pseudomorphids

A pseudomorphid is a term used in mineralogy to describe a mineral that has been replaced by another mineral while retaining the original crystal shape. This process, known as pseudomorphism, occurs when the original mineral undergoes a chemical alteration, such as dissolution and subsequent precipitation of a new mineral. The resulting crystal is then a pseudomorph of the original mineral, meaning it is a "false form."

Pseudomorphism can happen through various mechanisms. One common method is inversion, where a mineral transforms into another mineral with the same chemical formula but a different crystal structure due to changes in temperature or pressure. Another is paramorphism, where a mineral changes to another with a different chemical formula and crystal structure. Surface replacement or overgrowth can also lead to pseudomorphs.

The original crystal form is preserved because the replacement process often occurs in situ, without destroying the external morphology. This makes pseudomorphs valuable for identifying minerals that may have been altered or are no longer stable under current geological conditions. For example, quartz pseudomorphs after calcite are common, where the external rhombohedral shape of calcite is retained by quartz. Other examples include limonite after pyrite and goethite after marcasite. The study of pseudomorphs provides insights into the geological history and chemical environments of mineral formation and alteration.