Formula: Al2SiO5Orthorhombic
Description:
Sillimanite forms during relatively high temperature metamorphism and is common in schist, gneisses and quartzites.
FOREST COUNTY: Tiny radiating fibers and clusters of sillimanite are intergrown with quartz in the McCaslin Quartzite at Deer Lookout Tower and adjacent outcrops extending into Oconto and Marinette Counties. (Olson, 1964). These rocks are in the contact metamorphic zone of the High Falls Granite (Mancuso and Motten, 1980).
IRON COUNTY: Tiny sillimanite needles are found in kyanite schist exposed in the road cut on Hwy 182 west of Powell (SE SW sec. 28 T.42N. R.4E.) (Mudrey, 1979).
JACKSON COUNTY: Sillimanite was tentatively identified as fibrous grains in talc schist exposed at the Jackson County Iron Mine east of Black River Falls (SE sec. 15 T.21N. R.3W.) Associated minerals are anthophyllite, almandine, cummingtonite, andalusite and kyanite. (Jones, 1978). Crowns (1976) describes the sillimanite as “embedded bundles of slender brown xls. resembling the bristles of a paint brush.”
—Sillimanite needles in quartz occur in gneiss exposed along the gorge below the dam at Lake Arbutus near Hatfield (DuBois, 1981).
MARATHON COUNTY: Sillimanite occurs in large quartzite inclusions in the Wausau intrusive complex. Some reported localities are the quarry at the south end of Rib Mountain (NW SW sec. 8 T.28N. R.7E.) and the quartzite exposed on a ridge in the S 1/2 sec. 21 T.29N. R.7E. (Sood, Myers and Berlin, 1980) and outcrops in SE NE sec. 29 T.29N. R.7E. (Myers et al., 1984).
—Fine-grained sillimanite occurs in tiny grains in the center of muscovite books in the 2 mica granite exposed at Mosinee (Anderson, Cullers and Van Schmus, 1980).
MARINETTE COUNTY: Tiny radiating fibers and clusters of sillimanite are intergrown with quartz in the McCaslin Quartzite (Olson, 1964). These rocks are in the contact metamorphic zone of the High Falls Granite (Mancuso and Motten, 1980).
OCONTO COUNTY: Tiny radiating fibers and clusters of sillimanite are intergrown with quartz in the McCaslin Quartzite at Thunder Mountain and adjacent outcrops extending into Forest and Marinette Counties. (Olson, 1964). These rocks are in the contact metamorphic zone of the High Falls Granite (Mancuso and Motten, 1980).
—Sillimanite is reported as a contact metamorphic mineral formed in metasedimentary rocks of the Waupee Formation adjacent to the Hagar Granite near Mountain. Associated minerals include muscovite, biotite and andalusite (Lahr, 1972).
PRICE COUNTY: Sillimanite occurs with almandine and biotite in schist in a drill core from near Park Falls. (Holm et al., 2001).
WAUPACA COUNTY: -Fine-grained sillimanite occurs in tiny grains in the center of muscovite books in the 2 mica granite exposed at Waupaca (Anderson, Cullers and Van Schmus, 1980).