Wiktionary
birnessite
n. (context mineral English) A soft mineral, mostly containing manganese oxides, that occurs in manganese nodules.
Wikipedia
Birnessite
Birnessite (NaCaK)(Mn,Mn)O ยท 1.5 HO is an oxide mineral of manganese along with calcium, potassium and sodium. It has a dark brown to black color with a submetallic luster. It is also very soft, with a Mohs hardness of 1.5. Birnessite is formed by precipitation in lakes, oceans and groundwater and is a major component of desert varnish and deep sea manganese nodules.