Mineral Pseudomorphs: When a Crystal Keeps Another Mineral’s Shape
A crystal's shape is a useful clue, but it does not always reveal the material now inside it. Sometimes one mineral changes or is replaced while its old outer form survives. The result is called a pseudomorph.
An old shape with a new identity
The word means false form. A pseudomorph can preserve the appearance of an earlier mineral even though its composition or internal structure has changed.
Replacement can happen gradually. Other processes involve material coating an older crystal or filling a space left behind. The details matter when describing a particular specimen.
A museum example
The Australian Museum holds flower-like specimens of chrysocolla replacing azurite from the Dorothy Mine in Queensland. Their striking forms record a mineral change, rather than simply the normal growth shape of chrysocolla.
What does “after” mean on a label?
A label such as “chrysocolla after azurite” names the present material first and the earlier mineral second. It describes a relationship between the two.
This is a good reason to avoid identifying a specimen by shape alone. Colour, physical properties, the locality, and sometimes laboratory analysis are needed as well.
For a collector, a pseudomorph is more than an unusual shape. It is evidence that a mineral specimen can keep part of its history even after the original material has changed.
Sources: New Mexico Bureau of Geology: Introduction to pseudomorphs; Australian Museum: Chrysocolla replacing azurite.
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