Limonite after pyrite is a pseudomorph, meaning one mineral replaces another while preserving the original mineral’s external crystal shape. In this case, the original mineral Iron Pyrite was replaced with Limonite (a mixture of iron oxyhydroxides, mostly goethite + jarosite + amorphous FeO(OH) phases). The result is a specimen that looks like a pyrite crystal, but its internal chemistry and structure have transformed into iron oxides/hydroxides.

It’s literally “rusted pyrite” — but in a scientifically fascinating way.

People sometimes call it “Rust pyrite”, “Burnt pyrite”, “Oxidized pyrite”, and single cubes are refered to as "Devils Dice.

During oxidation, pyrite reacts with oxygen + water, sulfur becomes sulfuric acid, Iron oxidizes and hydrates, limonite + goethite replace the pyrite. So these pseudomorphs are physical evidence of a major environmental/geochemical process.