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

Franklinite (black) with Zincite (red)
CategoryOxide minerals
Spinel group
Spinel structural group
(repeating unit)
Strunz classification4.BB.05
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Crystal habitoctahedral
Fractureirregular/uneven, conchoidal
Mohs scale hardness5 12-6
Lustermetallic, often dull
Streakreddish brown to black
Specific gravity5.07 - 5.22
SolubilityHCl soluble
Other characteristicsmagnetic

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

See alsoEdit


External linksEdit

  Media related to Franklinite at Wikimedia Commons