MARCASITE FeS2 Orthorhombic

Description: Marcasite is common in Wisconsin, often found in well-formed crystals in cavities and veins in the Paleozoic carbonate rocks. It is one of the most abundant minerals in the zinc-lead mines in the southwest part of the state where it forms in low temperature hydrothermal veins associated with sphalerite, galena, pyrite, calcite and dolostones in the Upper Mississippi Valley zinc-lead district. It also occurs commonly in cavities in the Ordovician Prairie du Chien group and in the Silurian Niagara Formation of eastern Wisconsin. Here it occurs as a groundwater precipitate with pyrite, quartz, calcite and dolomite. The relationships of these fluids and those producing the zinc-lead deposits are unknown. Marcasite resembles its polymorph pyrite in color, hardness, and luster and may be found with pyrite. Its crystal habit, however, allows its distinction. Pyrite is isometric, showing some generally the cube, octahedron and pyritohedron forms or their combinations. Marcasite has orthorhombic symmetry forming elongated modified prismatic crystals with a rectangular outline. Marcasite is frequently twinned in distinctive cockscomb or radial clusters.

Marcasite crystals

Marcasite crystal cluster, Lutz Quarry, near Oshkosh, Winnebago County, WI. Main crystal is about 2 cm. long. Pete Rodewald specimen and photograph.

Marcasite is inherently unstable under atmospheric conditions. Often primary marcasite is incipiently to completely replaced by iron oxides, especially goethite, forming pseudomorphs. In other cases, the marcasite is only incipiently replaced, giving it a coating iridescent in green, blue, red and/or purple. Fresh marcasite may deteriorate rapidly to a mixture of iron sulphates and sulphuric acid as it resides in storage or specimen cases in periods of time measured in months or years. The breakdown is exothemic. In the Etna Mine, near Benton, for example, the heat in the rocks in the marcasite-rich zones was said to be high enough to cook eggs (Bain, 1907) Marcasite specimens should be carefully cleaned and coated in order to prevent their reaction with the air. The author has found that tarnished marcasite does not deteriorate rapidly, as the tarnish provides an attractive "weather-stabilized" coating.

Only some of the more typical or notable marcasite occurrences are given below.

COLUMBIA COUNTY: Marcasite associated with sphalerite is found in the Decorah Formation in a small quarry on County K near the center of Sec. 22 T.10N R.12E, near Columbus (Heyl and West. 1982).

CRAWFORD COUNTY: Large rosettes of twinned cockscomb crystals, now largely replaced by goethite, are found on chert in Oneota dolostone from a quarry near Steuben. Large goethite pseudomorphs after cubo-octahedral pyrite crystals are also found here. (Phil Burgess, 1989, personal communication).

DANE COUNTY: Large clusters of xls. with individuals up to 5 cm. long, now replaced by goethite, were found near Waunakee. (Specimens on display in the geology museum at UW Madison.)

DODGE COUNTY: Vertical veins of pyrite and marcasite up to 30 cm. thick cut Galena and Decorah formation sediments in a quarry in sec. 8 T.10N. R.14E. (Brown and Maas, 1992).

DOOR COUNTY: Marcasite occurs as small xls. with pyrite, fluorite, calcite and dolomite in a quarry in Sturgeon Bay. (Carl Cochrane, 1999, pers. com.).

FOREST COUNTY: Marcasite is a minor mineral intergrown with pyrite in the massive sulfide ore of the Crandon deposit, near Little Sand Lake (Lambe and Rowe, 1989).

FOND DU LAC COUNTY: Marcasite occurs as tiny crystals, sometimes iredescent, on joint surfaces at the Ripon Lime and Materials Company Quarry, NW sec. 20 T.16N R.14E., just west of Ripon.

GRANT COUNTY: Marcasite is found at virtually all of the zinc-lead mines, many of which are listed under sphalerite and galena. Heyl et. al. (1959) described several major habits for marcasite in the district. These include lustrous brassy yellow masses and crystals, often beautiful striated and twinned; grayish-green colloform reniform masses of radiating crystals; and conical to cylindrical stalactitic forms. The last two habits of marcasite deteriorate the most rapidly upon exposure to the air.

