Formula: Al2Si4O10(OH)2 Monoclinic.
Description: Pyrophyllite is a micaceous mineral formed as a result of the relatively low grade metamorphism of aluminum-rich rocks. It can also form as a hydrothermal replacement of aluminous minerals such as feldspars. The mineral may be in fine-grained aggregate and is easily mistaken for other phyllosilicates.
DODGE COUNTY: Pyrophyllite is a component of the argillites interbedded with quartzite in the Michal Materials Quarry (sec. 33 and 34 T.9N. R.13E, near Portland. (Brown et al. 2001).
RUSK COUNTY: Pyrophyllite is a major component of the meta-volcanic wall rock in the Flambeau copper deposit, near Ladysmith.
SAUK COUNTY: Pyrophyllite is a fine-grained component of the phyllites and argillites interleaved with the Baraboo Quartzite (Dott and Dalziel, 1970, Brown, 1987). Some outcrops of pyrophyllite-rich phyllites they describe as listed below. These outcrops are world famous and are studied each year by hundreds of geologists and geology students. For this reason NO HAMMERING OR SAMPLING should be done on any of them.
— Pyrophyllite occurs in complexly folded phyllites in roadcuts on U.S. Hwy. 12 in the NW sec. 15 T.11N. R.6E. It is also found at the LaRue quarry, 1 mile south of LaRue (SW NW sec. 22 T.11N. R.5E.) and in the Happy Hill area (NW SE sec. 25 T.11N. R.5E.)
— Gunderson (1986) describes the pipestone of the Devil’s Lake area as an argillite consisting of pyrophyllite, rutile, hematite and quartz.
— Pyrophyllite is a component of the metasaprolite horizon at the base of the Baraboo Quartzite, occurring both in the ground mass and in veins with kaolin, muscovite and diaspore. (Medairs et al., 2001).
SAWYER COUNTY: An argillite from the Radisson area having the same properties as pipestone was found to consist of a mixture of quartz, kaolinite and pyrophyllite (Gunderson, 1986).