Pyrite decay
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The defining conservation problem of mineral collecting, and the one that destroys neighboring specimens as well as its own.
The specimen consumes itself
A pyrite specimen develops a white or yellowish powdery bloom, begins to smell faintly of sulfur, cracks along the grain and eventually falls apart into powder.
It is not something growing on the specimen. It is the specimen turning into something else.
Marcasite is worse than pyrite. Pyrrhotite and several other sulfides do the same thing.
The chemistry
Pyrite oxidizes in the presence of oxygen and moisture, converting to iron sulfates that occupy more volume than the original, which is what splits the specimen apart.
The reaction produces sulfuric acid.
Relative humidity is the controllable variable. Pyrite-bearing material should not be kept above about 60% RH, and 45% or lower is the working target. Below that the reaction effectively stalls.
Not every pyrite specimen is susceptible. Susceptibility depends on the crystal structure and on whether the material has already begun, which is why one specimen from a locality fails and another from the same pocket does not.
It takes the label and the box with it
The acid attacks paper, cardboard and adhesive. A decaying specimen frequently destroys its own documentation before it destroys itself, which converts a conservation problem into a total loss.
It also reaches neighbors. An affected specimen in a closed drawer is a source of acid vapor for everything else in it, and calcareous material next to it will start to effloresce.
What to do
Isolate anything showing signs, immediately, in its own sealed container.
Dry storage: a sealed box with conditioned silica gel, held low.
Keep the label outside the sealed container, or photograph it now.
Inspect sulfides annually. This is slow and it is not reversible once advanced, so the check is the whole defense.
What not to do
Do not wash it. Water accelerates the reaction.
Do not seal an actively decaying specimen in with its own moisture and consider it solved.
Do not coat it hoping to exclude air, which traps what is already there and makes later treatment harder.
And do not keep it with the rest of the collection while you decide.