Ferri-kaersutite is a rare member of the amphibole supergroup characterized by high iron and titanium content. It typically forms dark, prismatic crystals within alkalic volcanic rocks and is often difficult to distinguish from standard kaersutite without chemical analysis.
Is this ferri-kaersutite?
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 ferri-kaersutite with a known reference. Ferri-kaersutite 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. Ferri-kaersutite leaves a brownish streak.
- 3Read the lusterHold the specimen under a strong light. Ferri-kaersutite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: black, dark brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: prismatic crystals.
Often confused with
Ferri-kaersutite vs. its common look-alikes — and how to tell them apart in the field.
How to tell apart: Streak differs — Ferri-kaersutite leaves brownish, Kaersutite leaves white.
How to tell apart: Streak differs — Ferri-kaersutite leaves brownish, Hornblende leaves grayish-white.
How to tell apart: Streak differs — Ferri-kaersutite leaves brownish, Arfvedsonite leaves grey to bluish-grey.
Often found alongside ferri-kaersutite
Minerals reported to co-occur with ferri-kaersutite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaCa₂ (Mg₃Fe³⁺Ti) (Si₆Al₂) O₂₂O₂
- Mohs hardness
- 5-6
- Density
- 3.3-3.4 g/cm³
- Colors
- Streak
- Brownish
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Prismatic Crystals
- Cleavage
- Perfect Prismatic
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Alkalic Volcanic and Subvolcanic Rocks
- Typical price
- $20-100 per specimen
Where rockhounds find ferri-kaersutite
Classic worldwide localities
- Kaersut, Greenland
- Eifel, Germany
- Kerguelen Islands
- Arizona, USA
Field-hunting tip
Look in alkalic volcanic and subvolcanic rocks country — that is the host setting where ferri-kaersutite typically forms. If you start seeing augite, olivine, magnetite in float, you are in the right ground. Field specimens usually show a prismatic crystals habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify ferri-kaersutite?+
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Find ferri-kaersutite on the map
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