Esperite is a rare lead-calcium silicate mineral famous for its brilliant yellow-green fluorescence under shortwave UV light. It is primarily found in the metamorphosed ore deposits of Franklin and Sterling Hill, New Jersey, where it often occurs in massive or granular form alongside willemite.
Is this esperite?
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 esperite with a known reference. Esperite sits at Mohs 5-5.5, 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: dull to slightly greasy.
- 4Match the color rangeCompare against the expected color range: white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive.
Often confused with
Esperite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Luster reads dull to slightly greasy on Esperite and vitreous to resinous on Willemite.
How to tell apart: Esperite is noticeably harder (Mohs 5-5.5 vs. 3-4); luster reads dull to slightly greasy on Esperite and vitreous on Hardystonite.
Often found alongside esperite
Minerals reported to co-occur with esperite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- PbCa₂(ZnSiO₄)₃
- Mohs hardness
- 5-5.5
- Density
- 4.28-4.42 g/cm³
- Colors
- Streak
- White
- Luster
- Dull to slightly greasy
- Transparency
- Opaque, transparent in small grains
- Crystal system
- Monoclinic
- Crystal habit
- Massive
- Cleavage
- Distinct on {010} and {100}; poor on {101}
- Fluorescence
- Bright lemon yellow under SW UV
- Rarity
- Rare
- Uses
- Collector, Fluorescent specimen
- Host rock
- In a metamorphosed stratiform zinc orebody
Where rockhounds find esperite
Classic worldwide localities
- Franklin, Sussex Co., New Jersey, USA
Field-hunting tip
Where it forms: In a metamorphosed stratiform zinc orebody. If you start seeing willemite, zincite, hardystonite in float, you are in the right ground. Typical habit: massive. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify esperite?+
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Find esperite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
