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Franklinite is named after the locality of Franklin, New Jersey, where it is found. To date it is only found in Franklin and the neighboring town of Ogdensburg, in Sussex Co., New Jersey. There are no other localities for this mineral, but in those two localities it is quite abundant. However, the large crystals of the past are no longer found, and are considered classics that are highly sought-after by collectors.
Franklinite is usually associated with brilliantly fluorescent Calcite and Willemite, and may also occur with orange Zincite, providing specimens of nice color contrasting.
Franklinite is sometimes classified as a variety of the mineral Magnetite, but it is in fact a distinct mineral species.
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Chemical Formula |
(Zn,Fe2+)(Fe3+)2O4
This is the pure formula for Franklinite. However, since Franklinite usually contains a significant amount of manganese as both the first and second element, its formula can also be recognized as:
(Zn,Fe2+,Mn2+)(Fe3+,Mn3+)2O4 |
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Composition |
Zinc iron oxide, usually with a significant amount of manganese |
| Variable Formula |
(Zn,Fe2+,Mn2+)(Fe3+,Mn3+)2O4 |
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Color |
Black |
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Streak |
Dark gray to black |
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Hardness |
5.5 - 6.5 |
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Crystal System |
Isometric |
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Crystal Forms
and Aggregates |
Crystals are most commonly octahedral crystals. Complex crystals with multiple faces, as well as dodecahedral crystals, are very commons. Most often in small rounded or grainy crystals with distorted faces. Also massive, as rounded pebbles, and in groups of octahedral crystals.
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Transparency |
Opaque |
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Specific Gravity |
5.0 - 5.2 |
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Luster |
Metallic |
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Cleavage |
None. May exhibit parting. |
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Fracture |
Subconchoidal to uneven |
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Tenacity |
Brittle |
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Other ID Marks |
Is paramagnetic, thus is slightly attracted to magnetic fields. |
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In Group |
Oxides; Multiple Oxides |
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Striking Features |
Locality, mineral associates, and weak attraction to magnetic fields. |
| Environment |
Only found in the zinc ore zone of the metamorphic marble deposit of Franklin and Ogdensburg, New Jersey.
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| Rock Type |
Metamorphic |
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Popularity (1-4)
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2 |
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Prevalence (1-3)
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2 |
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Demand (1-3) |
2 |
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Franklinite was a very important ore of zinc, together with the other zinc minerals at the Franklin and Sterling Hill mines. Now, however, those mines are no longer active, so Franklinite is no longer used as an ore of zinc.
Large crystals of Franklinite are popular among collectors.
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The only places where this mineral exists is Franklin and Ogdensburg (Sterling Hill), both in Sussex Co., New Jersey. In these two localities it occurs in abundance.
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Magnetite - Strongly attracted to magnetic fields.
Spinel - Not attracted to magnetic fields, has a white streak.
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