Chestermanite is a rare borate mineral belonging to the ludwigite group, typically found in contact metamorphic skarns. It usually appears as fine, needle-like or fibrous dark crystals that are best identified via micro-analysis. Collectors primarily source it from specific geological occurrences where boron-rich fluids have reacted with magnesium-rich host rocks.
Is this chestermanite?
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 chestermanite with a known reference. Chestermanite sits at Mohs 6, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: pale grayish green.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous to silky.
- 4Match the color rangeCompare against the expected color range: black, gray green, black green.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic.
Often confused with
Chestermanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Chestermanite leaves pale grayish green, Ludwigite leaves black to greenish black; luster reads vitreous to silky on Chestermanite and submetallic on Ludwigite.
How to tell apart: Chestermanite is noticeably harder (Mohs 6 vs. 5); streak differs, so Chestermanite leaves pale grayish green, Vonsenite leaves brownish black; luster reads vitreous to silky on Chestermanite and metallic to submetallic, silky if fibrous on Vonsenite.
Often found alongside chestermanite
Minerals reported to co-occur with chestermanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₂(Fe,Mg,Al,Sb)O₂BO₃
- Mohs hardness
- 6
- Density
- 3.72 g/cm³
- Streak
- Pale grayish green
- Luster
- Vitreous to silky
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Rare, in veins and disseminated in a brucite marble lens contained in a multiply metamorphosed roof pendant between two plutons in a granodiorite batholith
Where rockhounds find chestermanite
Classic worldwide localities
- Twin Lakes region, Fresno County, California, USA (type locality)
Field-hunting tip
Where it forms: Rare, in veins and disseminated in a brucite marble lens contained in a multiply metamorphosed roof pendant between two plutons in a granodiorite batholith. If you start seeing magnesian–aluminian ludwigite, magnesian calcite, fluoborite in float, you are in the right ground. Typical habit: prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify chestermanite?+
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Find chestermanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
