Franklinite is a zinc-iron-manganese spinel-group oxide essentially restricted to the unique Franklin and Sterling Hill Zn-Fe-Mn deposit in New Jersey. It is the dominant ore mineral of that historic district and the ferromagnetic black “third leg” of the Franklin fluorescent triad.
Properties
- Formula: (Zn,Mn,Fe)(Fe,Mn)2O4
- Crystal system: Isometric (Cubic)
- Hardness: 5.5 – 6.5
- Color: Black to brownish-black
- Streak: Reddish-brown
- Luster: Metallic to sub-metallic
- Cleavage: None
- Density: 5.0 – 5.2 g/cm³
Occurrence
Franklin and Sterling Hill (New Jersey) are essentially the world’s only economic source. Crystals of franklinite — sharp octahedra to several centimeters — are classic mineralogical specimens of American collecting tradition.
Identification
Black + octahedral habit + reddish-brown streak + weakly magnetic + Franklin association. Distinguish from magnetite (strongly magnetic, black streak) by streak color and weaker magnetism.
Collector Notes
Franklinite octahedra in white calcite + green willemite + red zincite is the classic “Franklin combination” specimen — one of the most collected American mineralogical assemblages.
Found at these Localities
- Broken Hill (布罗肯希尔铅锌银矿)
- Franklin and Sterling Hill (富兰克林矿(新泽西))
