Franklinite is an oxide mineral belonging to the normal spinel subgroup's iron (Fe) series, with the formula ZnFe3+2O4.
As with another spinel member magnetite (2+) and ferric (3+) iron may be present in Franklinite samples. Divalent iron (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. 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.