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The Mineral franklinite

Large Octahedral Franklinite Crystal

Franklinite is named after the locality of Franklin, New Jersey, where it is found. To date, this mineral is found only at Franklin and neighboring Ogdensburg, New Jersey. There are no other localities for Franklinite, but at these two localities it is quite abundant. Large crystals of previous mining eras 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.

Chemical Formula

(Zn,Fe2+)(Fe3+)2O4
This is the chemical formula for pure Franklinite. However, Franklinite usually contains a significant amount of manganese as both the first and second elemental groups, and its chemical formula is therefore also recognized as:
(Zn,Mn2+,Fe2+)(Fe3+,Mn3+)2O4

Color

Black

Crystal System

Isometric

Properties

Streak
Dark gray to black
Hardness
5.5 - 6.5
Transparency
Opaque
Specific Gravity
5.0 - 5.2
Luster
Metallic
Cleavage
None. May exhibit parting.
Fracture
Subconchoidal to uneven
Tenacity
Brittle
Other ID Marks
Is paramagnetic, thus is slightly attracted to magnetic fields.

Crystal Habits

Crystals are usually octahedral. Complex crystals with multiple faces, as well as dodecahedral crystals, are much less common. Most often in small rounded or grainy crystals with distorted faces. Also massive, in dense crystal veins, and in groups of octahedral crystals.

3D Crystal Atlas

Additional Information

Composition
Zinc iron oxide, usually with a significant amount of manganese
In Group
Oxides; Multiple Oxides
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.
Rock Type
Metamorphic

Uses

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 Now, however, those mines are no longer active, so Franklinite is no longer used as an ore of zinc.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.

Noteworthy Localities

The only location where this mineral was ever found was in the zinc ore deposit of the Franklin Mining District, in both Franklin and Ogdensburg (Sterling Hill), Sussex Co., New Jersey.

Common Mineral Associations

Calcite, Willemite, Zincite, Magnetite

Distingushing Similar Minerals

Magnetite - Strongly attracted to magnetic fields.
Spinel - Not attracted to magnetic fields, has a white streak.

franklinite Photos



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