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Chromium in jewels
  
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Tom Mazanec




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PostPosted: Jan 25, 2021 12:57    Post subject: Chromium in jewels  

Why does the same element (chromium) color beryl green (emerald) and corundum red (ruby)?
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lluis




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PostPosted: Jan 25, 2021 13:04    Post subject: Re: Chromium in jewels  

Interesting question...

Answer

For red in ruby
http://www.webexhibits.org/causesofcolor/6AA.html
(link normalized by FMF)

For green in emerald
http://www.webexhibits.org/causesofcolor/6AB.html
(link normalized by FMF)

Hope it serves (chemistry... it is odd, as chemists are (or so it is said... :-) )

With best wishes

Lluís
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Riccardo Modanesi




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PostPosted: Jan 26, 2021 05:07    Post subject: Re: Chromium in jewels  

Hi to everybody!
The difference is due to different combinatons and coordinations of chrome with other elements. Andradite, grossular and uvarovite garnets get green (uvarovite, demantoid and tsavorite), chrysoberyl gets changing (alexandrite), tourmaline (elbaite and dravite) get green as well, etc.
Greetings from Italy by Riccardo.

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Hi! I'm a collector of minerals since 1973 and a gemmologist. On Summer I always visit mines and quarries all over Europe looking for minerals! Ok, there is time to tell you much much more! Greetings from Italy by Riccardo.
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Roger Warin




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PostPosted: Jan 29, 2021 09:04    Post subject: Re: Chromium in jewels  

CORUNDUM Al2O3 versus EMERALD Be3Al2(Si6O18).).

The origin of colors in mineralogy is even more difficult to understand than crystallography.
It can be of chemical origin or, more subtly, of physical origin.
It is the latter case which concerns the presence of chromium in corundum, giving ruby, or in beryl giving emerald.

Some Impurities may be present :
• in ruby: Al2O3:Cr
• in emerald: Be3(Al,M)2(SiO3)6 with M = Cr, Fe, V.
In corundum, O2- anions combine with Al3+ cations to form the structure of this oxide in a compact hexagonal assembly. This structure thus has available octahedral cavities, 2/3 of which are occupied by Al atoms.
In sapphire, another variety of corundum, under the incidence of light radiation a charge transfer occurs on a pair of Fe2 + and Ti4 + ions.
The explanation is different for the ruby. It is no longer produced by a very fast redox equilibrium. These main effects are attributed to the electric field generated in the vicinity of the Cr3+ site for the ruby.

In the case of emerald there remains a fundamental unresolved question. Despite recent EXAFS measurements (RX absorption spectra), such a color difference (for the same chromophore) can hardly be explained solely on the basis of a different mean distance between the Cr3+ cation and the six oxygen ligands.
The surprising difference in color of the two gemstones results primarily from the distinct electrostatic potential imposed by the other ions remaining in the lattice on the active electrons of the CrO69- unit.
In fact, in ionic materials, there are fields acting on the active electrons located within the complexed cation Cr3+ which cannot be neglected.
These main effects are thus attributed to the electric field generated in the vicinity of the Cr3+ site for the ruby.
As for the emerald, for reasons of symmetry, such an electric field cannot be generated in the vicinity of the Cr3+ site. Then the color induced by this chromophore differentiates.
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