Zincite with Franklinite Franklin, Sussex Co., New Jersey, USA Small Cabinet, 6.4 x 4.2 x 3.7 cm Photo Copyright © Arkenstone

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.

Physical Properties

Cleavage: None
Color:    Black, Brownish black.
Density: 5.07 – 5.22, Average = 5.14
Diaphaneity: Opaque
Fracture: Uneven – Flat surfaces (not cleavage) fractured in an uneven pattern.
Hardness: 5.5-6 – Knife Blade-Orthoclase
Luminescence: Non-fluorescent.
Luster: Sub Metallic
Magnetism: Naturally weak
Streak: reddish brown

Photos :

This sample of franklinite is displayed in the Smithsonian Museum of Natural History. The sample at left is franklinite with calcite. It is about 8 cm across and is from Franklin, New Jersey.
Hedyphane, Willemite, Franklinite, Calcite 10.0×9.0x7.0 cm Franklin, New Jersey, USA Copyright © David K. Joyce Minerals
Franklin Mine, Franklin, Franklin Mining District, Sussex Co., New Jersey, USA
Franklinite (black) with Zincite (red) Locality: Franklin, New Jersey, USA – Exposed in the Mineralogical Museum, Bonn, Germany Photo Copyright © Ra’ike