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. Chestermanite leaves a brown streak.
- 3Read the lusterHold the specimen under a strong light. Chestermanite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: black, brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: fibrous, acicular, radiating masses.
Often confused with
Chestermanite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Chestermanite is noticeably harder (Mohs 6 vs. 5); streak differs — Chestermanite leaves brown, Ludwigite leaves black.
How to tell apart: Chestermanite is noticeably harder (Mohs 6 vs. 5); streak differs — Chestermanite leaves brown, Vonsenite leaves black.
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(Mg₀.₅Al₀.₅)(BO₃)O₂
- Mohs hardness
- 6
- Density
- 3.84 g/cm³
- Streak
- Brown
- Luster
- Submetallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Fibrous, Acicular, Radiating Masses
- Cleavage
- Poor
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Metasomatic Rocks and Skarns
- Typical price
- $20-150 for micro-mounts and small specimens
Where rockhounds find chestermanite
Classic worldwide localities
- Brooks Mountain, Alaska, USA
- Kola Peninsula, Russia
Field-hunting tip
Look in metasomatic rocks and skarns country — that is the host setting where chestermanite typically forms. If you start seeing calcite, diopside, forsterite in float, you are in the right ground. Field specimens usually show a fibrous, acicular, radiating masses habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find chestermanite on the map
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