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

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