What is a pseudomorph in mineralogy ?

Pseudomorph : definition

Pseudomorph is the replacement of a mineral by one or more minerals, with conservation of the original external shape of the replaced mineral.

The famous cuprite octahedra from Chessy (Rhône, France), curiously green, are o malachite pseudomorph after cuprite. Similarly, goethite cubes are pseudomorphs after goethite during surface oxidation : goethite then adopts a cubic habitus, in apparent contradiction with its orthorhombic crystal system.

Please note, this term should not be confused with epigenesis : the replacement of a fossil by a mineral (helix in lussatite, pyritized ammonites, etc...).
Malachite pseudomorph after cuprite from Chessy, Rhône, France
Plumbogummite pseudomorph after pyromorphite from China
Copper pseudomorph after aragonite from Coro Coro Mine, Bolivia
Acanthite pseudomorph after argentite from Imiter, Morocco