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	ZnFe2O4
IMA symbol	Frk[1]
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
Cleavage	None; indistinct octahedral parting in four directions
Fracture	irregular/uneven, conchoidal
Tenacity	Brittle
Mohs scale hardness	5.5–6
Luster	metallic, often dull
Streak	reddish brown to black
Diaphaneity	opaque
Specific gravity	5.07–5.22
Ultraviolet fluorescence	None
Solubility	HCl soluble
Other characteristics	magnetic
References	[2]

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.

In 2023, Franklinite was designated the state mineral of New Jersey.^[3]

A microscopic picture of Franklinite

See also

• Earth sciences portal

• Classification of minerals
• List of minerals
• Spinel