How is the iridescent tarnish made on chalcopyrite?

There is a chemical treatment that imparts an iridescent tarnish to chalcopyrite, making it resemble "peacock ore". Does anyone know the recipe?

I would also like to know if anyone would care to share a reliable formula for a solution to create the peacock color on chalcopyrite. It is rare that mineral dealers want to change the color of their chalcopyrite specimens. Some wholesalers treat massive chalcopyrite on a regular basis if and when they can get their hands on a supply of nice bright natural chalcopyrite. They sell it as "color rock". One of the suppliers has his chalcopyrite "color rock" broken to size in Mexico and acid treated there so he won't have to use the chemicals here in the USA where using chemicals of any kind has gotten to be difficult because of the many regulations regarding the safe use and disposal of the chemicals. The people I have talked to tell me that the solution is usually a mixture of concentrated sulfuric and phosphoric acids. This is not all however. The successful treatment also depends on conditions that they do not fully understand. I am told that one day the solution works wonderfully batch after batch and some days it doesn't work well at all. They think it may have to do with the current humidity, temperature and the amount of sun light the specimens get. If someone knows exactly what is going on they will most likely not tell us because they are making money off their knowledge and do not want to break their own rice bowl. Certainly not an unreasonable attitude. No I won't tell you who the guys are that are using the chemicals because I am sure they would not want the attention.
Rock

By action of concentrated sulfuric acid (oleum) on phosphoric acid, phosphorous pentoxide (P2O5, the anhydride of H3PO4) is formed which is a strong oxidant in certain conditions. It oxidizes not only the Fe(2+) - this could be more easily accomplished with less aggressive chemicals - but also the sulphide (S[-2] -> S[+6]).

This mixture is utterly hygroscopic. When sufficiently diluted with water however, the oxidizing properties are lost. Concentrated sulfuric acid is an "oxidizing acid" (i.e. it exhibits a higher oxidation power than protons), even able to attack metallic copper. When slightly diluted with water however, this ability is lost. Likewise, P2O5 is a condensation catalyst, but phosphoric acid is not. It is therefore important to keep the mixture in a tight container. To use a fair excess of the sulphuric acid in the mixture is also important, because concentrated phosphoric acid has only 85 weight percent of H3PO4. The residual water will dilute the sulfuric acid, successively, depraving it of its ability to catalyse condensation reactions (and thus P2O5 formation).



Edited 1 time(s). Last edit at 05/04/2008 09:38AM by Peter Haas.

Thanks but I still don't know how to change the colour of Chalcopyrite. Could we have an answer that everyone will understand and be able to use please. Can this also be achieved by heating the chalco in vinegar and adding a piece of copper? This method was given to me some time ago with no specific instructions, eg. how long to heat it or at what temp. etc. Thank you.

Sometimes common bleach will do it (sodium hypochlorite solution).

Hello,

Detergents (with perborate) such as the German washing powder brandmark Persil and vinegar have been used in Germany in the 1960-ies. The chalkopyrite is cooked in the solution.

Regards,

Sebastian Möller

Hello!

May I reverse the question?Does anyone knows how to remove this tarnish?


Thanks!
-Kostas.

Prolonged treatment (several hours) with dilute citric acid, or short treatment with a strongly alkaline solution (prolonged treatment in these conditions has detrimental effects on chalcopyrite), depending on what the tarnish consists of.

The colours are interference colours, so it does not matter what compound is deposited on the surface. However, the film must be sufficiently thin. There is a multitude of chemicals that can corrode chalcopyrite, but the trick is, to have a mixture that reacts just slowly enough to have full control of the process. No need to say, that the result also sensitively depends on concentration of the chemicals and on temperature. Hence, optimization of the process requires many tests and is time-consuming.



Edited 1 time(s). Last edit at 05/08/2008 02:15PM by Peter Haas.