Franklinite Identification Guide
How to identify franklinite by its black octahedral spinel crystals, weak magnetism, reddish-brown streak, and its signature association with zincite and willemite.
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What Franklinite Looks Like
Franklinite is a zinc-iron-manganese oxide of the spinel group (ideally ZnFe2O4 with Mn), famous from the Franklin/Sterling Hill zinc deposits of New Jersey.
- Color: iron-black to brownish-black.
- Luster: metallic to submetallic.
- Transparency: opaque.
- Crystal habit: octahedra (often with rounded edges), also massive and granular grains embedded in white calcite marble, classically alongside red zincite and green willemite.
Step-by-Step Field-ID Checklist
- Look for black octahedra or rounded black grains in white calcite.
- Check the streak — a reddish-brown to dark-brown streak, not black.
- Test magnetism: franklinite is weakly magnetic (a strong magnet tugs it; it is far less magnetic than magnetite).
- Note the host: white calcite marble that fizzes in acid, often with the red and green companion minerals.
- Test fluorescence of the matrix (calcite/willemite glow) to confirm the Franklin association.
Key Diagnostic Tests
- Mohs hardness: ~5.5–6.5. Scratches glass; harder than a knife.
- Streak: reddish-brown to dark brown — key separator from magnetite (black streak).
- Magnetism: weakly to moderately magnetic; clearly less strongly attracted than magnetite.
- Cleavage: none; uneven to conchoidal fracture (octahedral parting may show).
- Density: high, ~5.0–5.2 g/cm3 — it feels heavy.
- Acid: franklinite itself does not fizz, but its calcite host does.
- Fluorescence: franklinite is non-fluorescent, but it sits with willemite (green glow) and calcite (red glow) under UV — a diagnostic context clue.
Common Look-Alikes and How to Tell Them Apart
- Magnetite: the closest look-alike — black, octahedral, metallic. But magnetite is strongly magnetic and has a black streak, whereas franklinite is weakly magnetic with a reddish-brown streak. Streak plus magnet strength is decisive.
- Chromite: black, weakly magnetic, but has a brown streak and occurs in ultramafic rocks, not zinc-bearing calcite marble. Locality and association separate them.
- Ilmenite: black, weakly magnetic, but platy/tabular and with a black-to-brownish streak; different setting.
- Hematite: can be black (specular) but has a distinctive red-brown streak and is non-magnetic to weakly magnetic; hardness similar. Franklinite's octahedral habit and zinc-deposit association help.
- Black spinel: harder (8) and not from the Franklin assemblage.
The diagnostic combination is black octahedra in white calcite + reddish-brown streak + weak magnetism + high density + association with red zincite and green willemite.
Where Franklinite Is Found
Franklinite is essentially a one-locality mineral, abundant almost exclusively at Franklin and Sterling Hill, Sussex County, New Jersey (USA), in the metamorphosed zinc orebody, with minor occurrences elsewhere. Its tight association with zincite and willemite in fluorescent calcite marble is itself an identification fingerprint.
Frequently asked questions
How can you tell franklinite from magnetite?
Both are black, metallic, octahedral oxides, but franklinite is only weakly magnetic and has a reddish-brown streak, while magnetite is strongly magnetic and has a black streak. Franklinite also typically occurs in white calcite marble alongside red zincite and green willemite.
Is franklinite magnetic?
Yes, but only weakly. A strong magnet will tug at franklinite, but it is far less magnetic than magnetite. This weaker magnetism, combined with its reddish-brown streak, helps distinguish it from magnetite.
What does franklinite look like?
It looks like iron-black to brownish-black, metallic octahedral crystals or rounded grains, usually set in white calcite marble and often accompanied by red zincite and green willemite.
Where is franklinite found?
Franklinite is found almost exclusively at the Franklin and Sterling Hill zinc deposits in Sussex County, New Jersey, USA, which is essentially its type and only major locality.
Does franklinite glow under UV light?
Franklinite itself does not fluoresce, but it occurs with minerals that do: the willemite glows green and the calcite glows red under shortwave UV. That glowing host with black non-glowing grains is a strong clue that the black mineral is franklinite.