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"Pyrite Disease"

Posted by Robert Simonoff  
avatar "Pyrite Disease"
August 19, 2009 02:54PM
    
Hi everyone,
A while ago I went collecting with a club and found some nodules of pyrite or marcasite. They are beginning to catch "pyrite disease," cracking and crumbling. Will putting them in a closed box so that they are exposed less to sunlight help prevent further cracking and crumbling? Is there anything else I can do to prevent cracking/crumbling?
Re: "Pyrite Disease"
August 19, 2009 03:21PM
I don't think light has anything at all to do with it. A closed box isn't going to help unless you have a dessicant in it too. Extreme dryness will help slow the decomposition.
avatar Re: "Pyrite Disease"
August 19, 2009 04:13PM
    
Hi,
Thank you Alfredo, that was a very quick response! :)
What is a dessicant, and how do you achieve extreme dryness?
avatar Re: "Pyrite Disease"
August 19, 2009 04:22PM
    
silica gel packets that you get with shoes and other such materials are dessicants- they absorb water from the air. A dehumidifier can be used to maintain dryness as well.
avatar Re: "Pyrite Disease"
August 19, 2009 04:27PM
    
Oh I see, thank you. Now I will have to try to find at least one of those silica gel packets I saved up for various science experiments and the like... ;)
Jessica



Edited 1 time(s). Last edit at 08/19/2009 04:27PM by Jessica and Robert Simonoff.
Re: "Pyrite Disease"
August 19, 2009 04:41PM
    
Jessica,

I think that you will find that you need more than just a single packet. Also remember that the piece will have to be kept in an airtight container to prevent constant refreshment of humid air. The silica gel can only absorb a certain amount of water before becoming useless. At this stage it is useless.

Barry
Re: "Pyrite Disease"
August 19, 2009 04:48PM
I use the calcium oxide (lime) packets that come with japanese seaweed. I think it's a more aggressive dessicant than silica gel.
Re: "Pyrite Disease"
August 19, 2009 04:59PM
    
Alfredo,

What do you think the effects of keeping a specimen under a nitrogen atmosphere or a noble gas would be? Of course keeping it dry at the same time. I was thinking that perhaps the nitrogen would inhibit the respiratory functions of the thiobacillus ferrooxidan bacterias?

Barry



Edited 1 time(s). Last edit at 08/19/2009 04:59PM by Barry Flannery.
avatar Re: "Pyrite Disease"
August 19, 2009 05:11PM
    
Wow!
How often should I change the silica gel pouches? (I don't have calcium oxide like Alfredo gets)
avatar Re: "Pyrite Disease"
August 19, 2009 05:21PM
    
just gently reheat it to drive off the water, then it can be reused.
Re: "Pyrite Disease"
August 19, 2009 05:30PM
Ian, won't the silica gel crystallize and then lose its hygroscopic properties?

Barry, you might be right; I haven't tried it.

Some museums use sophisticated vacuum techniques to suck out all moisture and then inject lacquer into deep cracks to prevent future decomposition. Seems like a lot of work for a mineral as common as pyrite. I'd rather just throw out the unstable ones and concentrate on finding relatively stable ones. Unfortunately, most lesser preservation efforts seem to only slow the process down but not stop it completely.
avatar Re: "Pyrite Disease"
August 19, 2009 05:30PM
    
There is also a desiccant that changes from blue to pink that can be regenerated by heating in an oven.

There are some chemical treatments that tend to inhibit pyrite disease, but they use some nasty chemicals and are often much more costly than the specimen is worth (mainly used on fossils). You should isolate these specimens from the rest of your collection.
Re: "Pyrite Disease"
August 19, 2009 06:02PM
    
Keeping the pyrite/marcasite dry is key to its survival. Water is essential for the reaction. As the reaction proceeds sulphuric acid is generated as a by-product. This is hygroscopic (absorbs moisture from the atmosphere) thus drawing in more water and causing more reaction. The reaction is therefore self-perpetuating and keeps on going until the pyrite is all consumed. From what I have read it seems unlikely that bugs, like T. ferroxidans, are involved, and may just be incidental, if present. Chemistry alone is probably enough. The finer grained, framboidal, pyrites are the worst, and I have seen many an example from the Irish zinc mines disintegrate. In fact so reactive is finely divided pyrite it has even been known to catch fire, as happened once in Tara mine, when a pile of such produced by drilling was left over the Christmas vacation, and ignited. To extinguish it that section of the mine was sealed to starve it of air. When everything had cooled down and it was re-entered the walls were coated with sulphur crystals distilled out of the pyrite.

As a research chemist for what used to be Europe's biggest desiccant gel manufacturer I am, perhaps, in a good position to figure out how best to keep my pyrites dry. Storing in a desiccator with a suitable desiccant, is my preferred method, but laboratory desiccators are big and expensive. Recently I have begun experimenting with cheap, plastic, air-tight food containers, which seem to be just as good. Fill them to about a fifth or a quarter of their volume (the more the better) with dry silica gel (can be in a sachet for convenience) and place the specimen next to it. There are many other desiccants available too. For very low humidities molecular sieves (zeolites) available as little grey pellets, are particularly effective, although I've found silica gel to suffice (at least my pyrites stopped falling apart once I began using it). Clay based ones are cheap, but generally not as good as silica gel. These can all be regenerated by heating, although zeolites need a good cooking at about 400 degrees C to get them really dry. Silica gel can be freshened in the kitchen oven at about 110 - 150 degrees C for an hour. Indicating varieties are available that change colour when used up. The traditional one uses cobalt chloride and turns from blue to pink. Turns blue again when baked in an oven to refresh it and can be used many times over. As cobalt salts are slightly carcinogenic I invented one based on iron which goes from amber to near colourless (United States Patent Application 20040209372). A variant I came up with later uses a Fe-bromo complex and gives a better colour change at even lower humidity and is manufactured under licence by Engelhard (as "Sorbead Orange Chameleon" ), although I am not sure if it is available to non-industrial consumers, but would be the best one for the job. Both can be refreshed in the oven many times over. Some people use a microwave oven, although temperature control is harder. There are other indicating silica gels, but those based on organic indicators (commonly blue/yellow colour change, although there are others) tend not to withstand many drying/redrying cycles. I have no qualms about using the traditional blue/pink cobalt-based one for my samples however, and it is probably still the most readily available. Just don't eat it!

