We sometimes find fossils preserved by pyrite. They are prized as much for their pleasing gold colouring as for their scientific value as windows into the past. If you have pyrite specimens and want to stop them from decaying, you can give them a 'quick' soak in water (hour max) then wash them off, and dry them thoroughly in a warm oven. Cool, then soak in pure acetone for a couple of days. Then soak in paraloid, a thermoplastic resin surface coating or acetone for a couple of days. Keep them in a sealed container with a desiccant pack afterwards to keep them dry — or leave them out on display to enjoy knowing that the decay will come in time. We do this with cut flowers so why not fossils sometimes? A friend gives her pyrite fossils on display a quick thumb wipe with vaseline or petroleum jelly. I'm not sure if the hydrocarbons there will play nice over time but they will act as a protective barrier.