SEE BLOG POSTING OF 5-12-13 FOR BETTER DESCRIPTION
Causal collectors of minerals often find pyrite, iron disulfide (FeS2), appealing as a display specimen; most give little thought to keeping it protected over the years. On the other hand, paleontologists, but especially vertebrate paleontologists and museum workers, are quite concerned about “pyrite disease” eating up their specimens. If fossil “bones” (as an example) contain even small amounts of pyrite and are exposed to conditions of high humidity, then the mineral begins to oxidize and forms iron sulfate (perhaps the mineral melanterite), FeSO4. Iron sulfate is of greater volume that iron sulfide (pyrite) and so causes expansion in the specimens and breakage and crumbling soon follow. In addition, sulfuric acid forms and discolors the specimens. Once pyrite disease begins the process seems irreversible. That is one reason museum collections are stored in dry conditions, 45% humidity or less. And, individual collectors should keep their prized specimens in a sealed container along with a package of silica gel, a desiccant. Without proper storage they may begin to oxidize within a few months. Larger specimens of pyrite cubes are slower to oxidize and most collectors will only see some “tarnishing” on their specimens. This is the same chemical reaction, plus water and perhaps microbes, that produces the very acidic (sulfuric acid) acid mine drainage.