1911 Encyclopædia Britannica/Pyrolusite

by Leonard James Spencer

PYROLUSITE, a mineral consisting essentially of manganese dioxide (MnO₂), of importance as an ore of manganese. It is a soft, black, amorphous mineral, often with a granular, fibrous or columnar structure, and sometimes forming reniform crusts. It has a metallic lustre, and a black or bluish-black streak, and readily soils the fingers. The specific gravity is about 4.8. Supposed crystals of pyrolusite have been proved to be pseudomorphs after manganite; in fact the mineral often results by the dehydration and oxidation of manganite (Mn₂O₃⋅H₂O), and for this reason it frequently contains a little water. True crystals of manganese dioxide are referred to the rare species polianite: they are tetragonal and isomorphous with cassiterite. Pyrolusite is an alteration product of other manganese minerals—manganite, rhodochrosite, rhodonite, &c. It occurs as irregular masses and nodules in the residual clayey materials resulting from the decomposition of various rocks, for example, limestone. That it is readily deposited from solution is shown by the frequent occurrence of black dendritic markings in the crevices of rocks, excellent examples of which are seen in mocha stone (q.v.) and in the lithographic stone of Solenhofen in Bavaria. It is deposited from the waters of some springs, and manganiferous nodules are dredged from the Hoor of the deep sea.

As an ore it is extensively mined at Ilmenau and several other places in Thuringia, at Vorderehrensdorf near Prossnitz in Moravia, Platten in Bohemia, in North Wales, at several places in the United States (Vermont, Virginia, Arkansas, &c.), Nova Scotia and Brazil. Pyrolusite, together with the rather less important ore, psilomelane, has various economic applications. It is extensively used for the manufacture of spiegeleisen and ferromanganese, and of various alloys, such as manganese-bronze. As an oxidizing agent it is used in the preparation of chlorine and disinfectants (permanganates), and for decolorizing glass: when mixed with molten glass it oxidizes the ferrous iron to ferric iron, and so discharges the green and brown tints, hence the name pyrolusite, from Gr. πῦρ (fire) and λύειν (to wash). As a colouring material, it is used in calico printing and dyeing; for imparting violet, amber and black colours to glass, pottery and bricks; and in the manufacture of green and violet paints. (L. J. S.)