I would like to cab some chalcopyrite but that will eliminate the iridescence and attractiveness of the stone. Does anyone have any recommendation regarding the best type of acid to use and the procedure for accomplishing this without hurting myself. So far, I have tried plain white vinegar with no real result. I think it could result in a marketable product down the road. Any help would be much appreciated.
When this material is polished, the metallic portions can reach a mirror-like surface. This may reduce the saturation of color when rebuilding the oxidized layer. You may be disappointed by the subtle hues achieved afterwards.
With that being said:
- Diluted citric acid has been mentioned in some forums way back in early 2000’s. It will have to soak in the solution for several hours at elevated temperature (90 - 100F) I do not remember the ratio of distilled water to citric acid.
Caution: The acid solution may cause visible pitting which may reduce the finished stone’s luster. (But this may help intensify/restore the color saturation of the oxidation)
- Second type of process uses sodium hypochlorate (regular chlorine bleach). Soak specimens for 10 - 15 minutes at room temperature. Remove from solution and let air dry. Then rinse with plenty of water. This process can be repeated several times.
If one doesn’t work, try the other. Thoroghly rinse the stones before applying the other process.
NEVER MIX THE TWO SOLUTIONS TOGETHER!
Thanks for the response. I will give both methods a try and let you know my results.
I’m not familiar with treatments that create iridescence but I would caution against any acid treatment of mineral sulfides. Most are inherently unstable because of the impurity presence of sulfides such as pyrrhotite that break down in moist air with eventual production of very acid ferric sulfate which then accelerates the decomposition process. Cabbed sulfides should ideally be protected by a sealer that prevent entry of moisture. However, this does not work if there are fissures or moisture in the sulfides already: witness early problems in preserving some museum meteorites.
Ivan, I agree with the cautionary against acid, however, I came across a dilute citric acid treatment during my research into the treatment processes. Not many resources were available or forthcoming.
My curiosity got the better of me, and I should have provided links to the processes I did find. (note to self… 
)
Chalcopyrite does exhibit a weak valent bond at the sulfide. The natural process forming the oxide is via galvanic interaction of mineralized water (alkaline) on the exposed surfaces. Some of the sulfide ions recombine with the Fe oxide, developing the optical iridescence.
Oxalic acid may act faster than citric in removing iron oxide. If you have plenty of material it may be useful to try oxalic both hot and cold. After any acid treatment I recommend using either ammonia or sodium bicarbonate to neutralise any retained acid. Then soaking in hot water. For drying my zircon specimens I use a microwave oven on low but zircons are virtually indestructible unlike sulfides!
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Ivan, Is there any particular reason you use a microwave oven to dry the specimens? There is significant risk for arc generation, especially if there is sufficient metal content present in the remaining matrix material. Just curious…
I have not had any problems with arcing even when samples contain magnetite, hematite and ilmenite. It might be different if the minerals contain metals such as iron, copper and aluminum. Samples containing water or water of crystallization heat up appreciably as do the oxides magnetite, hematite and ilmenite which do not possess water of crystallization. Microwaves have been used to rapidly heat silicon carbide crucibles. If unsure of how mineral samples react I usually experiment with a low power setting first.
Cracking of samples may be a problem if you already have small cracks in crystals that hold water. For example, a good party trick is to get some bloating hydromica that is used to make vermiculite and to zap it for a couple of minutes in a microwave. It usually expands to five times its volume due to the heating of intercalated water to steam. Maybe the method may be useful for making micromounts!