Formula: Mixture of hydrate manganese-barium oxides
Description: Psilomelane is used as a blanket term to denote a variety of hard massive mixtures of manganese oxides. Most, but not all, psilomelane is the mineral romanechite (BaMn2+Mn4+8O16(OH)4 ). The term is so loosely used, and the substance referred to usually so superficially examined that all reported psilomelane and romanechite localities are grouped together below. Psilomelane forms as a weathering product of other manganese minerals, as concretions or dendrites deposited by ground water or as nodules precipitated from lake water. Such nodules have been found at various places in Green Bay, for example (Rossman et. al., 1972). Psilomelane as “manganese dendrites” are ubiquitous along joint surface in much of the bedrock of the state. Psilomelane is associated and intergrown with iron oxides such as hematite and goethite and other manganese oxides such as pyrolusite.
DUNN COUNTY: Psilomelane is found as bog manganese ore in the SE sec. 16 T.31N. R.14W. north of Connorsville. The ore ran 22-31% manganese and occurred in layers up to 15 cm. thick (WGNHS files, 1942).
IOWA COUNTY: Psilomelane 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: Romanechite 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 spectacular large botryoidal banded masses lining cavities in the iron formation. Beautiful specimens are found in museums, including the Seiman museum at Michigan Tech (Dickey, 1938; LaBerge, 1984).
LAFAYETTE COUNTY: Psilomelane 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.
— Psilomelane is associated with pyrolusite 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.)
— Psilomelane is “abundant” in drill core from NW SW sec. 21 T.2N. R.1E. near Meeker’s Grove (Heyl et al., 1959).
MARATHON COUNTY: Psilomelane commonly forms dendrites and joint coatings throughout the Wausau Pluton (Falster, 1987).