This post concerns chemical weathering that can occur to the mineral pyrite [= iron sulfide FeS2 = “fool’s gold].” The weathering transforms the pyrite molecules into another mineral, but the external shape of the pyrite crystal remains. This phenomenon, which is called a “pseudomorph after pyrite,” is not restricted to pyrite, as it can occur in other minerals, as well.
A pseudomorph is when one mineral is replaced by another mineral without any change in the external form. In the examples shown here, the original pyrite has largely not been replaced, but the surfaces of the original pyrite crystals have been altered by weathering. Thus, these altered specimens are only partial pseudomorphs. This kind of distinction is not commonly discussed in the literature.
Unaltered pyrite crystals are cubic, shiny, heavy (dense), and leave a black-color streak when scratched on a streak plate (see one of my previous posts). Also, the crystal surfaces of pyrites can be striated.
The last image, which is of two co-joined crystal faces of pseudomorphs, is the most weathered. It has some areas of a soft-chalky coating of white, red, or brown material; its striations are still present but are not pristine. The composition of the coating material is a variable mixture of two minerals: goethite [FeO(OH)], a hydrated iron oxide-hydrous oxide and limonite [FeO(OH)•nH2O], a hydrous iron oxide. Goethite typically is the dominant mineral.
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