Franklinite
Franklinite is a zinc iron manganese oxide, (Zn,Mn,Fe)(Fe,Mn)2O4, belonging to the spinel group and crystallizing in the isometric system, usually as black octahedral crystals or granular masses. It is virtually unique to the zinc deposits of Franklin and Sterling Hill, New Jersey, where it occurs in an assemblage found almost nowhere else on Earth. Though weakly magnetic, it is chemically distinct from true magnetite.
Franklinite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | (Zn,Mn,Fe)(Fe,Mn)2O4 |
| Mohs hardness | 5.5–6.5 |
| Crystal system | Isometric |
| Luster | Metallic to submetallic |
| Streak | Reddish brown to black |
| Cleavage | None (octahedral parting) |
| Fracture | Uneven to conchoidal |
| Specific gravity | 5.1–5.2 |
| Transparency | Opaque |
| Common colors | Black, Brownish black |
| Rarity | common |
Geology & background
Franklinite formed within a body of zinc-, iron-, and manganese-rich sediment that was later regionally metamorphosed to high grade, recrystallizing the original minerals into a unique ore assemblage alongside willemite and zincite. This combination at Franklin and Sterling Hill in New Jersey is considered one of the most mineralogically diverse ore deposits known, and it has yielded hundreds of distinct mineral species. Franklinite itself was historically mined as an ore of both zinc and manganese. Collectors typically encounter franklinite as black, submetallic octahedral crystals or as granular masses embedded in white to pink calcite matrix alongside orange-red zincite and gray-green willemite. Unlike its famous neighbors, franklinite does not fluoresce strongly under ultraviolet light, so identification usually relies on its color, weak magnetism, and typical crystal form rather than a glowing display.
How to identify Franklinite
Franklinite forms black octahedral crystals or granular masses with a submetallic to metallic luster and a reddish-brown to black streak. Hardness runs 5.5 to 6.5, there is no distinct cleavage, and the mineral is weakly attracted to a strong magnet, unlike ordinary magnetite, which is more strongly magnetic. It is most reliably identified by its association with willemite and zincite in New Jersey specimens, since visually it can resemble magnetite or chromite from other localities.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Franklinite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Franklinite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Franklinite comes almost entirely from Franklin and Sterling Hill, New Jersey, a deposit famous for producing more fluorescent minerals than anywhere else known.
- Though it looks like magnetite, franklinite is only weakly magnetic and contains significant zinc and manganese instead of pure iron oxide.
- It was mined historically as a valuable ore of both zinc and manganese before the New Jersey deposits were largely worked out.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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