"Pyrite Iron Cross" – Pseudomorphosis of goethite after pyrite – Colombia

These crystals, originally composed of pyrite (iron sulfide), have been chemically altered by hydrothermal activity. Pyrite was replaced by goethite, a hydrated iron oxide. Nevertheless, goethite pseudomorphic crystals retain the shape and form of the original pyrite crystals.

Pseudomorphogenesis is a phenomenon in which one mineral replaces another but retains its original crystal shape. This process is often the result of chemical reactions in which the original mineral reacts with the surrounding environment and is gradually replaced by a new mineral. Pseudomorphosis can occur as a result of various geological processes, including hydrothermal activity, which is a common mechanism for this type of transformation.

Goethite is a common mineral that often occurs as needle-like crystals and is typically brown to yellow in colour. It is a hydrated iron oxide that forms in the presence of water and oxygen, often as a product of weathering of iron minerals.

Pyrite, also known as cat gold, is a common sulphide mineral with a metallic lustre and characteristic gold colour.
Pyrite crystallizes in a cubic system and often forms cubic or pyritohedral crystals.

Pseudomorphosis of goethite after pyrite

In the pseudomorphosis of goethite after pyrite, the following process occurs:

1. Initiation of transformation: pyrite crystals are exposed to hydrothermal or other geochemical conditions that favor iron oxidation and the presence of water.
2. Oxidation of pyrite: Pyrite begins to oxidize and sulfur is released while iron remains and goes into an oxidation state, leading to the formation of goethite.
3. Goethite formation: Hydration and oxidation of iron leads to the formation of goethite, which gradually replaces the original pyrite crystal.
4. Shape retention: Even when the original pyrite is chemically replaced by goethite, the crystal structure and shape of the original pyrite crystals are retained.
   The result is a goethite pseudomorph that has the shape of the original pyrite.

Significance and uses

Pseudomorphs of goethite after pyrite are of interest not only to mineralogists and geologists, but also to mineral collectors because of their aesthetic and scientific significance. These minerals can provide valuable information about the geological processes and conditions that led to their formation. This process is a fascinating example of how natural processes can change one mineral into another while preserving the external form of the original mineral.