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?
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1$\begingroup$ I feel like this question is better suited to Chemistry.SE. See discussion at meta.earthscience.stackexchange.com/questions/164/… $\endgroup$– naught101Commented Apr 23, 2014 at 1:39
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$\begingroup$ "It is well known"... to most people, maybe. I just learned something new! :) $\endgroup$– RichardCommented Dec 19, 2014 at 12:18
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$\begingroup$ @naught101 I agree. Though this phenomenon does occur naturally, it is a set of chemical reactions that is mostly responsible for this. $\endgroup$– unbindallCommented Dec 23, 2014 at 15:23
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$\begingroup$ @DominatorX please read the accepted answer on that meta question -mineralogy (which this question fits under) is on topic here. $\endgroup$– user889Commented Dec 23, 2014 at 20:42
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2 Answers
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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
(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
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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:
- The presence of an element essential to the mineral composition
- The presence of a minor chemical impurity
- Physical defects in the crystal structure
- The mechanical mixture of very fine impurities
- The presence of finely-spaced structures in the mineral
See these pages for more information about the causes of color in minerals.
If you search for "Color Centers in Quartz" you will find a large literature on this subject. Some color centers in quartz can be created or destroyed by irradiating crystals with light or X-rays.
The quantitative measurement and study of mineral color is called Mineral Spectroscopy.
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5$\begingroup$ How does this answer the question? $\endgroup$– GimelistCommented Nov 8, 2014 at 11:50