Formula: (K,Ca,Na)(Fe, Mg,Al)8(Si,Al)12(O,OH)36.nH2O Triclinic
Description: Stilpnomelane is a brown micaceous to velvety to fibrous mineral found in various metamorphic rocks, especially those rich in iron. It often occurs with chlorite, epidote, magnetite, hematite and goethite and is easily mistaken for biotite mica.
ASHLAND COUNTY: Stilpnomelane is common in the Ironwood Iron formation mined through the Gogebic Range. It is here found with quartz, siderite, hematite, magnetite and chamosite (Huber, 1959, USGS, 1976; Schmidt, 1980).
BAYFIELD COUNTY: Ferrostilpnomelane is reported from a felsite porphyry exposed south of Grandview in the SE SW sec. 2 T.44N. R.6W. (LaPoint, 1976).
FLORENCE COUNTY: Dark green to brown lamina of ferrostilpnomelane are common near the base of a complex quartzite-schist unit within the Michigamme slate near Commonwealth in sec. 31 and 32 T.40N. R.18E. Garnet, graphite, quartz and chlorite are associated with the ferrostilpnomelane (Dutton, 1971).
— Stilpnomelane with quartz, chlorite and hematite occurs in metagreywackes northwest of Florence in T.40N. R.17E. (Dutton, 1971).
— Stilpnomelane is a component of the iron formation found in and near the Dunkel test shaft and adit in sec. 25 T.40N. R.17E. It is associated with quartz and magnetite. (Dutton, 1971).
— Stilpnomelane occurs as radiating crystals with magnetite, hematite, goethite and cherty quartz in shafts, trenches and prospect pits in sec. 23, 24 and 25 T.40N. R.17E. (Dutton, 1971).
IRON COUNTY: Stilpnomelane is common in the Ironwood Iron formation mined through the Gogebic Range. It is here found with quartz, siderite, minnesotaite, hematite, magnetite and chamosite.(Huber, 1959, USGS, 1976; Laybourne, 1979, Schmidt, 1980). Laybourne describes the stilpnomelane as occurring in “lath-like plates usually arranged in radial, sheath-like aggregates”. Laybourne cites stilpnomelane-bearing rock as exposed in outcrops on the east side of the Potato River (SE SE sec. 19 T.45N. R.1E.) and along Tyler’s Fork (SE NE sec. 33 T.45N. R.1W.). LaBerge (1984) also reports it from the Montreal Mine (NE NE sec. 33 T.46N. R.2E.).
MARATHON COUNTY: Stilpnomelane is found as brown crystals in crude clusters as much as 3 mm. across in cavities in pegmatite of the Wausau pluton, as exposed in the rotten granite quarries in sec. 19 and 20 T.28N. R.7E. (Falster, 1987).
MARINETTE COUNTY: Stilpnomelane is found locally in the iron formation associated with the Duval massive sulfide deposit in sec. 2 and 3 T.35N. R.18E. and sec. 24 and 28 T.36N. R.18E. It is associated with magnetite, pyrrhotite, grunerite and other amphiboles (Hollister and Cummings, 1982).
— Found in amygdules in the Quinnesec Formation metabasalts in the quarry at Kremlin. Associated minerals are epidote, quartz, chlorite and actinolite (Depangher, 1982).
ONEIDA COUNTY: Stilpnomelane is a metamorphic and wall-rock alteration mineral in the Pelican River massive sulfide deposit (sec. 29 T.36N. R.9E.). It is associated with chloritoid, biotite and actinolite (Bowden, 1978).