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. Streak: blue-gray.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: blue, black.
- 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: Streak differs, so Magnesio-riebeckite leaves blue-gray, Riebeckite leaves greenish brown.
How to tell apart: Streak differs, so Magnesio-riebeckite leaves blue-gray, Arfvedsonite leaves deep bluish gray, gray-green.
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
- 3.12-3.29 g/cm³
- Streak
- Blue-gray
- Luster
- Vitreous
- Transparency
- Semitransparent
- Crystal system
- Monoclinic
- Crystal habit
- Fibrous, acicular, asbestiform
- Cleavage
- Perfect on {110}, intersecting at ~58° and ~122°; partings on {010}, {001}
- Rarity
- Uncommon
- Uses
- Collector, Scientific study
- Host rock
- In granulites, ironstones, ferruginous metacherts, greenschist facies schists, and carbonatites
Where rockhounds find magnesio-riebeckite
Classic worldwide localities
- Franklin, Sussex Co., New Jersey, USA
- Ternovsk mine, Krivoy-Rog basin, Ukraine
- Norra Kärr complex, near Gränna, Sweden
- Chapare, Cochabamba Province, Bolivia
- Over a large area from Koegas, Cape Province, to Botswana, and to the east of Pietersburg, Transvaal, South Africa
- Lusaka, Zambia
Field-hunting tip
Where it forms: In granulites, ironstones, ferruginous metacherts, greenschist facies schists, and carbonatites. If you start seeing stilpnomelane, epidote, quartz in float, you are in the right ground. Typical habit: fibrous, acicular, asbestiform. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify magnesio-riebeckite?+
Where is magnesio-riebeckite found?+
Is magnesio-riebeckite safe to handle?+
What rocks look like magnesio-riebeckite?+
What minerals are found with magnesio-riebeckite?+
What kind of rock does magnesio-riebeckite form in?+
What is magnesio-riebeckite used for?+
Find magnesio-riebeckite on the map
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