Franklinite

(mineralogy) ZnFe₂O₄ Black, slightly magnetic mineral member of the spinel group; usually possesses extensive substitution of divalent manganese and iron for the divalent zinc, and limited trivalent manganese for the trivalent iron.

A natural member of the spinel structure type, with composition Zn²⁺Fe₂³⁺O₄. The habit is octahedral, often modified by the cube and dodecahedron, but the mineral usually occurs as bands of isolated rounded grains, blebs, or compact masses. It is black with a metallic luster and red internal reflections; its hardness is 6 (Mohs scale); specific gravity is 5.3; and it is weakly magnetic.

Franklinite is confined in its occurrence to the unique ore bodies at Franklin and Sterling Hill (Ogdensburg), Sussex County, New Jersey. Franklinite is a major ore mineral and is still mined at Sterling Hill for spiegeleisen and zinc. See also Willemite; Zinc.