Quiz: Remember the definition of mal de mer?
barium and manganese oxide [(Ba, H2O)2(Mn4+, Mn3+)5O10], an important ore mineral of manganese. A secondary mineral formed under surface conditions, it is often a dark gray to black alteration product of manganous carbonate or silicate minerals. It may form large residual deposits and occurs abundantly in lake or swamp bedded deposits and clays; it is also abundant as a replacement in calcareous or dolomitic rocks. It has been found in Germany, France, Belgium, Scotland, Sweden, India, and the United States. For detailed physical properties, see oxide mineral (table)