Pseudomorphism

Mineralogical change at the Earth’s surface and within the crust does not always destroy the outward shape of the original mineral. In pseudomorphism, one mineral is completely replaced by another - or by a mixture of minerals - while the external crystal form of the original is perfectly preserved. The result is a crystal whose outward geometry records what was once there, while the material filling that form is chemically and structurally entirely different from the original. ^[1] A pseudomorph is therefore simultaneously a record of the original mineral and a record of the conditions under which replacement occurred - it is geological history preserved in crystal form.

Mechanism and Products

The replacement process involves dissolution of the original mineral and precipitation of the new mineral or minerals in the same volume of space. Because the volume is constrained by the original crystal form, the replacement product inherits that form exactly. The secondary material may be a single mineral, or it may be a fine-grained intergrowth of several minerals that collectively fill the original crystal outline. ^[1]

The most familiar example is goethite [\hspace{0pt}FeO(OH)\hspace{0pt}] pseudomorphically replacing pyrite (FeS₂). Pyrite crystals are cubic and frequently form perfect cubes. When pyrite weathers - for instance, in an oxidising supergene environment or at the Earth’s surface - the iron sulfide is unstable and is replaced by the iron oxyhydroxide goethite. The replacement is chemical and structural, but goethite fills exactly the cubic outline that the pyrite once occupied. The result is a cube-shaped mass of yellowish-brown goethite: a pseudomorph after pyrite. The shape tells you what was there; the chemistry tells you what replaced it. ^[1]

Paramorphs

A special category of pseudomorphism occurs when the replacing mineral has the same chemical composition as the original but a different crystal structure - that is, when the replacing mineral is a polymorph of the original. These are called paramorphs. ^[1] Paramorphism is a reconstructive polymorphic transition that preserves external form. An aragonite crystal converted to calcite at low temperature retains its original crystal outline even though the internal structure has reorganised from the aragonite arrangement to the calcite arrangement. Because both minerals are CaCO₃, the composition is unchanged and no material needs to enter or leave - only the atomic arrangement changes, and the form survives the transition.

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