Gerstmannite is a rare silicate mineral found exclusively in the Franklin mining district of New Jersey. It typically occurs as small, pale pink to colorless prismatic crystals or as granular masses associated with willemite and zincite.
Is this gerstmannite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch gerstmannite with a known reference. Gerstmannite sits at Mohs 4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to subadamantine, silky when matted.
- 4Match the color rangeCompare against the expected color range: white, light pink.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic crystals, massive, granular.
Often confused with
Gerstmannite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Willemite is the harder of the two (Mohs 5.5 vs. 4.5).
How to tell apart: Clinohedrite is the harder of the two (Mohs 5.5 vs. 4.5); luster reads vitreous to subadamantine, silky when matted on Gerstmannite and brilliant, glassy; pearly on {010} on Clinohedrite.
Often found alongside gerstmannite
Minerals reported to co-occur with gerstmannite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mg,Mn)₂Zn(SiO₄)(OH)₂
- Mohs hardness
- 4.5
- Density
- 3.68 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous to subadamantine, silky when matted
- Transparency
- Translucent to opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic crystals, massive, granular
- Cleavage
- Good on {010}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In a hydrothermal vein cutting franklinite-willemite ore in a metamorphosed stratiform zinc orebody
Where rockhounds find gerstmannite
Classic worldwide localities
- Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
Field-hunting tip
Where it forms: In a hydrothermal vein cutting franklinite-willemite ore in a metamorphosed stratiform zinc orebody. If you start seeing calcite, manganpyrosmalite, sphalerite in float, you are in the right ground. Typical habit: prismatic crystals, massive, granular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find gerstmannite on the map
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