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Pyrite Disease (Rot)

Posted by Anonymous User  
Anonymous User April 25, 2006 01:41PM
Anybody have any information on this? In particular, cause, identification and treatment.

Barry Flannery April 25, 2006 03:23PM

The common iron sulphide minerals marcasite, pyrrhotite and pyrite are all rather unstable and tend to oxidise when exposed to the atmosphere. They turn into ferrous sulphate then ferric sulphate and sulphuric acid. The acid accelerates the reaction, so it becomes autocatalytic, with the result that once it gets started it speeds up and there's no stopping it. As the reaction products have a larger volume than the starting material the whole specimen just disintegrates and ends up as a handful of crumbs. Meanwhile the acid generated eats through everything around it.

Instead it is a function of surface area and humidity. The larger the surface area, the more reactive the sample. Thus, the framboidal/granular pyrites that are so typical of the big zinc mines, are very unstable. They are composed of many fine grains, and have pores or fissures penetrating deep inside them, so they have a large reactive area available. They also tend to contain some marcasite, which only worsens things. Big chunky xls, like the cubes in matrix from Spain, have a very low surface area to volume ratio, and so last indefinitely.

I have tried soaking in water to remove the acid and ferrous sulphate, then in dilute potassium hydroxide solution to make it alkaline. This seems to slow it down, but still they fall apart eventually. There is a complex and expensive vacuum impregnation method that is employed in museums to try and save pyritised fossils, but its success rate is variable. Sealing them with epoxy does not work either - they actually explode after a while as the pressure builds up!

Avoid fine grained, granular pyrite. Store your specimens in as dry a place as possible. Dryness is the key to success.


Barry Flannery April 25, 2006 03:39PM

In regard to identifying the disease, simply a fowl smell and ''crumbliness'' of the specimenw will tell.
Also, I strongly suggest you put your specimen into an alkaline solution until it stops fizzing (you'd be surprised!).


Chester S. Lemanski, Jr. April 26, 2006 12:49AM
Pyrite disease is nasty stuff. The decomposition products include the stinky sulphur but also probably contain melanterite, other iron sulphates, and the nasty stuff known as sulfuric acid - very bad stuff. ALWAYS wear gloves and wash your hands when handling diseased pyrite, marcasite and pyrrhotite. The only safe storage from the perspective of safety is sealable plastic bags. Once the decomposition process begins, it is unlikely that the specimen can be salvaged except as a melanterite specimen. Encapsulation in acrylic spray may be effective if done in time (after neutralization of the acid). Unstable and metastable pyrite should be avoided if possible. Always store pyrite in a low humidity environment at moderate temperatures.
Alfredo Petrov April 27, 2006 12:39PM
I find that marcasite/pyrite always continues rotting after using alkalis to neutralize the acid; any respite is only temporary. It may sound counterintuitive, but I have much longer-lasting results by dipping the specimens in strong HCl. (Assuming no carbonates or other soluble minerals are present.)
Phil B. April 27, 2006 01:35PM
The process starts with oxidization right? Because i recently etched off calcite from a pyrite specimen and the color went from yellowish to a more white color... Would that be oxidization?

Thanks, Phil.
Alfredo Petrov April 27, 2006 02:32PM
No, you dissolved off a very thin surface film of oxides which had intensified the yellow color of the pyrite. Now you are temporarily back to the very light yellow pure pyrite color. It will eventually tarnish deeper yellow again.
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