Formula: (Fe,Mg)3Si4O10(OH)2 Triclinic
Description: Minnesotaite is found frequently as small grains in iron formations that have experienced low grade metamorphism. It is often associated with stilpnomelane, cummingtonite, chlorite and siderite. Wisconsin localities for this minerals are restricted to the Proterozoic Ironwood iron formation in Ashland and Iron counties. Since the mineral is relatively indistinct, it may well be present but unreported in other localities around the state.
ASHLAND COUNTY: Minnesotaite associated with quartz, iron oxides and siderite is found in the Ironwood Iron Formation throughout the county (USGS, 1976).
— Laybourn (1979) describes fibrous sheaths of minnesotaite 0.1 mm across in the Ironwood Iron Formation where it is in contact with the Mellen complex on the east bank of Ballou Creek, NW NE Sec. 11, T.44N R.2W. Associated minerals are stilpnomelane, chlorite, grunerite and cummingtonite.
IRON COUNTY: Minnesotaite associated with quartz, iron oxides, and siderite is found in the Ironwood Iron Formation throughout the county (USGS, 1976).
— LaBerge (1984) reports minnesotaite as occurring at the Montreal Mine (NE NE Sec. 33 T.46N R.2E).
— Schmidt (1980) reports minnesotaite from outcrops near Pence.
— Fibrous sheaths 0.1 mm long interwoven with quartz, iron oxides, siderite, stilpnomelane and chlorite occur where the Ironwood Iron Formation is in contact with the Mellen Complex. Some localities include the east side of the Potato River (SE SE Sec. 19 T.45N R.1E) and along Tyler’s Fork, SE NE Sec. 333 T.45N R.1W (Laybourn, 1979).