Magnesio-arfvedsonite is a rare member of the amphibole supergroup typically found in alkaline igneous environments. It usually occurs as dark, prismatic to acicular crystals often associated with other rare alkaline minerals in syenite complexes.
Is this magnesio-arfvedsonite?
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-arfvedsonite with a known reference. Magnesio-arfvedsonite 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: grayish.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: black, gray.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals, acicular, fibrous, columnar.
Often confused with
Magnesio-arfvedsonite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Magnesio-arfvedsonite leaves grayish, Arfvedsonite leaves deep bluish gray, gray-green.
How to tell apart: Streak differs, so Magnesio-arfvedsonite leaves grayish, Riebeckite leaves greenish brown.
How to tell apart: Streak differs, so Magnesio-arfvedsonite leaves grayish, Hornblende leaves grayish-white.
Often found alongside magnesio-arfvedsonite
Minerals reported to co-occur with magnesio-arfvedsonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaNa₂(Mg₄Fe³⁺)Si₈O₂₂(OH)₂
- Mohs hardness
- 5-6
- Density
- 3.17 g/cm³
- Streak
- Grayish
- Luster
- Vitreous
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic crystals, acicular, fibrous, columnar
- Cleavage
- Perfect on {110}, intersecting at ~56° and ~124°
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As exsolution lamellae in cummingtonite from metamorphosed iron formation; in felsic pegmatites; from sodium metasomatism (fenitization) around some carbonatites
Where rockhounds find magnesio-arfvedsonite
Classic worldwide localities
- Green River Formation, head of Avintagnia Canyon, Duchesne Co., Utah, USA
- Norra Kärr complex, near Gränna, Sweden
- Gardiner complex, beyond the head of the Kangerdlugssuaq Fjord, Greenland
- Khibiny and Lovozero massifs, Kola Peninsula, Russia
- Wabush Iron Formation, southwestern Labrador, Newfoundland, Canada
- Perkins, and at Mont Saint-Hilaire, Quebec, Canada
Field-hunting tip
Where it forms: As exsolution lamellae in cummingtonite from metamorphosed iron formation; in felsic pegmatites; from sodium metasomatism (fenitization) around some carbonatites. If you start seeing aegirine, braunite, winchite in float, you are in the right ground. Typical habit: prismatic crystals, acicular, fibrous, columnar. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify magnesio-arfvedsonite?+
Where is magnesio-arfvedsonite found?+
What rocks look like magnesio-arfvedsonite?+
What minerals are found with magnesio-arfvedsonite?+
What kind of rock does magnesio-arfvedsonite form in?+
What is magnesio-arfvedsonite used for?+
Find magnesio-arfvedsonite on the map
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