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 — 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 — 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 — 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 — that is 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
How do you identify birnessite?+
Where is birnessite found?+
How much is birnessite worth?+
What rocks look like birnessite?+
What minerals are found with birnessite?+
What kind of rock does birnessite form in?+
What is birnessite used for?+
Find birnessite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
