Chemical Formula: Zn2+Fe3+2O4
Locality: Dominant ore mineral at Franklin and Sterling Hill, New Jersey, USA.
Name Origin: Named after its locality which was named after Benjamin Franklin (1706-1790), American scientist and inventor.
Franklinite is an oxide mineral belonging to the normal spinel subgroup’s iron (Fe) series, with the formula Zn2+Fe3+2O4.
As with another spinel member magnetite, both ferrous (2+) and ferric (3+) iron may be present in Franklinite samples. Divalent iron and/or manganese (Mn) may commonly accompany zinc (Zn) and trivalent manganese may substitute for some ferric iron.
At its type locality, Franklinite can be found with a wide array of minerals, many of which are fluorescent. More commonly, it occurs with willemite, calcite, and red zincite. In these rocks, it forms as disseminated small black crystals with their octahedral faces visible at times. It may rarely be found as a single large euhedral crystal.
Franklinite was a minor ore of zinc, manganese, and iron. It is named after its local discovery at the Franklin Mine and Sterling Hill Mines in New Jersey.
Color: Black, Brownish black.
Density: 5.07 – 5.22, Average = 5.14
Fracture: Uneven – Flat surfaces (not cleavage) fractured in an uneven pattern.
Hardness: 5.5-6 – Knife Blade-Orthoclase
Luster: Sub Metallic
Magnetism: Naturally weak
Streak: reddish brown