Crocidolite is the asbestiform variety of riebeckite, known for its distinct deep blue, fibrous, and silky appearance. It occurs primarily in iron-rich metamorphic rocks and is famously the precursor for tiger's eye, where it is replaced by silica. Due to its toxicity as an amphibole asbestos, collectors should store it in sealed containers to prevent fiber inhalation.
Is this crocidolite?
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 crocidolite with a known reference. Crocidolite sits at Mohs 4 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Crocidolite leaves a light blue streak.
- 3Read the lusterHold the specimen under a strong light. Crocidolite typically shows a silky luster.
- 4Match the color rangeCompare against the expected color range: blue, lavender-blue, indigo.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, asbestiform.
Often confused with
Crocidolite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Riebeckite is the harder of the two (Mohs 5-6 vs. 4); streak differs — Crocidolite leaves light blue, Riebeckite leaves blue-gray; luster reads silky on Crocidolite and vitreous on Riebeckite.
How to tell apart: Glaucophane is the harder of the two (Mohs 6 vs. 4); streak differs — Crocidolite leaves light blue, Glaucophane leaves white; luster reads silky on Crocidolite and vitreous on Glaucophane.
How to tell apart: Crocidolite is noticeably harder (Mohs 4 vs. 2.5-3); streak differs — Crocidolite leaves light blue, Chrysotile leaves white.
Often found alongside crocidolite
Minerals reported to co-occur with crocidolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₂(Fe²⁺₃Fe³⁺₂)Si₈O₂₂(OH)₂
- Mohs hardness
- 4
- Density
- 3.2-3.4 g/cm³
- Colors
- Streak
- Light Blue
- Luster
- Silky
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, Asbestiform
- Cleavage
- Perfect Prismatic
- Rarity
- Common
- Uses
- Collector
- Host rock
- Metamorphosed Banded Iron Formations
- Typical price
- $10-100 per specimen depending on size and fibrous quality
Where rockhounds find crocidolite
2 mapped spotsClassic worldwide localities
- Cape Province, South Africa
- Western Australia
- Bolivia
- Russia
Field-hunting tip
Look in metamorphosed banded iron formations country — that is the host setting where crocidolite typically forms. If you start seeing quartz, magnetite, hematite in float, you are in the right ground. Field specimens usually show a fibrous, asbestiform habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in California, Utah — start trip planning there.
Common questions
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Find crocidolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
