PYROLUSITE, a mineral consisting essentially of manganese dioxide 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 pseudo morphs after manganite ; in fact the mineral often results by the dehydration and oxidation of manganite and for this reason it frequently contains a little water. True crystals of man ganese dioxide are referred to the rare species polianite : they are tetragonal and isomorphous with cassiterite. Pyrolusite is an alter ation product of other manganese minerals—manganite, rhodo chrosite, rhodonite, etc. 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 depos ited 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, Bavaria. It is deposited by some springs and man ganiferous nodules are dredged from the floor of the deep sea.
As an ore pyrolusite is extensively mined at Ilmenau and other places in Thuringia, in North Wales, at several places in the United States (Vermont, Virginia, Arkansas, etc.), Nova Scotia, Brazil, India, etc. Together with the rather less important ore, psilome lane, it 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 (perman ganates) ; and 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. riv (fire) and Xbfai (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.