psilomelane, Barium and manganese hydrous oxide, BaMnMn8O16(OH)4, an important ore mineral of manganese. What was formerly called psilomelane is now known to be a mixture of several manganese oxides of which romanechite is a major constituent. Such manganese oxide mixtures may form large deposits and may occur in lake or swamp bedded deposits and clays; it has been found in Germany, France, Belgium, Scotland, Sweden, India, and the U.S. The name psilomelane (from the Greek words for “smooth” and “black”) refers to its typical black, smooth-surfaced, grape-bunch-shaped or stalactitic masses.