The image left is a closeup of a lovely pyrite balls on calcite from Broken Hill, New South Wales. However, there is something sinister happening. Have a close look at the circled area in the second image. The grey area is also pyrite, but it has what is sometimes known as 'pyrite disease'. That is, the pyrite is breaking down and causing the whole specimen to slowly disintegrate. If left with other sulphide minerals, they too will likely break down.
Barry Flannery describes the process in a Mindat forum thus: "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."