Franklinite

Franklinite is a member of the spinel group, an oxide of zinc, iron, and manganese with the general formula (Zn,Mn,Fe)(Fe,Mn)2O4. It crystallizes in the cubic system, typically forming black octahedral crystals or rounded grains.

The mineral is named after Franklin, New Jersey, the locality where it was discovered and is essentially confined. There it occurs in the famous metamorphosed zinc orebody alongside red zincite and green willemite in white calcite.

Franklinite is one of the defining minerals of the unique Franklin-Sterling Hill assemblage and was historically a major ore of both zinc and manganese.

Franklinite formed through the high-grade metamorphism of an ancient zinc- and manganese-rich sedimentary deposit at Franklin and Sterling Hill, New Jersey. This unusual protolith and its metamorphism produced a mineral assemblage found almost nowhere else on Earth.

It occurs as black octahedral crystals and granular masses embedded in white calcite marble, intimately associated with zincite, willemite, and a remarkable variety of rare minerals.

Because its formation required such specific and unusual conditions, franklinite is one of the most locality-restricted of all reasonably common ore minerals.

Franklinite is identified by its black to brownish-black color, metallic to submetallic luster, octahedral crystal habit, and reddish-brown to black streak. It has a hardness of 5.5 to 6.5 and is weakly magnetic, unlike the strongly magnetic magnetite it resembles.

The single most diagnostic feature is its association: franklinite occurring as black grains and crystals in white calcite with red zincite and fluorescent green willemite is essentially unique to Franklin, New Jersey.

The weak magnetism and Franklin assemblage together distinguish it from magnetite and chromite.

Franklinite was historically an important ore mined at Franklin and Sterling Hill, yielding both zinc and manganese; the manganese-bearing residues were used to make spiegeleisen and zinc-white pigment.

The Franklin mines were among the most productive zinc operations in U.S. history before closing in the mid-20th century.

Today franklinite's primary value is as a classic mineral specimen, especially in association with zincite and fluorescent willemite. It has no major current industrial role, but it remains scientifically significant as a defining mineral of a one-of-a-kind deposit.