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.
