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.
Associated minerals are anthophyllite, almandine, cummingtonite, andalusite and kyanite.
Sillimanite occurs in large quartzite inclusions in the Wausau intrusive complex. 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.