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. Streak: yellow-brown, greenish brown.
- 3Read the lusterHold the specimen under a strong light. Luster: submetallic.
- 4Match the color rangeCompare against the expected color range: greenish brown, golden brown, reddish brown.
- 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: Goethite is the harder of the two (Mohs 5-5.5 vs. 3-4); streak differs, so Guyanaite leaves yellow-brown, greenish brown, Goethite leaves brownish yellow, yellow-orange, ocher-yellow; luster reads submetallic on Guyanaite and imperfect adamantine metallic to dull earthy; silky when fibrous on Goethite.
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 dull orange.
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
- 4.53 g/cm³
- Streak
- Yellow-brown, greenish brown
- Luster
- Submetallic
- Transparency
- Semitransparent
- Crystal system
- Orthorhombic
- Crystal habit
- Platy, granular
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In fine-grained intergrowth with other chromium oxide-hydroxide minerals (Merume River, Guyana); in chromium-rich tremolite skarns, metaquartzites, and chlorite veins (Outokumpu, Finland)
Where rockhounds find guyanaite
Classic worldwide localities
- Merume River and its tributaries, Mazaruni district, Guyana
- Outokumpu, Finland
Field-hunting tip
Where it forms: In fine-grained intergrowth with other chromium oxide-hydroxide minerals (Merume River, Guyana); in chromium-rich tremolite skarns, metaquartzites, and chlorite veins (Outokumpu, Finland). If you start seeing bracewellite, eskolaite, grimaldiite in float, you are in the right ground. Typical habit: platy, granular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find guyanaite on the map
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