Guyanaite is a rare chromium oxyhydroxide mineral typically found as tiny, dark green to black platy crystals. It is primarily identified in weathered chromium-rich deposits and requires microscopic examination for positive identification.
Is this guyanaite?
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 guyanaite with a known reference. Guyanaite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Guyanaite leaves a yellow-brown, greenish brown streak.
- 3Read the lusterHold the specimen under a strong light. Guyanaite typically shows a submetallic luster.
- 4Match the color rangeCompare against the expected color range: dark green, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: platy, granular.
Often confused with
Guyanaite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Iron Ore is the harder of the two (Mohs 5-6.5 vs. 3-4); streak differs, so Guyanaite leaves yellow-brown, greenish brown, Iron Ore leaves reddish-brown to black; luster reads submetallic on Guyanaite and metallic to submetallic on Iron Ore.
How to tell apart: Lepidocrocite is the harder of the two (Mohs 5 vs. 3-4); streak differs, so Guyanaite leaves yellow-brown, greenish brown, Lepidocrocite leaves orange-red.
Often found alongside guyanaite
Minerals reported to co-occur with guyanaite. 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
- 3-4
- Density
- 4.2-4.5 g/cm³
- Colors
- Streak
- Yellow-brown, Greenish Brown
- Luster
- Submetallic
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, Granular
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Weathered Chromitite Deposits
Where rockhounds find guyanaite
Classic worldwide localities
- Guyana
- Turkey
- Russia
Field-hunting tip
Look in weathered chromitite deposits country, the host setting where guyanaite typically forms. If you start seeing goethite, chromite, quartz in float, you are in the right ground. Field specimens usually show a platy, granular habit, so train your eye for that shape before scanning the outcrop.
Common questions
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