Bannisterite is a complex sheet silicate that typically appears as thin, platy crystals in shades of brown or pink. It is most famous as a rare component of the unique mineral assemblages found in the Franklin and Sterling Hill mines of New Jersey. Collectors should look for its distinctive pearly luster on cleavage surfaces when examining specimens from manganese-rich deposits.
Is this bannisterite?
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 bannisterite with a known reference. Bannisterite sits at Mohs 4, 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: resinous.
- 4Match the color rangeCompare against the expected color range: dark brown, black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: aggregates, bladed, prismatic.
Often confused with
Bannisterite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Bannisterite leaves white, Stilpnomelane leaves gray white; luster reads resinous on Bannisterite and pearly to vitreous on cleavage surface, may be submetallic on Stilpnomelane.
How to tell apart: Streak differs, so Bannisterite leaves white, Ganophyllite leaves brownish yellow; luster reads resinous on Bannisterite and vitreous, brilliant on Ganophyllite.
Often found alongside bannisterite
Minerals reported to co-occur with bannisterite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaMn₁₀[(Si₁₄Al₂)O₃₈(OH)₈]·nH₂O
- Mohs hardness
- 4
- Density
- 2.83-2.84 g/cm³
- Colors
- Streak
- White
- Luster
- Resinous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Aggregates, bladed, prismatic
- Cleavage
- Perfect on {001}; a second prismatic cleavage is noted microscopically
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In metamorphosed Mn-Zn orebodies
Where rockhounds find bannisterite
Classic worldwide localities
- Franklin, Sussex Co., New Jersey, USA
- Benallt mine, Rhiw, Lleyn Peninsula, Wales
- Nyberget, Sweden
- Ananai mine, Kochi Prefecture, Japan
- Broken Hill, New South Wales, Australia
Field-hunting tip
Where it forms: In metamorphosed Mn-Zn orebodies. If you start seeing rhodonite, sphalerite, quartz in float, you are in the right ground. Typical habit: aggregates, bladed, prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
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Find bannisterite on the map
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