Armbrusterite is a rare manganese-bearing silicate mineral found in alkaline igneous complexes. It typically presents as platy or tabular crystals and is notable for its distinct bright yellow fluorescence under short-wave ultraviolet light.
Is this armbrusterite?
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 armbrusterite with a known reference. Armbrusterite sits at Mohs 3.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: light brown.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: dark reddish brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: platy, spherical.
Often confused with
Armbrusterite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Labuntsovite is the harder of the two (Mohs 6 vs. 3.5); streak differs, so Armbrusterite leaves light brown, Labuntsovite leaves white; luster reads vitreous on Armbrusterite and greasy on Labuntsovite.
How to tell apart: Streak differs, so Armbrusterite leaves light brown, Astrophyllite leaves golden; luster reads vitreous on Armbrusterite and submetallic, pearly, greasy on Astrophyllite.
Often found alongside armbrusterite
Minerals reported to co-occur with armbrusterite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na₆K₅MnMn₁₄[Si₉O₂₂]₄(OH)₁₀·4H₂O
- Mohs hardness
- 3.5
- Density
- 2.78 g/cm³
- Colors
- Streak
- Light brown
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Platy, spherical
- Cleavage
- Perfect on {001}
- Fluorescence
- Bright yellow under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Modulated manganese phyllosilicate
Where rockhounds find armbrusterite
Classic worldwide localities
- Mt. Kukisvumchorr, Kola Peninsula, Russia (type locality)
Field-hunting tip
Where it forms: Modulated manganese phyllosilicate. If you start seeing aegirine, microcline, nepheline in float, you are in the right ground. Typical habit: platy, spherical. Train your eye for that shape before scanning the outcrop.
Common questions
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Find armbrusterite on the map
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