Pyrolusite

Pyrolusite is a mineral composed mainly of manganese dioxide (MnO₂) and serves as a key source of manganese ore. It appears black and often amorphous, with textures that can be granular, fibrous, or columnar, and it sometimes forms kidney-shaped crusts. The mineral has a metallic shine, leaves a black or bluish-black mark on a streak plate, and easily smudges the fingers. Its density is roughly 4.8. The name comes from Greek words for "fire" and "to wash," referring to its historical use in removing color from glass.

Pyrolusite and romanechite are among the most widespread manganese minerals. It forms under oxidizing conditions in hydrothermal deposits alongside minerals such as manganite, hollandite, hausmannite, braunite, cryptomelane, chalcophanite, goethite, and hematite. It also appears in bogs and often results from the alteration of manganite. Additionally, it can be found with rhodonite, which is typically pink.

The metal manganese is obtained by reducing the oxide with sodium, magnesium, aluminum, or through electrolysis. Pyrolusite is widely used to produce spiegeleisen, ferromanganese, and alloys like manganese-bronze. As an oxidizing agent, it helps prepare chlorine—indeed, chlorine gas was first described by Karl Scheele in 1774 from the reaction of pyrolusite with hydrochloric acid. Natural pyrolusite has been used in batteries, though high-quality batteries require synthetic versions. It is also employed to make disinfectants (permanganates) and to decolorize glass. When mixed with molten glass, it oxidizes ferrous iron to ferric iron, removing green and brown tints—a classic use for glassmakers. As a coloring agent, it appears in calico printing and dyeing, imparts violet, amber, and black shades to glass, pottery, and bricks, and is used in green and violet paints.

Black manganese oxides that form dendritic (branch-like) crystals on rock or fracture surfaces are often mistaken for pyrolusite, but careful analysis of many such dendrites shows that none are actually pyrolusite; they are other manganese oxide minerals.

Some of the earliest famous cave paintings in Europe were made using manganese dioxide. Blocks of pyrolusite are frequently found at Neanderthal sites. It may have been kept as a pigment for cave art, but it has also been suggested that it was ground up and mixed with tinder fungus for starting fires. Manganese dioxide, in the form of umber, was one of the earliest natural substances used by human ancestors, serving as a pigment at least since the Middle Paleolithic. Neanderthals may have also used it for fire-making.

The ancient Greeks used the term μάγνης or Μάγνης λίθος ("Magnes lithos"), meaning stone from the region of Magnesia (either in Thessaly or Asia Minor). Two minerals were called μάγνης: lodestone and pyrolusite (manganese dioxide). (This lodestone is not to be confused with the current mineral called lodestone, which is naturally magnetized magnetite, FeFe₂O₄, and contains no manganese.) The term μαγνησία was used for manganese dioxide. In the sixteenth century, it was called "manganesum." It was also known as Alabandicus (from the Alabanda region of Asia Minor) and Braunstein. Eventually, the element manganese took its name from "manganesum," while "magnesia" came to refer to the oxide of a different element, magnesium.

Other manganese oxides include birnessite and psilomelane.