Wisconsin Geological & Natural History Survey » Pyrolusite
Formula: MnO 2 Tetragonal Description: Pyrolusite forms as a weathering product of other manganese minerals, as concretions or dendrites deposited by ground water or as nodules precipitated from lake water. Pyrolusite is often soft and soils the fingers, but when well crystallized it is harder than glass. Pyrolusite and other manganese minerals are a component of the "manganese dendrites ...