Hobbs (1895) did an extensive crystallographic study of euhedral marcasite crystals from the district. He delineated 5 separate types. Type 1 crystals are tabular, flattened on the c axis and dominated by 1st and 3rd order prisms. This type commonly forms brassy cockscomb aggregates with common twins on {101}. Type 2 marcasites are lath-shaped crystals, elongated parallel to the b crystal axis with many 1st order prisms developed, giving the crystals rounded edges. Type 3 marcasites are thick, tabular stellate twins on {101}. Type 4 marcasite are short thick prisms with diamond-shaped cross-sections dominated by third order prisms and deeply striated basal pinacoid forms. Finally, type 5 marcasites are square tabular brassy crystals with square cross-sections on the a crystal axis. They are dominated by the forms {hkl}, {hk0}, {0kl} and often form twinned trillings on {101}.

Marcasite in these deposits is usually associated with sphalerite, calcite, galena, dolomite and pyrite. Some of the more prominent reported occurrences for marcasite in Grant county are:
- in the Platteville district with crystals and masses found in mines in T.3N R.1W such as the Lyght, Hibernia and Trego. Particularly fine crystals reportedly occur in the Rasque ore body in the NW Sec. 36 T.3N R.1W (Agnew, 1963).
- Found abundantly at the Cardiff #1 mine in the N 1/2 Sec. 25 T.3N R.3W near Potosi (Heyl et al, 1959).
- Found at the Piquette #2 mine near Tennyson in the SW NE Sec. 36 T.3N R.3W (Heyl, 1964).
- Found in the south ore body at the Last Chance Mine and in the Lafollette Mine, both near Arthur in the NE NW Sec. 3 T.4N R.1W (Taylor, 1964).
- Common at the Beetown Mine at the east quarter corner of Sec. 29 T.4N R.4W (Heyl et. al., 1959).
- Common with pyrite at the John's iron mine in the S 1/2 SE Sec. 26 T.6N R.1W east of Preston (Heyl et. al., 1959).

IOWA COUNTY: Marcasite is present at most of the extensive zinc-lead mines in Iowa County. Habits described for Grant County are also typical of Iowa County. A few of the more prominent marcasite localities in Iowa County are:
- Raasch (1924) describes both botryoidal, reniform marcasite and slender stalactitic marcasite several centimeters long from the mines near Mineral Point. The stalactites have open tubes in the center, and some occasionally curl up the tip, forming helectites. Similar habits are found in Lafayette County, where they are described in more detail.
- Marcasite is abundant in the Coker #3 ore body in the SE Sec. 29 T.5N R.1E near Mifflin (Taylor, 1964).
- Goethite pseudomorphs after marcasite are common at the Ohlerking Mine, near Highland, in the NW SE Sec. 31 T.7N R.1E. Associated minerals are galena, goethite pseudomorphs after pyrite, drusy quartz and calcite (Heyl et al., 1959).

JEFFERSON COUNTY: Small crystals. with calcite, pyrite, dolomite and sphalerite occur in vugs in Galena Formation at the Hausz Brothers Quarry, Sec. 11 T.5N R.14E near the intersection of Hwy. 12 and county M south of Fort Atkinson (U.W. Whitewater, 1980).

