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December 07, 2007
Rubies!
Ruby is a variety of the common mineral corundum, which has the simple formula Al2O3. Its color is due to the incorporation of small amounts of chromium (an impurity which also happens to be responsible for the green color of emerald, a sexy variant of the mineral beryl). How does this work? An electron attached to chromium can be excited to a higher energy level by absorbing a photon with the appropriate energy. Two electron transitions are possible for chromium in corundum, one of which is achieved by absorbing violet light, and the other by absorbing green light. When a ruby is illuminated by white light, the violet and green portions of the spectrum are removed, and what's left is "ruby red". Blue sapphire gems are also corundum, but they have Fe and Ti impurities rather than chromium--electron transfers between the Fe and Ti are responsible for the blue color.
Most gemstones are natural minerals, but synthetic versions are also produced. In this year's Mineralogy (GEOL 341) lab we decided to have a try at growing synthetic rubies. Large synthetic corundum (sapphire) crystals are produced commercially, through precipitation from a melt, but the very high melting temperature requires a specialized (and expensive) furnace that we did not have access to. So the first order of business was to find a way around this problem. We decided to try a flux method, using a compound with lower melting temperature, in this case lithium molybdate, to dissolve a powder consisting of aluminum oxide with a trace of chromium oxide added, then lowering the temperature to precipitate (hopefully) ruby crystals. Our first experiment failed--the temperature was too low, and not all of the aluminum oxide dissolved. Tiny crystals of ruby may have precipitated, but they were too small to be recovered (at least by eye). On our second try, we cranked up the temperature to 1200 degrees celsius, then cooled the mix slowly to 1120 degrees over two weeks. After dissolving the flux (by boiling in concentrated ammonia--do not try this at home unless you have a good hood), we recovered several small--but magnificently colored and faceted--ruby crystals! An example is shown below, in a not-so-high-quality image taken with a digital camera looking down the ocular of a binocular microscope (I'll add better images later if I can get around to taking them). This crystal is a few hundred micrometers across, and the largest was around a cubic millimeter.
[Note Added 4/26/08: The large, low-quality image that was originally here has been deleted--please see RubiesII entry for ruby pictures of much higher quality]
Posted by jav12 at December 7, 2007 01:32 AM
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Comments
Useful site. Thank you:-)
sidekick
Posted by: sidekick at March 20, 2008 06:15 PM
Could you reduce the image? It's breaking the blog and hard to see as it's so large.
Posted by: Rubies at April 26, 2008 06:01 PM