The deceptive identity of the smooth, black manganese ore

Nature · Published 2 October 2026 Swipe more like this →

In short: Nature often hides complex mixtures behind a single name. Psilomelane appears as smooth, grape-bunch-shaped masses, but modern mineralogy reveals it is less of a single mineral and more of a sophisticated chemical cocktail. Discover why this

While the name psilomelane dates back to 1758, its chemical identity is surprisingly fluid. Derived from the Greek words for "smooth" and "black," the term describes a group of hard manganese oxides. It is primarily composed of hydrous manganese oxide, featuring variable amounts of barium and potassium. Its generalized formula is often represented as Ba(Mn2+)(Mn4+)8O16(OH)4, though it is sometimes categorized as a hydrous manganese manganate of uncertain composition.

Geologically, psilomelane is a significant manganese ore, often found in the same environments and used for the same commercial purposes as pyrolusite. It occurs in diverse locations globally, including Vermont, Virginia, Arkansas, and Nova Scotia. It frequently forms in botryoidal or stalactitic masses with a submetallic lustre. However, modern research suggests that what was once called psilomelane is actually a mixture of several manganese oxides, with romanechite serving as a primary constituent.

Distinguishing psilomelane from other hydrous manganese oxides like manganite or wad relies on its physical properties. It possesses a greater hardness of 5 to 6 and a specific gravity ranging from 3.7 to 4.7. It is also notable for its reaction to hydrochloric acid, which triggers the release of chlorine gas. Despite its dark appearance, it is frequently confused with black hematite, though the two are unrelated; hematite is an iron oxide, whereas psilomelane is manganese-based.

The mineral's presence in lake or swamp bedded deposits and clays highlights its varied geological origins. It has been documented in regions as far-reaching as Germany, France, Belgium, Scotland, Sweden, India, and the United States. Its complex nature—containing admixed impurities like iron hydroxides—makes it a fascinating study in how minerals can transition from simple oxides to complex, multi-component deposits.

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