Mineralogy

By the origin of their color, minerals are divided in three types: idiochromatic, allochromatic, and pseudochromatic.

For idiochromatic minerals, color is a fundamental and unchangeable property that stems from their chemical compositions. In other words, idiochromatic minerals have a constant color produced by transition metals in their crystal structure called chromophores, which include Cr, Mn, Fe, Ni, Co, Ti, V, Cu. The production of color is depended on the absorption of light waves by liable electrons in the atoms of the mentioned elements. For instance, chromium produces the intense green color in emeralds, while manganese colors lepidolite in violet. The image features cinnabar (HgS), which is always red:

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Pseudochromatic minerals are of the color that is induced not by their chemical composition, but by physical effects such as the play of light. Light refracted from layers of slightly different refractive index produces iridescent colors. One example of a pseudochromatic mineral is labradorite:

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Allochromatic minerals are minerals whose color may vary. Alochromatic minerals do not have chromophores as an integral part of their chemical composition but they can contain interstitial inclusions that affect their color. For example, quartz (SiO2), which is colorless if chemically pure, can become violet, green yellow, white, pink, or gray.

• The inclusions of tiny air bubbles in quartz turn it white (milky quartz.)
• The inclusions of rutile (TiO2), which is a separate mineral itself, turn quartz yellow (rutilated quartz.)
• Radiation creates holes in the molecular structure of SiO2 by removing electrons, which results into absorption of a different wavelength of light, turning quartz gray (smoky quartz.)
• The addition of very small amount of iron turn quartz violet (amethyst, also a type of quartz.)

In my project, I am mainly interested in species of allochromatic minerals that gained their colour from impurities (like in the case of amethyst). It is important to note that very small amounts of contaminations in the molecular structure, usually less than 1%, affect the color of the entire mineral, which makes this area of study even more complicated and interesting.

The picture below features amethyst: