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. Streak: brownish-black.
- 3Read the lusterHold the specimen under a strong light. Luster: dull.
- 4Match the color rangeCompare against the expected color range: black, dark brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: earthy, massive.
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 metallic to dull; silky in aggregates on Todorokite.
How to tell apart: Hausmannite is the harder of the two (Mohs 5.5 vs. 1.5); streak differs, so Birnessite leaves brownish-black, Hausmannite leaves dark reddish brown; luster reads dull on Birnessite and submetallic on Hausmannite.
How to tell apart: Psilomelane is the harder of the two (Mohs 5-6 vs. 1.5); streak differs, so Birnessite leaves brownish-black, Psilomelane leaves brownish black; luster reads dull on Birnessite and submetallic on Psilomelane.
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
- 3.0 g/cm³
- Colors
- Streak
- Brownish-black
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Earthy, massive
- Cleavage
- None
- Rarity
- Common
- Uses
- Collector, Scientific research
- Host rock
- A major manganese-bearing mineral of many soils; a common alteration product of manganese-rich mineral deposits; a component of bacterially-precipitated manganese oxides
Where rockhounds find birnessite
Classic worldwide localities
- Sterling Hill, Sussex Co., New Jersey, USA
- Cummington, Hampshire Co., Massachusetts, USA
- Kramer borate deposit, Boron, Kern Co., California, USA
- Silver Cliff, Custer Co., Colorado, USA
- Birness, Aberdeenshire, Scotland (type locality)
- Treburland mine, Altarnun, and the Penberthy Croft mine, St. Hilary, Cornwall, England
Field-hunting tip
Where it forms: A major manganese-bearing mineral of many soils; a common alteration product of manganese-rich mineral deposits; a component of bacterially-precipitated manganese oxides. If you start seeing rhodonite, rhodochrosite, tephroite in float, you are in the right ground. Typical habit: earthy, massive. 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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