Formula: CaSO4 Orthorhombic
Anhydrite is a relatively common mineral usually formed as a chemical precipitate from saline water in limestone, dolostone, and salt beds. Its presence is obscured by its tendency to dissolve or be altered to gypsum when exposed to groundwater or surface weathering. Its true extent is most often documented in areas that have been drilled extensively during petroleum exploration. The author contends that anhydrite may be widespread in Wisconsin within such rocks as the Prairie du Chien group, but that insufficient subsurface exploration has been done to document its presence. Anhydrite can also be found as a hydrothermal mineral or as late stage mineral within igneous rocks.
MARATHON COUNTY: Anhydrite forms rare small inclusions in quartz in pockets in the pegmatites of the Wausau pluton such as are exposed in the “rotten granite” quarries in Sec. 19 and 20 T.28N R.7E., south of Rib Mountain (Falster, 1987; Falster et al., 2000).
RUSK COUNTY: Anhydrite and gypsum occur with serpentine, chlorite and carbonates in an unusual metamorphosed mafic tuff in the Eisenbrey deposit, north of Ladysmith (NW sec. 17 T.35N. R.6W.) (May, 1996).
SAUK COUNTY: Adams (1978) reports bladed and lobate pores in the Oneota dolostone in the Madison area, which he suggests could be from dissolution of evaporitic anhydrite.