Pirolusita

Pyrolusite is manganese dioxide (MnO₂), an oxide and one of the principal ores of this element.

It forms and crystallises in oxidising environments, particularly in the oxidation zones of manganese deposits. It commonly occurs as fibrous aggregates with acicular growth. It also develops short, lustrous prismatic crystals in dense aggregates, typical of secondary formations in altered volcanic zones. That habit matches the type of pyrolusite that formed at La Unión.

At Imini, many pieces sold as 'pyrolusite' are actually manganite (MnO(OH)) partially replaced by pyrolusite. The transformation takes place as manganite oxidises: it loses the hydrogen of the hydroxyl group and progresses to pyrolusite, retaining part of the prior shape. The chemical change can preserve the original fibrous or acicular habit. It often produces a contrast between bright metallic areas and a black, silky, microcrystalline interior.

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