Birnessite

This sample of birnessite is displayed in the Smithsonian Museum of Natural History. Birnessite is an oxide of manganese and magnesium along with sodium, calcium, and potassium with the composition (Na,Ca,K)(Mg,Mn)Mn₆O₁₄.5H₂O . This sample is about 3x13 cm and is from Mont Saint-Hilaire, Quebec, Canada.