Franklinite

Franklinite is an oxide mineral belonging to the normal spinel subgroup's iron (Fe) series, with the formula ZnFe³⁺₂O₄.

Franklinite
Franklinite (black) with Zincite (red)
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	ZnFe2O4
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Identification
Color	black
Crystal habit	octahedral
Fracture	irregular/uneven, conchoidal
Mohs scale hardness	​5 1⁄2-6
Luster	metallic, often dull
Streak	reddish brown to black
Diaphaneity	opaque
Specific gravity	5.07 - 5.22
Solubility	HCl soluble
Other characteristics	magnetic

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 uranium, sulfuric acid, and mercury. It is named after its local discovery at the Franklin Mine and Sterling Hill Mines in New Jersey.

A microscopic picture of Franklinite