LAFAYETTE COUNTY: Marcasite is abundant at all of the hundreds of zinc-lead mines and "diggings" in Lafayette County. All of the localities described under sphalerite and galena for this county are also localities where marcasite is found. The habits described for marcasite in Grant County also occur in Lafayette County. A number of authors (Peck, 1979; Heyl et. al., 1959; and George, 1926) point out unusual stalactitic to helectitic sphalerite in Lafayette County. Only a few of the many marcasite occurrences in the county are mentioned below.
- Klemic and West (1964) report marcasite crystals and reniform masses as abundant in the Belmont and Calamine Quadrangles, especially at the Dall Mine in Sec. 22 T.2N R.1E.
- Agnew (1963) reports dull gray-green reniform marcasite and brassy bladed marcasite as common in the Platteville district, especially in the Depp (or Fever River) Mine in the S 1/2 NE Sec. 18 T.2N R.1E. Heyl et. al.(1959) reports marcasite at the Vandeveter Mine nearby in the SE NE Sec. 26 T.2N R.1E.
- Many localities occur in T.1N R.1E in the region near Benton and New Diggings. Some specific localities given by Heyl et. al. (1959) include the Hird #1 in the SE SW SE Sec. 8; The Jug Handle Mine in the NW NW NE Sec. 14; the Jack of Diamonds Mine in the NW Se sec. 22 and the New Occidental Mine in NW NE NE sec. 34. Other localities in the township include the Teasdale Mine in Sec. 21 (Zimmermann, 1969) and the Thompson-Temperley Mine in Sec. 27 (Hostermannet. al., 1964). As of fall of 1988, the author collected attractive clusters from mine dumps in the SW SW Sec. 24 and purple tarnished crystals in calcite from dumps in the NE Sec. 25 in this township (WS Cordua field notes).
Marcasite is abundant at the Etna Mine NW NE sec. 11 near Benton where it forms large masses that rapidly altered on exposure to the air. Mine tunnels became coated with colorless to green efflorescences of iron sulfates. Since the alteration of marcasite to these sulfates was exothermic, temperatures in the marcasite rich zones were said to be high enough to cook an egg. (Bain, 1907).
- Cockscomb marcasite groups, associated with pyrite, can be found in near Yellowstone Lake State Park, east of Fayette (Virginia Frayer, 1988, oral communication).
- Stalactitic marcasite comes from several mines in T.1N R.1E near New Diggings. George (1926) carefully describe a number of specimens from the C.A.T. and Longhorn mines where the largest stalactites were "about the size and shape of an ordinary lead pencil" while others were "horn" or corkscrew-shaped helectites. The marcasite was found in vugs in the Galena Formation with calcite, galena and sphalerite. George reports that the stalactites made the vugs look like "the back of a porcupine". The stalactites extend in all directions in the vugs, but were mostly vertical and were aligned along joints in the vug's roofs and walls. Some stalactites were solid, but many had small tubes along their axes. George appealed to currents of water flowing through the vug to account for the horn and corkscrew habit. Heyl et. al. (1959) also described stalactitic forms with conical or cylindrical shapes from the Monroe Mine in the NW Sec. 25 T.1N R.1E. These growths extend upward from the base of the vug, suggesting the structures are actually stalagmites.

Marcasite helectite

Marcasite helectite, New Diggings area, Lafayette County, Wi. Sample is about 8 cm. long.

MANITOWOC COUNTY: Marcasite occurs as attractive iridescent crystals and clusters with calcite, fluorite, strontianite, pyrite and sphalerite at a quarry on Highway 151 near Valders. (WGNHS files; Carl Cochrane, 1999, pers. com.)

MILWAUKEE COUNTY: Bladed, globular and concretionary marcasite, occasionally showing iridescence, forms in vugs in the Milwaukee Formation with pyrite, millerite, calcite and sphalerite at Estabrook Park, Sec. 5 T.7N R.22E in Milwaukee (Bagrowski, 1940).

OUTAGAMIE COUNTY: Marcasite occurs as dull xls. and rosettes at the Mackville quarry. (Carl Cochrane, 1999, pers. com.)

PEPIN COUNTY: Marcasite, replaced by goethite, occurs in vugs in Oneota dolostone in a quarry N. of Stockholm in NW NW Sec. 7 T.23N R.15W. Marcasite was as twinned cockscomb, radial and stellar cluster and as geode- like masses over 12 cm. across In places complex twinned marcasite crystals form epitaxial overgrowths on pyrite This unusual overgrowth results in cluster with crystals radiating at 90 degrees to each other in three dimension. The marcasite and pyrite are now replaced by lustrous brown goethite. Other associated minerals are drusy quartz, dolomite and calcite (WSC field notes).

