Moonstone is a sodium rich plagioclase feldspar. It demonstrates a flash of colour when moved under the light. This is called labradorescence. The optical effect is due to exsolution. Plagioclase is part of a solid solution series -- it has different proportions of calcium and sodium within its structure. Sodium and calcium ions have different ionic charges, but a similar ionic radius. At high temperatures the mineral is more concerned with the size of the radius. However, once it has cooled down to make a solid, the differences in charge make the material energetically unstable. In order to form a more stable structure, the plagioclase separates out into a more stable structure. This creates small gaps which have the same wavelength as blue light.
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