What is epigenesis in mineralogy ?

Epigenesis : definition

Epigenesis is the slow, small-scale replacement within a rock of a form (usually a fossil) by a mineral. The classic pyritization of ammonites in marly soils or the molding of ramondi helix shells from Dallet (Puy-de-Dôme, France) by lussatite (photo on the right) are examples of epigenies. In the literature, this term is often confused with "pseudomorph" : the replacement of one mineral by another with the preservation of the original external form of the replaced mineral.