Goethite replacing Pyrite

specimen number:
9771510

location:
Hermosa, La Plata Co., Colorado, USA

description:
A loose floater cluster of cubic pyrite crystals, this in the process of being replaced by goethite.

The process of goethite replacing pyrite is a natural weathering phenomenon known as pseudomorphism. When pyrite crystals are exposed to oxygen and water near the Earth surface, they undergo oxidation and hydration. Over time, the original iron sulfide mineral dissolves and is chemically replaced by goethite, an iron-bearing oxide hydroxide. While the internal chemical composition changes completely, the new mineral preserves the exact external shape and crystal form of the original pyrite, resulting in a stable replica.

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