As Alfredo says calcium oxide is an even more powerful desiccant (drying agent), however, unlike silica gel, it cannot be regenerated (except in a furnace). It is also very alkaline. Where I have found it useful, however, is in treating specimens in the early stages of pyrite rot. For this one needs a large air-tight container (a desiccator is ideal, although I guess a particularly large food box will do as long as it is air-tight). Fill a small beaker or pot (100 ml) with calcium oxide ("quicklime", best as lumps) and place it in the container. Place the specimens nearby in the container. Add a few ml of strong ammonia solution to the calcium oxide and put the container lid on fast. The calcium oxide reacts with the water in the ammonia solution, setting the ammonia free as gas. Leave for several days. Being a small molecule ammonia can penetrate into the tiniest spaces in the specimens, neutralising any sulphuric acid within them. It will thereby bring to a halt any pyrite rot, but will not repair the damage already done. Do this treatment in a well ventilated space (NOT in the house) as ammonia stinks. I use my garden shed. When done take the lid off (outside) and leave for 20 minutes for the smell to go then get the specimens into an air-tight container with desiccant as discussed above. If the specimen is already badly affected you may see some brownish colouration (hydrated iron oxides) where the products of the pyrite rot have reacted with the ammonia. In that form they will do no more harm, but rot will resume if the specimen is ever exposed to atmospheric moisture again, so keep it in its container and only bring it out briefly for ceremonial occasions.



Edited 1 time(s). Last edit at 08/19/2009 06:13PM by Stephen Moreton.
avatar Re: "Pyrite Disease"
August 19, 2009 06:09PM
    
Thank you everyone! :)
Re: "Pyrite Disease"
August 19, 2009 07:35PM
    
Stephen,

Thanks that is a pretty interesting comment! I love hearing stories about mine fires. Do you know about the fire in Jerome, AZ? The sulphide body caught on fire, burned for about 30 years, and created a whole suite of interesting new minerals.. Pretty cool history! Thanks for sharing that all that information!

Ben Kirchner
avatar Re: "Pyrite Disease"
August 20, 2009 07:37AM
    
Actually , it sounds like a pretty HOT history to me.
Thanks Stepen!
You and Alfredo always seem to come up with
interesting and thorough answers.
avatar Re: "Pyrite Disease"
August 21, 2009 04:42AM
Stephen,
I think that is the best thing written to date on treating degenerating pyrite and Marcasite. Would you consider writing it up in a somewhat more formal sense for an article here on mindat. I would like to post it up in the welcome section of this forum to refer people to when they ask about this problem. If enough people address the more common aspects of cleaning and preparing mineral specimen, we will eventually have a pretty good manual for cleaning and preparation.

Rock Currier
Crystals not pistols.
avatar Re: "Pyrite Disease"
August 21, 2009 06:00AM
    
Just a little clarification here. Are all FeSO family minerals at issue here? If I have a pyrite cube do I need to worry about it becoming a mineral killer? Or do I need to worry about that cube becoming FeO2 dust? Or are we really talking more about the forms witrh a very fine structure - very large surface area as in "The finer grained, framboidal, pyrites are the worst, and I have seen many an example from the Irish zinc mines disintegrate. In fact so reactive is finely divided pyrite "

Thanks
Bob
avatar Re: "Pyrite Disease"
August 21, 2009 07:12AM
All iron sulphides are problematic. The worst specimens are probably fine grained or poorly crystalline sulphides, esp. pyrite and marcasite, because of the higher effective surface area. Most greigite needs immediate storage in nitrogen when collecting. The reaction often appears to start inside of specimens, which can eventually expolode - even well formed pyrite crystals can do this if they have framboidal cores. It has been found that inclusions of fine thiosulphate minerals can occur in some of these sulphides and I wonder if these unstable minerals can catalyse the reactions? Pyrrhotite is a common problem, because it often has some fine grained alteration to pyrite and marcasite. I understand a nitrogen or other inert gas atmosphere will work, as the reaction needs oxygen as much as water. Freezing of well-dried specimens is another technique I have heard of, to slow the reaction down. But once the reaction has started is is very difficult to stop it completely, without constant monitoring. Perhaps move to the desert?

Regards,
Ralph
Re: "Pyrite Disease"
August 21, 2009 12:07PM
Pyrites decompose even in the driest desert on Earth, the Atacama, where there is still some moisture in the air, although I suppose pyrites do better in the desert than the jungle. But the most important thing collectors can do, especially in public display situations where it is difficult to seal them away with desiccants, is to concentrate on the more stable varieties. There are significant differences in stability with pyrites from different environments. The famous big cubes from Spain are relatively stable, as are the hydrothermal vein pyrites from the Huanzala mine in Peru.
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