Alunite, with pen for scale. 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: KAl3(SO4)2(OH)6 Hexagonal


Alunite is an inconspicuous grayish massive mineral formed by the alteration of pyrite and other sulfides during weathering or solfatara activity. It is probably widespread in Wisconsin but has so far been reported from only one locality. Its habit, fine-grained nature and common intergrowth with minerals such as hematite, goethite, jarosite, melanterite and pyrite are likely reasons for it being overlooked.

ROCK COUNTY: Seen as an alteration product of pyrite in the St. Peter sandstone at the sand quarry near Hanover. Here it is associated with jarosite, gypsum and lepidochrocite (Odom, Willand and Lassin, 1979).