Birnessite is a layered manganese oxide mineral that is a primary component of marine nodules and common in soil weathering environments. It typically appears as dull, black, earthy masses or crusts and is rarely found as well-defined macroscopic crystals due to its fine-grained habit.
Is this birnessite?
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 birnessite with a known reference. Birnessite sits at Mohs 1.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Birnessite leaves a brownish-black streak.
- 3Read the lusterHold the specimen under a strong light. Birnessite typically shows a dull luster.
- 4Match the color rangeCompare against the expected color range: black, brownish-black.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive, earthy, botryoidal, crusts.
Often confused with
Birnessite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Birnessite leaves brownish-black, Todorokite leaves black; luster reads dull on Birnessite and submetallic on Todorokite.
How to tell apart: Hausmannite is the harder of the two (Mohs 5-5.5 vs. 1.5); streak differs, so Birnessite leaves brownish-black, Hausmannite leaves brownish-red; luster reads dull on Birnessite and submetallic on Hausmannite.
How to tell apart: Romanèchite is the harder of the two (Mohs 5-6 vs. 1.5); streak differs, so Birnessite leaves brownish-black, Romanèchite leaves shiny brownish black; luster reads dull on Birnessite and submetallic to dull on Romanèchite.
Often found alongside birnessite
Minerals reported to co-occur with birnessite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Na,Ca,K)₀.₆(Mn⁴⁺,Mn³⁺)₂O₄·1.5H₂O
- Mohs hardness
- 1.5
- Density
- 2.8-3.0 g/cm³
- Colors
- Streak
- Brownish-black
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Massive, Earthy, Botryoidal, Crusts
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Scientific Research
- Host rock
- Sedimentary Deposits, Soil Weathering Crusts, Marine Manganese Nodules
- Typical price
- $10-50 for small specimens
Where rockhounds find birnessite
Classic worldwide localities
- Birness, Scotland
- Various oceanic manganese nodules
- Soil profiles worldwide
- Hydrothermal vent deposits
Field-hunting tip
Look in sedimentary deposits, soil weathering crusts, marine manganese nodules country, the host setting where birnessite typically forms. If you start seeing pyrolusite, manganite, goethite in float, you are in the right ground. Field specimens usually show a massive, earthy, botryoidal, crusts habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find birnessite on the map
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