What is a Pseudomorph?

A pseudomorph is created when a specific mineral replaces another mineral, leaving the shape or form of the original mineral in place, but creating a new stone entirely. In description, these specimens are generally noted as a replacement mineral after the original mineral. This word literally means “false form”, and many of the stones we encounter fall into this class.  There are five main types of pseudomorphs:

Alteration Pseudomorph:

The process of alteration is when the pseudomorph specimens which undergo alteration have experienced only partial replacement or substitution. This will usually occur in a situation where the original mineral undergoes a chemical reaction to another mineral of similar composition, but retains the original crystalline form. This typically occurs on exposed surfaces, which will result in a specimen containing a core of the unaltered mineral, and something different on the exposed exterior. An example of this is azurite, malachite.

Substitution Pseudomorph:

Also known as “infiltration pseudomorphs”, an alteration pseudomorph takes place when one mineral is replaced by another mineral. The shape of the original formation is preserved, but the original mineral goes through complete chemical substitution by another mineral, resulting in a specimen with the shape of the original, but the color, hardness, density, and properties of the second mineral. An example of this is petrified wood. 

Parimorph 

This type of specimens are also known as “incrustation pseudomorphs”. These are formed when one mineral coats another, and creates a crust of material, encasing the original. This new crust will remain intact, while the original material will dissolve. The end result is a mold like formation of the new material, displaying surfaces exhibiting the shape and characteristics of both materials. This process most commonly occurs through exposure to naturally occurring acidic solutions, although it can be done intentionally as well. 

Allomorphs

Sometimes called paramorphs, this type is a type of pseudomorph specimen that has experienced change only on the molecular level. Allomorphs will maintain their original, unaltered form and composition chemically, but carry a different molecular structure. This usually occurs when a mineral is replaced by another mineral with the same chemical structure (dimorphs), but a different structure molecularly. An example of this would be aragonite, and calcite, both of which are forms of calcium carbonate. Both have the same chemical formula CaCO3, but calcite is trigonal and aragonite is orthorhombic. Switching these two minerals in an allomorph would create specimens with the same chemical structure, but they would differ on a molecular level. 

Epimorph  

This is a type of pseudomorph whose shape has been formed by its environment, usually as a coating over an original material. The original may or may not have dissolved, but if it has, the specimens could be referred to as a “cast”. An example of this process is a geode. 

There are many factors that can contribute to the formation of a pseudomorph, they can occur under many conditions, and some minerals are more prone to this process than others. All of them are as unique as a fingerprint, and as we continue to learn more about the planet we live one, more and more of them are sure to be discovered. Some of the more common examples of pseudomorphs would include copper or malachite over azurite, calcite after aragonite, limonite or geothite after pyrite, and beta-quartz after alpha-quartz.