Why does the colour of amethyst fade if exposed to too much light?

It is well known that the purple hue that is characteristic of the amethyst quartz ($\ce{SiO2}$) will fade if the crystal is exposed to light for a prolonged period of time. What is the underlying mechanism of this change?

• I feel like this question is better suited to Chemistry.SE. See discussion at meta.earthscience.stackexchange.com/questions/164/… – naught101 Apr 23 '14 at 1:39
• "It is well known"... to most people, maybe. I just learned something new! :) – Richard Dec 19 '14 at 12:18
• @naught101 I agree. Though this phenomenon does occur naturally, it is a set of chemical reactions that is mostly responsible for this. – boxspah Dec 23 '14 at 15:23
• @DominatorX please read the accepted answer on that meta question -mineralogy (which this question fits under) is on topic here. – user889 Dec 23 '14 at 20:42

The reason why amethyst loses its colour in sunlight is due in no small part to what impurity in the $\ce{SiO2}$ crystal structure gives the vivid purple and violet tones of amethyst.

According to several sources (1),(2),(3), the main source of the colour is naturally irradation-borne tetravalent iron $\ce{Fe^{4+}}$, which is formed via naturally occurring gamma radiation altering trivalent iron $\ce{Fe^{3+}}$ during crystalisation (3). Other elements exist as impurities but are far less important considerations for the colour produced (3).

The tetravalent state of iron is inherently unstable in 'normal' surface conditions and the process is reversible due to prolonged exposure to natural sunlight (3). Some research and experiments have demonstrated that some of the bleaching is caused by the UV portion of sunlight (4).

References

(1) Amethyst, Mindat.org

(2) Dudushenko et al. 2003, What Oxidation State of Iron Determines the Amethyst Colour?, ICAME 2003

(3) Hatipoğlu et al. 2011, Spectral, electron microscopic and chemical investigations of gamma-induced purple color zonings in amethyst crystals from the Dursunbey-Balıkesir region of Turkey, Radiation Effects and Defects in Solids: Incorporating Plasma Science and Plasma Technology

(4) Nunes et al. 2013, Spectroscopic study of natural quartz samples, Radiation Physics and Chemistry

• Tetravalent iron! That's new to me. – Gimelist Dec 24 '14 at 7:16

The color of a mineral can be caused by a variety of mechanisms. This is also true of amethyst, which is a variety of quartz ($\ce{SiO2}$), and can be found in many colors.

The major factors responsible for the production of color in minerals fall into five categories:

1. The presence of an element essential to the mineral composition
2. The presence of a minor chemical impurity
3. Physical defects in the crystal structure
4. The mechanical mixture of very fine impurities
5. The presence of finely-spaced structures in the mineral