Goethite sprays, Montreal Mine, Iron County Wisconsin (Photo by W. Cordua.)

Formula: FeO(OH) Orthorhombic


Stalactitic goethite from Montreal Mine in Iron County, Wisconsin. (Photo by W. Cordua.)

Goethite is a common iron mineral. It often forms by weathering of other iron-rich minerals, thus is a common component of soils. It may form excellent pseudomorphs after the original minerals particularly pyrite or marcasite. Goethite may also be precipitated by groundwater or in other sedimentary conditions, or form as a primary mineral in hydrothermal deposits. When present in sufficient quantities, it constitutes an important iron ore mineral. Only a few of the most important goethite localities in the state will be mentioned here.

ASHLAND COUNTY: Fine-grained goethite is abundant throughout the Gogebic iron range as a weathering product of carbonate-bearing iron formations and ferruginous schists. In association with hematite, it replaces magnetite and iron carbonates such as siderite and ferrodolomite. It also forms as concentric and oolitic masses (Van Hise and Irving, 1892). An example in Ashland County is the old Berkshire Mine near Mellen in the SW SW SE Sec. 9 T.44N R.2W (Mudrey, 1979; Laybourn, 1979).

Pierce County “crosses” – Goethite replacing marcasite clusters, quarry near Maiden Rock , Pierce County Wisconsin. (Photo by Dan Behnke.)

BUFFALO COUNTY: Earthy to botryoidal goethite found in crevices and veins cutting Prairie du Chien group dolostones in a quarry south of Tarrant in the SW Sec. 27 T.24N R.12W (WSC field notes).
— Goethite occurs in a small quarry in Prairie du Chien Group dolostones in the NW NW Sec. 24 T.22N R.12W near the intersection of County N and County T. The goethite forms small pseudomorphs after pyrite crystals and as thin vuggy masses with coarsely fibrous to botryoidal coatings (WSC field notes).
— Goethite forms pseudomorphs after pyrite and marcasite on dolomite with calcite in quarry on County I 4 miles NE of Nelson. NW Sec. 35 T.22N R.13 W (WSC. Field Notes,1992).

CRAWFORD COUNTY: Goethite forms the matrix of a conglomerate of the Window Formation, which caps a hill in the NW Sec. 10 T.9N R.5W. It also fills crevices in the underlying carbonate bedrock. A shaft shows that this material was locally mined (Thwaites and Twenhofel, 1921).
— Goethite forms pseudomorphs after octahedral crystals of pyrite and large clusters of marcasite crystals on chert and chert breccia in a quarry near Steuben (Phil Burgess, 1989, personal communication).

DANE COUNTY: Goethite forms pseudomorphs after marcasite, with individual crystals up to 5 cm. long from a locality in Waunakee (Specimen in Geology Museum, U.W.-Madison).

DODGE COUNTY: Goethite is a major component of the oolitic iron ore at Iron Ridge, near Hubbard (Lawson, 1906). in Sec. 12 and 13 T.11N R.16W. It also occurs with hematite in mines E. of Neda NW Sec. 1 T.11N R.16E (Paull, 1977).

GRANT COUNTY: Botryoidal to concretionary goethite, possibly in part pseudomorphous after marcasite, is found in small pits in the SW NE Sec. 28 T.8N R.2W, along County M approximately 3 miles east of Boscobel. This material was apparently mined briefly for paint pigment (UW-Platteville, 1988).

GREEN LAKE COUNTY: Goethite pseudomorphs after marcasite occur in vugs in dolostone and silcrete in the St. Marie Quarry, Sec. 13 T.16N R.12E, near Princeton (UW Oshkosh, 1994).

IRON COUNTY: Goethite is a widespread mineral throughout the Gogebic Iron Range. It forms with hematite as a secondary alteration product of iron minerals in the iron formation and associated ferruginous schists. It often replaces primary minerals such as magnetite, ferrodolomite and siderite. It also forms concretionary and oolitic masses. Some secondary goethite may form attractive iridescent, botryoidal or drusy crusts.
— Goethite is found at the Montreal Mine NE NE Sec. 33 T.46N R.2E where it forms large botryoidal masses and satiny drusy crystals masses nicknamed “golden glint” (Dickey, 1938; LaBerge, 1984).
— Large botryoidal masses of goethite were found at the Cary Mine, NW SE Sec. 26 T.46N R.2E on the western edge of the Hurley (Specimens in the Seaman Museum collections, Michigan Tech).
— Goethite also is noted as occurring at the Tyler’s Fork Mine (E 1/2 Sec. 33 T.45N R.1W) and the Iron Belt Mine (NE Sec. 11 T.45N R.1E) (Van Hise and Irving, 1892).

