Franklinite

(repeating unit)ZnFe₂O₄IMA symbolFrk^[1]Strunz classification4.BB.05Crystal systemCubicCrystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)Space groupFd3mIdentificationColorblackCrystal habitoctahedralCleavageNone; indistinct octahedral parting in four directionsFractureirregular/uneven, conchoidalTenacityBrittleMohs scale hardness5.5–6Lustermetallic, often dullStreakreddish brown to blackDiaphaneityopaqueSpecific gravity5.07–5.22Ultraviolet fluorescenceNoneSolubilityHCl solubleOther characteristicsmagneticReferences^[2]

Franklinite is an oxide mineral belonging to the normal spinel subgroup's iron (Fe) series, with the formula Zn²⁺Fe₂³⁺O₄.

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 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 is a major ore of the element zinc. 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]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ Chesterman, Charles W. (1978). The Audubon Society field guide to North American rocks and minerals. New York: Alfred A. Knopf. p. 418. ISBN 0394502698.
^ "Office of the Governor | Governor Murphy Signs Legislation Designating Franklinite as State Mineral". www.nj.gov. Retrieved 2023-07-20.