital and the interaction between these so-called chromophores and the incident light profoundly influences wavelength energy absorption of the mineral. The necessity for electrical neutrality in a stable mineral results in a "crystal field" of negative anion charges around the transitional cation. Furthermore, the nature of this crystal field will differ depending on the crystallography. Octahedral coordination will yield a different crystal field interaction than hexagonal or tetrahedral coordination. This, in turn, depends partly on the nature of the bonding (more purely ionic versus more purely covalent) and the valence of the transitional cation. Extended to greater complexity, this explains why trivalent Cr in beryl results in a different color than trivalent Cr in corundum (emerald green versus ruby red).

Molecular orbital transitions occur when shared orbitals result from delocalized valence electrons. Using iron as an example, the energies involved in the reversible relationship between the divalent ferrous ion and the trivalent ferric ion generally correspond to the wavelength energy toward the red-infrared end of the spectrum. These wavelengths being absorbed results in a blue transmitted color. The same is true of the Fe²⁺ to Ti⁴⁺ transfer. These two examples of orbital dynamics explain the intense blue color of many common minerals such as sapphire, corundum, aquamarine beryl and kyanite.

Color centers, or F-centers (from the German Farbe=color), are structural defects such as missing ions in the crystal lattice or "holes" resulting from displaced electrons. The energy required to produce such displacements is often the result of natural (or laboratory simulated) irradiation or heat. The presence of color centers explains many of the wide variety of colors seen in minerals that are basically colorless or faintly colored. Examples are fluorite, calcite, quartz, apatite and the tourmaline group.

Impurities similarly can alter the color of otherwise colorless minerals. Quartz may be green because of finely dispersed chlorite or orange-red from included hematite. Note that the mechanism for these colors of quartz is different from amethyst and smoky quartz whose color results from radiation-induced color centers.

Finally, interference of light transmission at the surface or interior of minerals may produce spectacular color displays. Surface films produced mostly by oxidation accounts for the iridescence of such minerals as hematite, resulting in the intense play of colors in such varieties as turgite. Internal interference may be caused by light reflected from closely packed fractures, lamellae or parallel inclusions, as in the schiller of labradorite, or from regularly packed spheres of amorphous silica and water in varying sized patches throughout the body of precious opal, resulting in differential diffraction of light or "fire", composed of the entire spectrum of light.

The above is a crude summary of the causes of mineral colors from Dana's Mineral logy in the 21^st edition of the Manual of Mineralogy by Klein and Hurlbut and is intended to impart a sense of amazement, not understanding. It is hoped that this greater appreciation of the wonder of color will give you pause the next time you are mesmerized by the dramatic color of a "purple rock."