Formula: (Na,Ca)0.5Mn2O4.1.5H2O Monoclinic
Description: Birnessite is a black massive mineral deposited in lake and deep sea nodules and as a groundwater precipitate. It is usually intergrown with other manganese and iron minerals.
Birnessite is found in Wisconsin as components of nodules and crusts in several of the lakes. Bowser et al. (1970) reported it as forming part of the ferromagnesian crusts in Tomahawk and Green Lake, among others. Rossman et al. (1972) found birnessite in many nodules collected from Lake Michigan, especially in the Green Bay region. These nodules were oval to discoid, 0.5 cm to 0.5 mm in diameter and found within the upper 10 cm. of the sedimentary column.