Wightmanite is an extremely rare magnesium borate mineral that typically forms as delicate, white, radiating needles or fibrous masses. It is famously associated with the unique boron-rich mineral assemblage found at the Big Creek locality in California, where it occurs within contact-metamorphosed limestone.
Is this wightmanite?
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 wightmanite with a known reference. Wightmanite sits at Mohs 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: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, light green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: acicular or fibrous radial aggregates.
Often confused with
Wightmanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Wightmanite is noticeably harder (Mohs 5.5 vs. 2.5-3); luster reads vitreous on Wightmanite and dull, earthy to porcelaneous on Boromuscovite.
How to tell apart: Wightmanite is noticeably harder (Mohs 5.5 vs. ~3).
Often found alongside wightmanite
Minerals reported to co-occur with wightmanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₅(BO₃)O(OH)₂·nH₂O
- Mohs hardness
- 5.5
- Density
- 2.60 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Acicular or fibrous radial aggregates
- Cleavage
- On {010}, perfect; on {100}, fair
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Very rare in contact metamorphosed limestone
Where rockhounds find wightmanite
Classic worldwide localities
- Commercial quarry, near Crestmore, Riverside Co., and north of Huntington Lake, Twin Lakes region, Fresno Co., California, USA
Field-hunting tip
Where it forms: Very rare in contact metamorphosed limestone. If you start seeing fluoborite, szaibélyite, dolomite in float, you are in the right ground. Typical habit: acicular or fibrous radial aggregates. Train your eye for that shape before scanning the outcrop.
Common questions
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Find wightmanite on the map
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