Rasvumite is a rare potassium iron sulfide typically found as dark, metallic, needle-like or prismatic crystals in alkaline pegmatites. It is best known from the Khibiny Massif in Russia, where it occurs within hyper-agpaitic rocks associated with minerals like aegirine and nepheline.
Is this rasvumite?
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 rasvumite with a known reference. Rasvumite sits at Mohs 4-5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: steel gray, brownish black, gray, black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: acicular, encrustations.
Often confused with
Rasvumite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Pyrite is the harder of the two (Mohs 6-6.5 vs. 4-5); streak differs, so Rasvumite leaves black, Pyrite leaves greenish black to brownish black.
How to tell apart: Luster reads metallic on Rasvumite and submetallic on Djerfisherite.
How to tell apart: Streak differs, so Rasvumite leaves black, Chalcopyrite leaves greenish black.
Often found alongside rasvumite
Minerals reported to co-occur with rasvumite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- KFe₂S₃
- Mohs hardness
- 4-5
- Density
- 3.1 g/cm³
- Streak
- Black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Acicular, encrustations
- Cleavage
- Perfect on {110}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Typically an alteration product of pyrrhotite; in differentiated alkalic massifs (Russia); in an alkalic mafic diatreme (Coyote Peak, California, USA)
Where rockhounds find rasvumite
Classic worldwide localities
- Coyote Peak, near Orick, Humboldt Co., California, and from Point of Rocks, Colfax Co., New Mexico, USA
- Kola Peninsula, from Mts. Rasvumchorr (type locality), Koashva, and Yukspor, and in the Kirov apatite mine, Mt. Kukisvumchorr (type locality), Khibiny massif, Russia
- Lovozero massif, Russia
- Oslo rift, southern Norway
- Mont Saint-Hilaire, Quebec, Canada
- Oldoinyo Lengai volcano, Tanzania
Field-hunting tip
Where it forms: Typically an alteration product of pyrrhotite; in differentiated alkalic massifs (Russia); in an alkalic mafic diatreme (Coyote Peak, California, USA). If you start seeing djerfisherite, chlorbartonite, pyrrhotite in float, you are in the right ground. Typical habit: acicular, encrustations. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rasvumite?+
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What rocks look like rasvumite?+
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Find rasvumite on the map
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