This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
Occurs in brilliant needle-shaped crystals, or in compact masses, but is usually met with in the shops as a dull, earthy, dark-brown or black powder, which contains iron and other impurities. Treated with hydrochloric acid it causes evolution of chlorine, thusMnO2 + 4HCl = MnCl2 + 2H1O + Cl2
When heated to redness, it gives off oxygen and leaves a residue of red oxide of manganese 3MnO2(heated) = Mn3O4 + O2 Sulphuretted hydrogen causes in manganic solutions a flesh-colored precipitate of sulphide.