Formula: MgAl2O4 Isometric
Description: Spinel is found as an accessory mineral in mafic to ultramafic igneous rocks, in marbles and other metamorphic rocks and as a rare heavy mineral in placer deposits. It is usually in small, dark colored grains, generally showing octahedron faces. Larger gemmy crystals of red, blue or green color can be found in marbles.
IRON COUNTY: Klewin et al. (1989) report spinel in outcrops from the NW NW sec. 32 T.46N. R.1E. and SW SW sec. 2 T.45N. R.1W., near Upson where it occurs as tiny chromium-rich grains in the troctolite of the Potato River Intrusive of the Mellen complex.
KENOSHA COUNTY: Spinel occurs as a heavy mineral found in cores from a subsurface, diamond-bearing lamprophyre diatreme, discovered within the outskirts of Kenosha (Carlson and Adams, 1997).