Bracewellite is a rare chromium oxyhydroxide mineral that occurs as small, dark needle-like crystals. It is structurally related to diaspore and is typically found in association with chromite deposits within ultramafic rock environments.
Is this bracewellite?
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 bracewellite with a known reference. Bracewellite sits at Mohs 5.5-6.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: dark brown.
- 3Read the lusterHold the specimen under a strong light. Luster: adamantine to metallic.
- 4Match the color rangeCompare against the expected color range: dark reddish brown, reddish brown, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular crystals, prismatic crystals.
Often confused with
Bracewellite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Bracewellite leaves dark brown, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads adamantine to metallic on Bracewellite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
How to tell apart: Streak differs, so Bracewellite leaves dark brown, Diaspore leaves white.
Often found alongside bracewellite
Minerals reported to co-occur with bracewellite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CrO(OH)
- Mohs hardness
- 5.5-6.5
- Density
- 4.45-4.48 g/cm³
- Streak
- Dark brown
- Luster
- Adamantine to metallic
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular crystals, prismatic crystals
- Cleavage
- Perfect in one direction
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In fine-grained intergrowth with other chromium oxide-hydroxide minerals
Where rockhounds find bracewellite
Classic worldwide localities
- Merume River Basin, Mazaruni, Guyana (type locality)
Field-hunting tip
Where it forms: In fine-grained intergrowth with other chromium oxide-hydroxide minerals. If you start seeing eskolaite, grimaldiite, guyanaite in float, you are in the right ground. Typical habit: acicular crystals, prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify bracewellite?+
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What rocks look like bracewellite?+
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Find bracewellite on the map
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