Grimaldiite is an extremely rare chromium oxyhydroxide mineral that typically forms small, shiny black platy crystals. It is most frequently found in serpentinite environments and is highly sought after by advanced mineral collectors due to its scarcity and distinct trigonal symmetry.
Is this grimaldiite?
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 grimaldiite with a known reference. Grimaldiite sits at Mohs 3.5-4.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: red.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: dark red, reddish brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: platy crystals, pseudohexagonal.
Often confused with
Grimaldiite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Grimaldiite leaves red, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads metallic on Grimaldiite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
How to tell apart: Streak differs, so Grimaldiite leaves red, Lepidocrocite leaves dull orange; luster reads metallic on Grimaldiite and submetallic on Lepidocrocite.
Often found alongside grimaldiite
Minerals reported to co-occur with grimaldiite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CrOOH
- Mohs hardness
- 3.5-4.5
- Density
- 4.11-4.12 g/cm³
- Colors
- Streak
- Red
- Luster
- Metallic
- Transparency
- Transparent, translucent in intergrowths with mcconnellite
- Crystal system
- Trigonal
- Crystal habit
- Platy crystals, pseudohexagonal
- Cleavage
- {0001}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In fine-grained intergrowth with other chromium oxide-hydroxide minerals in alluvial gravels (Merume River, Guyana); in cracks and crevices in penroseite from a hydrothermal mineral deposit (Hiaco, Bolivia)
Where rockhounds find grimaldiite
Classic worldwide localities
- Basin of the Merume River, Guyana
- Hiaca mine, 30 km north-northeast of Colquechaca, Potosí, Bolivia
Field-hunting tip
Where it forms: In fine-grained intergrowth with other chromium oxide-hydroxide minerals in alluvial gravels (Merume River, Guyana); in cracks and crevices in penroseite from a hydrothermal mineral deposit (Hiaco, Bolivia). If you start seeing bracewellite, eskolaite, guyanaite in float, you are in the right ground. Typical habit: platy crystals, pseudohexagonal. Train your eye for that shape before scanning the outcrop.
Common questions
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Find grimaldiite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
