WGNHS
Wisconsin Geological and Natural History Survey

Jarosite

Jarosite from the mineral collection of Brigham Young University Department of Geology, Provo, Utah, courtesy of the U.S. Geological Survey Denver Library Photographic Collection. (Photo by Andrew Silver.)

Formula: KFe3(SO4)2(OH)6 Hexagonal

Description:

Yellow clear jarosite grains on matrix. Individual grains are about 0.05 mm across. (Dan Behnke specimen and image.)

Jarosite occurs as earthy pale yellow to brown coatings, crusts and granular masses formed by the alteration of pyrite, marcasite and other iron sulfides. It is likely much more common in the state than indicated, being likely at most localities listed for pyrite and marcasite. Its earthy, crumbly nature results in its being overlooked, cleaned off or discarded.

MARATHON COUNTY: Jarosite was found as an alteration product in pegmatites exposed in a temporary excavation in Stettin Township on County O near Evergreen Av, on the Tom O’Brien property. (Tom Buchholz, personal communication).

ROCK COUNTY: Jarosite is noted as an alteration product of pyrite associated with alunite, gypsum and lepidochrocite in the St. Peter Sandstone exposed in the silica sand quarry near Hanover (Odom, Willard and Lassin, 1979).