Magnesio-riebeckite is a fibrous amphibole mineral often found in metamorphosed banded iron formations. It is most recognized for its asbestiform, silky blue appearance and is often the precursor mineral to Tiger's Eye when replaced by silica.
Is this magnesio-riebeckite?
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 magnesio-riebeckite with a known reference. Magnesio-riebeckite sits at Mohs 5-6 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Magnesio-riebeckite leaves a blue-gray streak.
- 3Read the lusterHold the specimen under a strong light. Magnesio-riebeckite typically shows a silky luster.
- 4Match the color rangeCompare against the expected color range: blue, dark blue, lavender blue.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: fibrous, acicular, asbestiform.
Often confused with
Magnesio-riebeckite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Luster reads silky on Magnesio-riebeckite and vitreous on Riebeckite.
How to tell apart: Streak differs — Magnesio-riebeckite leaves blue-gray, Glaucophane leaves white; luster reads silky on Magnesio-riebeckite and vitreous on Glaucophane.
How to tell apart: Streak differs — Magnesio-riebeckite leaves blue-gray, Arfvedsonite leaves grey to bluish-grey; luster reads silky on Magnesio-riebeckite and vitreous on Arfvedsonite.
Often found alongside magnesio-riebeckite
Minerals reported to co-occur with magnesio-riebeckite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₂Mg₃Fe³⁺₂Si₈O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.0-3.4 g/cm³
- Streak
- Blue-gray
- Luster
- Silky
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, Acicular, Asbestiform
- Cleavage
- Perfect Prismatic
- Rarity
- Uncommon
- Uses
- Collector, Scientific Study
- Host rock
- Metamorphic Rocks
- Typical price
- $10-50 for small specimens
Where rockhounds find magnesio-riebeckite
Classic worldwide localities
- South Africa
- Western Australia
- Bolivia
- Bolivian Andes
Field-hunting tip
Look in metamorphic rocks country — that is the host setting where magnesio-riebeckite typically forms. If you start seeing quartz, hematite, magnetite in float, you are in the right ground. Field specimens usually show a fibrous, acicular, asbestiform habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find magnesio-riebeckite on the map
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