PIERCE COUNTY: Marcasite is widespread as small crystals in vugs in the Prairie du Chien group rocks exposed throughout the county. - Cockscomb twin marcasite crystals form epitaxial overgrowths on pyrite in vugs in dolostone in a quarry on County S, east of Maiden Rock in the NW Sec. 14 T.24N R.16W. This unusual overgrowth results in cluster with crystals radiating at 90 degrees to each other in three dimension. The marcasite and pyrite are now replaced by lustrous brown goethite. Other associated minerals are drusy quartz, dolomite and calcite (Cordua, 1989a).
- Small marcasite and octahedral pyrite crystals, now replaced by goethite occur in a quarry in dolostone on the north side of U.S. Hwy. 10 in the NW 1/4 Sec. 5 T.25N R.16W east of Ellsworth.
- Small marcasite clusters and octahedral pyrite crystals now replaced by goethite, occur in vugs in dolostone exposed in the outcrops at the dam and spillway of the Eau Galle Reservoir in the NW Sec. 5 and the NE Sec. 6 T.27N R.15W near Spring Valley (W.S. Cordua field notes).

RACINE COUNTY: Cook (1924) describes marcasite found in vugs in dolostone at the Consumers Co. Quarry, 3 miles north of Racine. The vugs are up to 1 foot across and contain single xls. and complex twins of marcasite in radiating, globular, cockscomb and "spear-like masses".
- Botryoidal and cockscomb marcasite clusters up to 5 cm. across is found with calcite in the Vulcan Materials (Ives Quarry), N of Racine. The marcasite may also occur as thick veins or as inclusions within the large calcite crystals. Other marcasite makes attractive iridescent coatings on fossil coral and brachiopods or may be pseudomorphous after calcite. (Eckard, 1980, WGNHS files, Scoville, 1994).

RICHLAND COUNTY: Large clusters of twinned marcasite crystals, now largely replaced by goethite, can be found on the surface or in clayey regolith along the bluffs north of the Wisconsin River. Clusters up to 14.5 cm. across have been reported (North, 1913).

ROCK COUNTY: Marcasite is found associated with galena and sphalerite in drill core and at a quarry in the center of Sec. 16 T.1N R.10E, about 1.5 km. southeast of Avon (Heyl and West, 1982).
- Marcasite veins up to 10 cm. wide are found along joints in the Platteville Formation in sec. 25 T.1N. R.14E. (Brown and Maas, 1992).

RUSK COUNTY: Marcasite crystals and clusters occur in vugs with dolomite in the carbonate zone at the Flambeau Copper Mine south of Ladysmith.

WINNEBAGO COUNTY: Marcasite occurs in excellent crystals clusters with pyrite in cavities in dolostone at the Lutz Quarry, west of Hwy. 44 in Sec. 29 T.18N R.16E south of Oshkosh. Associated minerals include calcite, sphalerite, pyrite, smithsonite, and hemimorphite (U.S.G.S., 1976).
- Marcasite occurs in clusters in cavities in the Platteville Formation at the Grundy Quarry in the NE NW NE Sec. 33 T.19N R.16E northwest of Oshkosh. Associated minerals include calcite, pyrite and, rarely, sphalerite (U.W.Oshkosh, 1989).
- Marcasite xls. and rosettes occur with calcite xls. in dolomitic limestone in the Schultz quarry at Neenah, on Tullar Road N of Hwy. 114 and west of Route 41 (Zeitner, 1988; Carl Cochrane, 1999, pers. com.).

WOOD COUNTY: Marcasite occurs as bladed xls. in fractures at the Frederick Schill Quarry, near Vesper (SW NW sec. 5 T.23N. R.4E.). (Buchholz, 1999)

Return to Wisconsin Mineral List