When I was a kid, the only thing I was good at (and my mum will probably stand testimony to that) was smearing colour all over me and anything around me. As children, everyone of us was fascinated by that chart on the last page of our drawing books which showed us what we could obtain if we mixed the basic colours. (if you don't remember, have a look here : http://www.siliconimaging.com/ARTICLES/CMOS%20PRIMER/image006.gif )
Yes, agreed, crystals are pretty but there is a cryptocrystalline variety of quartz that is sometimes as beautiful as the ones above. And, if you need proof, have a look below. Believe it or not, these are just some of the extraordinary range of colours that can be exhibited.
All in all, most of us retain this childish allure for anything that exhibits a multitude of colour. There are very few examples in nature that are as captivating when it comes to this. One among them are minerals. And, as if to prove me right, I was noticing my classmates when our professor was teaching us the reasons behind these wonderfully coloured crystals. All their eyes lit up when a gemstone was shown on the screen and when the professor actually handed out some from his collection, everyone forgot that there was a lecture going on. (believe me the stone hadn't even gone back into the paper packet and most of the class was already half-asleep.)
Now, you find a variety of colours in minerals. Thousands of hues and shades of all possible colours are seen. Over time, a geologist sees so many minerals that nothing amazes him. And there is nothing more omnipresent than the mineral that makes up the sandy beaches that mark our shores - quartz.
Quartz is such a mineral that does not evoke any curiosity. White and transparent - there is hardly anything that is interesting. But, sometimes even this oft forgotten member of the mineral world conjures up something that makes us sit up and notice it. Purple amethysts and yellow citrines (pictured below) serve to remind us not to write anyone off so easily.
Yes, agreed, crystals are pretty but there is a cryptocrystalline variety of quartz that is sometimes as beautiful as the ones above. And, if you need proof, have a look below. Believe it or not, these are just some of the extraordinary range of colours that can be exhibited.
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| (Photo credit : David Englund; http://www.davidenglundphotography.com/) |
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| (Photo credit : David Englund; http://www.davidenglundphotography.com/) |
Known as agate, these beautiful mineral specimens are classically associated with volcanic rocks. They are generally found in nodules or hollow cavities in volcanic rocks formed by volatiles previously present in the lava and it is their monochromatic or coloured bandings that are their most striking features. Now, the burning question is - how do these bands form or more precisely, how are agates formed?
It all begins with a cavity in the rock, preferably in a volcanic rock. Water containing SiO2 (silica - the composition of quartz) percolates through these holes and the mineral starts to crystallize. In the initial stages, the concentration is high and the silica in the solution is in a polymerized state. This is supposed to lead to rapid crystallization in the form of fibrous crystals which nucleate on the wall and grow inwards. This makes the first layer very fine grained. It is followed by a comparatively coarser layer of quartz. However, the solution is an open system with continuous variation in the solute and the solvent due to external sources of water and silica. Thus, when the concentration gets high enough again, the silica crystallizes rapidly and voila! You have your peculiar bandings, all the result of these periodic changes in the concentration of the silica in the fluid.
And, the vivid colours that make agates so valuable and appealing to the eye are the result of trace impurities in the form of transition metals. Iron, manganese, copper are some of the ions that impart these colours to them. Iron oxides make the bands go red, brown, black or green depending on the various factors like oxidation state etc. Oxides of manganese colour the layers pink, violet or black. There are various combinations of these ions that bring about the other shades that are seen.
Sometimes, however, the concentrations don't vary as time passes and the silica keeps getting depleted as the crystalliztion proceeds inwards. This results in progressively coarser quartz grains towards the centre and we have what we call a geode (they are some impressive freaks of nature - fully formed crystals enclosed in a rock. Do check them out!)
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| Notice how the layers become coarser towards the centre. (Photo credit : David Englund; http://www.davidenglundphotography.com/) |
But this is not the place to be talking about geodes. For now, focus on the hypnotic beauty of the agates. I'll be back soon with some other stuff...
