A common variety of sillimanite is known as fibrolite, so named because the mineral appears like a bunch of fibres twisted together when viewed in thin section or even by the naked eye.

Both the fibrous and traditional forms of sillimanite are common in metamorphosed sedimentary rocks.

It occurs with andalusite, kyanite, potassium feldspar, almandine, cordierite, biotite and quartz in schist, gneiss, hornfels and also rarely in pegmatites.

[7] Sillimanite has been found in Brandywine Springs, New Castle County, Delaware.

[9] As of 1998[update], sillimanite was just 2% of all aluminosilicate mineral production in the western world.