Franklinite

Franklinite
Franklinite (black) with willemite (red) and calcite (white)
General
CategoryOxide minerals
Spinel group
Spinel structural group
Formula
(repeating unit)
ZnFe2O4
IMA symbolFrk[1]
Strunz classification4.BB.05
Crystal systemCubic
Crystal classHexoctahedral (m3m)
H-M symbol: (4/m 3 2/m)
Space groupFd3m
Identification
Colorblack
Crystal habitoctahedral
CleavageNone; indistinct octahedral parting in four directions
Fractureirregular/uneven, conchoidal
TenacityBrittle
Mohs scale hardness5.5–6
Lustermetallic, often dull
Streakreddish brown to black
Diaphaneityopaque
Specific gravity5.07–5.22
Ultraviolet fluorescenceNone
SolubilityHCl soluble
Other characteristicsmagnetic
References[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]

A microscopic picture of Franklinite
  1. ^ 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.
  2. ^ Chesterman, Charles W. (1978). The Audubon Society field guide to North American rocks and minerals. New York: Alfred A. Knopf. p. 418. ISBN 0394502698.
  3. ^ "Office of the Governor | Governor Murphy Signs Legislation Designating Franklinite as State Mineral". www.nj.gov. Retrieved 2023-07-20.