In the late 90’s I ran across some faceted Pyrite stones. I thought they looked neat and wanted a few for my collection so I got 14 stones at auction. Received the stones and thought they looked great. Bagged and boxed them and added them to my collection.
Haven’t looked at them since then.
Pulled them out today and they have tarnished and turned almost black with age. I would like to clean these up but can’t use any acid cleaners on them and not sure If soap and water will do the trick.
Would like your ideas on how to clean these up without doing damage to the surface.
THANKS.
Hi there,
On pyrite you cán use acid, no problem at all… doing it all the time myself and it is fast. The results are always amazing!
You can either use hydrochloric acid or phosforic acid, both diluted, but not necessarily heavily diluted. In a plastic or glass container of course…
Make sure you are wearing rubber gloves and safety glasses.
Leave the stones in for 24 hours, rinse and admire your new stones again!
Have fun 
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Oh… btw… do not use nitric acid, that one wíll dissolve the stones…
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all sulfides oxidize in air and moisture… I’ve picked up some nice pyrite samples off of mine dumps and all of them have slowly disintegrated in humid air… old mine dumps themselves are significant sources of ground water and stream pollution… sulfides oxidize creating sulfuric acid which will leach out arsenic, cadmium and other toxic heavy metals… cleaning oxidized pyrite should be done gently with acids…organic reducing acids like citric acid, oxalic acid will slowly clean oxidized sulfides not only by acid cleaning but also by reducing ferric iron which is black (hematite and magnetite) into ferrous iron that is water soluble… stay away from oxidizing acids!.. Nitric acid is a very powerful oxidant and will burn up virtually everything that is oxidable…preventing oxidation is more important that letting pyrite blacken and having to clean it up over and over again until there’s nothing left… using a light oil such as sewing machine oil, or even a light wax to seal out oxygen once cleaned will help keep it from blackening… this is the same problem that jewelers face in keeping their tools at a high polish and rust free…
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The literature always refers to oxidation of pyrite etc as producing sulfuric acid which then blackens and destroys the surface. This is not entirely correct. The main corrosive damage is eventually done by ferric sulfate in the presence of moisture.
Initially, moisture and micro-organisms oxidise the sulfides present to sulfurous acid. This releases iron which catalyses oxidation of sufurous acid to sulfuric acid and also ferrous ion to ferric ion and then both form ferric sulfate. The latter material is strongly acid in water and thus is sold as an etch agent for electronic circuit boards.
Simply put, eliminating oxygen from pyrite and other sulfides with a varnish only works if all moisture is also excluded/removed and there is no ferric sulfate already present in the sample.
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you are correct in stating that water is necessary to oxidize pyrite. The over all reaction does include water as a reactant with Ferric hydroxide and sulfuric acid being the endproducts. pyrite and other metal sulfides oxidize on old abandoned mine dumps throught the west, causing polluted water runoff which enters streams. It’s s significant problem as the run off contains toxic heavy metals as well as being acid.
for practical purposes, keep your pyrite jewelry dry. once cleaned, dry thoroughly, use a hair dryer if necessary… store them in a closed plastic bag like a zip lock one or in a plastic display box that is sealed to air… you can add a dessicant capsule, like the ones that come with certain types of medications or foods… keeping them away from moist humid air will help them maintain their shine.
So far as acids for cleaning, HCl does work quickly, but might cause surface damage… reducing organic acids such as oxallic acid, citiric acid are gentler and also promoted reduction of oxidized iron. Phosphoric acid will not cause oxidation,
Super Iron Out is supposed to work very well. Oxalic acid is another prefered reducing acid… Once cleaned, make sure to neutralize the acid by using baking soda and rinse well.
Looking up how to clean pyrite on the internet will give you a half dozen different techniques… all of them work. The main thing is that once cleaned, the pyrite has to be kept very dry…
I think you missed my point and that is sulfuric acid does not hang around. It quickly becomes ferric sulfate which is the more active agent in furthering decomposition. Ferric sulfate is a strong oxidiser, more so than sulfuric acid. In contact with reducing substances it forms the iron staining so often seen in mixed sulfide minerals, or the red ferric hydroxide in the streams that run from sulfide mines. I’m sorry about the chemistry lecture but the facts are that textbooks are wrong on acid mine drainage when they claim that it’s sulfuric acid rather than ferric sulfate. It’s ferric sulfate and not sulfuric acid that ceaselessly destroys minerals such as pyrrhotite and impure pyrites containing it. But, of course it all starts with moisture and oxygen that both kickstart the chemical cascade that occurs.
I completely agree that sulfuric acid in and of itself is not an oxidizing agent. Sulfur as sulfate is already fully oxidized and is stable…Ferric sulfate is corrosive to metals…it’s used to etch alumium in place of the more expensive sodium dichromate. Fe+3 in the form of sulfate is a weak oxidizing agent, with Fe+3 going to Fe+2, in the presence of weak reducing agents such as organic acids, and in the case of mine drainage, sulfides which are S -2, sulfur readily takes higher oxidation states, making sulfide susuceptible to oxidation in the presence of oygen which a strong oxidizer… In the presence of oxygen it will catalyse oxidation of reduced substances, mostly sulfides. Acid mine drainage leaches out mobilized metal ions… heap leaching mining sprays water on low grade sulfide ore, converting it in the presence of oxygen from sulfides which bind up metal ions into sulfates and sulfuric acid, freeing the metal ions. These are then treated with dilute cyanide solutions that chelate the metal ions, mainly gold and silver, which are then recovered. Mine drainage does have a how pH due to the sulfuric acid. It’s also loaded with oxidized free metal ions. The bottom line is that oxygen is required for oxidation reactions to occur. Water in necessary for oxygen to become active… the reactions take place in an aqueous solution.
One of the clever proposals from the American Geophysical Union was to recover rare earth elements from acid mine drainage. Rare earths or lanthanide metals are critical for technology. Neodymium comprizes up to 40% of ultra strong permanent magnets for electric vehicules. Other lanthanides such as terbium are essential for electronics. The US has only one operating rare earth mine, Mountain Pass in California. Carbonatitie intrusions are uncommon, these intrusions are rich in rare earths… China has 90% of the world reserves of rare earths…
I think we are on the same page when it comes to blackening of pyrite. water and oxygen are necessary for it to occur, as without water and oxygen, ferrous iron can’t be converted into ferric iron, unless another oxidizing agent is used (eg. nitric acid, chlorox)…removing water by keeping pyrite dry will help prevent blackening. This is a problem in areas that have high humidity… my own pyrite specimens have been neglected… some of them have crumbled away after being left outside in the rain… others are in better shape and could use a good but gentle cleaning.
Marcasite is even more unstable than pyrite, even though it has the same chemical composition, being a polymorph. To keep jewerly made of these minerals clean, putting them in sealed plastic bags, with a dessicant such as silica gel, calcium chloride or calcium sulfate is the best bet. the capsules that keep foods and medications dry can be recycled for further use.