Formula: MnO2 Tetragonal
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” that are ubiquitous along joint surface in much of the bedrock of the state. Pyrolusite is associated and intergrown with iron oxides such as hematite and goethite and other manganese oxides such as romanechite. It may be part of the mixture of manganese minerals generally referred to as psilomelane.
IOWA COUNTY: Pyrolusite and other manganese minerals are wide-spread as “black ocher” along joints in the oxidized zones of the many Upper Mississippi Valley zinc-lead deposits in the county. It is particularly notable as a component of earthy “wad” in the old lead pits in sec. 11 T.4N. R.1E. near Rewey (Heyl, et al., 1959).
IRON COUNTY: Pyrolusite was common at the Montreal Mine (NE NE sec. 33 T.46N. R.2E.) and nearby Cary Mine (NW SE sec. 26 T.46N. R.2E.) as velvety to powdery black material associated with other manganese minerals such as rhodochrosite and manganite. It has formed in these mine workings as a modern precipitate from saline mine waters coating not only the walls and floors of the mines, but also broken rocks, nails and steel railings. (Dickey, 1938).
LAFAYETTE COUNTY: Pyrolusite and other manganese minerals are wide-spread as “black ocher” along joints in the oxidized zones of the many Upper Mississippi Valley zinc-lead deposits in the county. (Heyl, et al., 1959). Agnew (1963) notes it as common in the Platteville district, especially in the SE portion.
— Pyrolusite forms masses 15- 20 cm. across mixed with psilomelane and a number of primary and secondary copper and iron minerals at the copper prospect 4 miles NW of Gratiot (SE NW sec. 36 T.2N. R.3E.). (Heyl et al., 1959).