Cobaltarthurite is a rare secondary mineral formed in the oxidized zones of cobalt-rich hydrothermal deposits. It typically appears as striking blue or straw-yellow to dark brown acicular crystal sprays or coatings associated with other arsenate minerals.
Is this cobaltarthurite?
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 cobaltarthurite with a known reference. Cobaltarthurite sits at Mohs 3.5-4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white to light brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous (crystals); silky (tufts).
- 4Match the color rangeCompare against the expected color range: straw yellow, dark brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: globular, massive, oolitic.
Often confused with
Cobaltarthurite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Cobaltarthurite leaves white to light brown, Arthurite leaves light green.
How to tell apart: Cobaltarthurite is noticeably harder (Mohs 3.5-4 vs. 2.5); streak differs, so Cobaltarthurite leaves white to light brown, Lavendulan leaves light blue.
Often found alongside cobaltarthurite
Minerals reported to co-occur with cobaltarthurite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CoFe₂³⁺(AsO₄)₂(OH)₂·4H₂O
- Mohs hardness
- 3.5-4
- Density
- 3.22 g/cm³
- Colors
- Streak
- White to light brown
- Luster
- Vitreous (crystals); silky (tufts)
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Globular, massive, oolitic
- Cleavage
- None observed
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Product of oxidation in a near-surface, sulfide-depleted lens
Where rockhounds find cobaltarthurite
Classic worldwide localities
- Dolores showing, near Pastrana, ~10 km east of Mazarrón, Province of Murcia, southeastern Spain
- Khder, Bou Azzer district, Morocco
Field-hunting tip
Where it forms: Product of oxidation in a near-surface, sulfide-depleted lens. If you start seeing pharmacosiderite, conichalcite, olivenite in float, you are in the right ground. Typical habit: globular, massive, oolitic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify cobaltarthurite?+
Where is cobaltarthurite found?+
Is cobaltarthurite safe to handle?+
What rocks look like cobaltarthurite?+
What minerals are found with cobaltarthurite?+
What kind of rock does cobaltarthurite form in?+
What is cobaltarthurite used for?+
Find cobaltarthurite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