JUNEAU COUNTY: Goethite occurs as a component of “bog iron ore” found under peat in SE Sec. 24 T.18N R.3E and elsewhere. The goethite is found as a nodular “shot ore” overlying a dark brown hard material with a “fibrous to silky luster”(Irving, 1877).

MARATHON COUNTY: Goethite is abundant in vugs in the pegmatites of the Wausau pluton. Falster (1987) and Falster et al. (2000) describes their habits as elongated crystals and sprays of a black to golden brown color, often as replacements of siderite. Also tiny xl. groups in granite at old thorium Mine near Stettin (Buchholz, 1994, personal communication).

MONROE COUNTY: Goethite is found as a cement in the conglomerates of the Windrow formation found in several areas in the county. It may also occur as powdery to botryoidal masses (Thwaites and Twenhofel, 1921). Some localities are on Windrow bluff near Tomah (NE NW Sec. 10 T.17N R.2W), and northwest of Norwalk (NW SW Sec. 18 T.16N R.2W and SW SW Sec. 17 T.16N R.2W).

PEPIN COUNTY: Goethite as pseudomorphs after euhedral pyrite and marcasite crystals occur in the Oneota Formation in a quarry on Highway J, NE of Stockholm (NW NW Sec. 7 T.23N R.15W). The pseudomorphs are associated with calcite, hematite and dolomite. The marcasite was originally in masses up to 12 cm. in diameter. The pseudomorphs are a lustrous brown, locally iridescent and preserve delicate primary crystals forms, such as twin lamellae (WSC field notes).

PIERCE COUNTY: Goethite forms pseudomorphs after both marcasite and pyrite in cavities in the Oneota dolostone in the quarry east on Maiden Rock on the north side of County S (NW Sec. 14 T.24N R.16W) (Cordua, 1989a).
Pseudomorphs of goethite after pyrite crystals in individuals over 2 cm. across and clusters over 10 cm. across occur in a quarry in the Oneota formation on Cady Creek, (SE SW SE Sec. 3 T.27N R.15W) SE of Spring Valley (WSC field notes). ,BR> Massive yellow-brown porous goethite with local botryoidal crusts occurs at the Cady Creek iron mine (SW NE SW Sec. 14 T.27N R.15W) S.E. of Spring Valley (Rosenberg, 1991, Allen, 1909).
Massive, yellow-brown goethite, with local botryoidal crusts occurs in the Gilman iron mine (NE SW SW Sec. 1 T.27N R.16W) west of Spring Valley (Rosenberg, 1991, Allen, 1909).
— Goethite occurs as cement in a peculair conglomerate exposed in a small pit in the NE sec. 35 T.28N. R.15W. Locally in vugs are small bortryoidal to reniformgoethite masses. (WSC field notes).

RICHLAND COUNTY: Large pseudomorphs of goethite after pyrite and marcasite single crystals and clusters were found on the surface and in clayey regolith on hilltops north of the Wisconsin River (North, 1913). The largest crystals were 14.5 cm x 7.5 cm. tall.

RUSK COUNTY: Goethite is a common component of the 5 meter thick gossan zone overlying the massive sulfide zone at the Flambeau deposit near Ladysmith (May, 1977). Some areas are cavernous with vugs up to 10 cm. wide with stalactitic to botryoidal goethite.

SAUK COUNTY: Many tons of goethite pseudomorphs after pyrite was mined from a mineralized fault zone in Cambrian sediments at the Ironton iron mine in the SW Sec. 10 T.12N R.3E, about 2 km. south of Ironton (Heyl and West, 1982).

WAUPACA COUNTY: Large masses of goethite after pyrite occurs in a pegmatite exposed in a highway expansion on County G near Weller Lake, east of Big Falls (Tom Buchholz, 2002,).

WOOD COUNTY: Goethite occurs as bog iron ore at several points along the Wisconsin River. Some localities reported in Irving (1877) include Point Bass, on the west bank of the Wisconsin River in Sec. 10 T.21N R.5E; and on the east bank at Sec. 4 T.22N R.6E and at Grand Rapids where a 50 cm. thick bed of porous goethite was found.
— Tiny sprays of golden brown goethite occur in calcite veins cutting serpentine exposed on Tower Hill, near Rudolph (Buchholz, 1994, personal communication).