Franklinphilite is a rare phyllosilicate mineral of the stilbite group, found almost exclusively in the famous zinc deposits of Franklin and Sterling Hill, New Jersey. Collectors should look for thin, platy crystals occurring as radiating clusters within metamorphosed zinc ores, often associated with willemite and franklinite.
Is this franklinphilite?
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 franklinphilite with a known reference. Franklinphilite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Franklinphilite leaves a light brown streak.
- 3Read the lusterHold the specimen under a strong light. Franklinphilite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: dark brown, black.
- 5Look at form & habitCrystal system: triclinic. Typical habit: platy crystals, radiating aggregates.
Often confused with
Franklinphilite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Franklinphilite leaves light brown, Stilbite leaves white.
How to tell apart: Streak differs, so Franklinphilite leaves light brown, Heulandite leaves white; luster reads vitreous on Franklinphilite and vitreous to pearly on Heulandite.
Often found alongside franklinphilite
Minerals reported to co-occur with franklinphilite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (K,Na,Ca)₃Mn₃Si₁₈O₄₄·12H₂O
- Mohs hardness
- 3-4
- Density
- 2.6 g/cm³
- Colors
- Streak
- Light Brown
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Triclinic
- Crystal habit
- Platy Crystals, Radiating Aggregates
- Cleavage
- Imperfect On {001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metamorphosed Zinc Ore Deposits
Where rockhounds find franklinphilite
Classic worldwide localities
- Franklin Mine, New Jersey, USA
- Sterling Hill Mine, New Jersey, USA
Field-hunting tip
Look in metamorphosed zinc ore deposits country, the host setting where franklinphilite typically forms. If you start seeing willemite, franklinite, calcite in float, you are in the right ground. Field specimens usually show a platy crystals, radiating aggregates habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find franklinphilite on the map
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