POPULAR PSEUDOMORPH MINERALS

More Than They Seem

- BOB JONES

The term pseudomorph, abbreviated ps or pseudo, means “false form.” This two-part article will describe a wide variety of minerals and fossils that have changed from their original form to something else and how that happens. Every good collection should have examples of common pseudomorphs.

The keyword, when discussing “pseudos,” is change. Minerals crystallize and exhibit a particular shape or crystal form. Such things as solution Ph, temperature, pressure, the richness of the solution, even location all play a role in crystallization. Some minerals, once formed, are unstable, so changes also occur as conditions change.

One typical example of a pseudomorph is iron rusting to iron oxide. The original shape remains but not the properties. Iron sulfide pyrite can also change, altering to hydrous iron oxide goethite. When pyrite changes to goethite, it retains the original crystal form of a cube or octahedron but is no longer brassy pyrite but dull dark brown goethite. Goethite after pyrite is a common form of a pseudomorph process called replacement.

UNDERSTANDING REPLACEMENTS

Replacements are substances in which the original entity is slowly replaced molecule by-molecule with a new substance while preserving the original form. Other forms of pseudomorphs include casts, paramorphs, and epimorphs. Plus, pseudomorph change scan happen in fossils as well as minerals.