removing pyrite from quartz

Dear all. I obtained an interesting sample of quartz but there is some remaining pyrite around tiny secondary quartz crystals that won't go off mechanically. What chemical procedure would you recommend me to use? Concentrated HCl and oxalic acid won't work even after several days. I thought about hot and concentrated sulfuric acid that would primarily oxidize and then dissolve pyrite but there should be a less dangerous way to do this. Any recommendations? Thanks! Albert

If the only thing on your quartz is some pyrite, you can remove the pyrite with nitric acid, but before you attempt this, be sure you know how to work with concentrated acids. Concentrated nitric acid is not a friendly character and you don't want to fool around with it unless you know what you are doing.

Rock Currier
Crystals not pistols.

Thanks! I have no more nitric acid left but do you think a mixture of HCl and hydrogen peroxide might trigger a similar oxidating environment, doing the job similarly like HNO3?

I don't know, but it would be an interesting experiment.

Rock Currier
Crystals not pistols.

If you're patient, you could just leave it out in the rain for ten years, and then remove the iron oxides with oxalic acid - Voila! Nice clean quartz and no pyrite. (:P)

Or you could hire the services of someone who has access to, and the skills to handle, things like concentrated nitric acid.

Thanks guys. I decided to give it a try with concentrated HCl and H2O2 after all. By now, about 90% of pyrite is gone (along with a few m2 of lawn around the bucket inside which the container was). Well, there was no no wind that would drift away the chlorine gas fast enough. So it is a possibility but don't try it at home unless you're a chemist. I am afraid I need a third round before the specimen is perfect! You may ask what I do with the acid afterwards. As always, I care about the environment. So I made a use for it to enlarge holes inside limestone rocks in my garden. The acid is neutralized and the loles are big enough for planting.

That mixture will most likely give off large amounts of highly toxic chlorine gas, in particular if catalyzed by iron compounds.
Do not do it.

Edit: oops - I should have read more carefully. You already did it and there was indeed chlorine gas. But luckily you just killed the gras around the bucket and not yourself...



Edited 1 time(s). Last edit at 05/19/2011 11:29AM by Amir C. Akhavan.

Hello!

Interesting results!

However,there are two more ways which are more safe.

1-As mentioned on the forum,the Pyrite should decompose when left in water for a long time.

2-Air abrasive would remove it without problems.Due to hardness difference.

An air abrasive and glass beads will not remove pyrite, but if you have something like sphalerite on its surface, ti will clean off the sphalerite nicely with no apparent harm to the pyrite. Even glass beads will have some effect on the quartz, even if you may not be able to see it. Quartz hit by glass beads disrupts some of the surface structure of the quartz, even though you can't see it, but put the beaded quartz in a little HF and you will see a big difference in how fast the HF will attack the surface beaded area vs. the un beaded surfaces. Crushed glass will probably take the luster of of a quartz crystal. Depends a lot on the velocity of the carrier gas. Probably all those angular sharp edges.

Rock Currier
Crystals not pistols.

Interesting observations, Rock, I didn't know that.
But makes sense.

Would the HCl + H2O2 combination work with diluted solutions that are more readily available like 10% HCl and store bought H2O2? Based on what Albert said, this method (at least with concentrated solutions) seems like a fairly viable method over using concentrated nitric acid, assuming one has access to a hood to avoid gassing oneself WWI style! Thanks for the input!

The redox potentials of H2O2 strongly depend on pH, and you simply don't get the reactivity in dilute HCl and with dilute H2O2. In any case, THIS IS NOT a fairly viable method. The active principle in this solution is free chlorine - very similar to aqua regia (which also contains free chlorine as the principle redox-active species). This mixture is as dangerous as aqua regia and far more dangerous than concentrated nitric acid. Upon inhalation, even small amounts of chlorine will damage your lungs irreversibly. Upon prolonged exposure, an oedema will form.

That mixture is certainly very dangerous, and I deffinatrly wouldn't recomend it to anyone!

I think you can remove the pyrite from the quartz with nitric acid. I once cleaned up several nice Bolivian cassiterite specimens using nitric acid to remove the pyrite from the cassiterite.

Rock Currier
Crystals not pistols.