A manganese-containing mineral that occurs as grey to black, platy, or finely crystalline masses, and is dimorphous with pyrolusite.
Ramsdellite is an oxide of manganese, MnO₂. Crystal system: orthorhombic.
1940s. From the name of Lewis S. Ramsdell, U.S. mineralogist, who described the mineral in 1932 (Amer. Mineral 17 143–9) + -ite.