The smoke in quartz One of the more common minerals in anyone’s collection will be a cryst
The smoke in quartzOne of the more common minerals in anyone’s collection will be a crystal of this type, though maybe not as perfect or beautiful as the 7.8 x 6 x 3.6 cm from the Bernese Oberland in the accompanying photo. Pure quartz is clear and transparent as river water and formed of pure silicon dioxide, while most of the well known coloured varieties are given their hue by tiny admixtures of impurities (see http://bit.ly/1gQbjajfor an explanation of how this happens). Amethyst (purple) and citrine (yellow) are both coloured by iron (see http://bit.ly/2Fr6pha) that is activated to absorb colour differently with varying temperatures baking it in the Earth while rose quartz contains a smidgen of manganese. The smoky variety known to the old English gem trade as Cairngorm (for the granitic mountains in Scotland where they were found) is much more interesting.Varying in colour from the faintest trace of darkness to an opaque black they owe their smokeyness to the interaction of natural radiation from the rock (often granite) that they formed in, said radiation coming from impurities of uranium and thorium distilled out of the Earth by molten rock. Aluminium is almost the same size as silicon, and sometimes it sneaks in and replaces some silicon atoms in the crystal structure as it forms. In normal circumstances nothing would result, the quartz would still be of the first river (as they used to say of the whitest diamonds).Add in the radiation from the above mentioned elements though and the crystal structure gets distorted due to the slight size difference between aluminium and silicon creating what are known as colour centres that absorb light and darken the stone in proportion to the amount of aluminium present and of radiation received. So there you have it, a wonderful interaction between nature’s forces producing a thing of beauty and a joy forever.LozImage credit: Saphira Minerals -- source link
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