Birnessite (delta MnO2-3H2O) in a large spherulite in obsidian near Silver Cliff, Colorado
Birnessite and cryptomelane were identified in the montmorillonitic border zone of a large spherulite in obsidian north of Silver Cliff, Colo. The manganese minerals contain no silver and probably formed more recently than argentian cryptomelane which is abundant in the manganiferous silver ores of the surrounding volcanic rocks.
Colorado↗