Woodruffite is a rare manganese zinc oxide mineral often found as dark, earthy, or botryoidal crusts in oxidized zones of ore deposits. It is most notable for its historical occurrence in the famous zinc-manganese mines of Franklin and Sterling Hill, New Jersey. Collectors should look for its distinctive brownish streak and association with other rare zinc minerals.
Is this woodruffite?
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 woodruffite with a known reference. Woodruffite 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: brown.
- 3Read the lusterHold the specimen under a strong light. Luster: dull.
- 4Match the color rangeCompare against the expected color range: brown, iron black, black brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, pulverulent.
Often confused with
Woodruffite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Woodruffite leaves brown, Psilomelane leaves brownish black; luster reads dull on Woodruffite and submetallic on Psilomelane.
How to tell apart: Hausmannite is the harder of the two (Mohs 5.5 vs. 4.5); streak differs, so Woodruffite leaves brown, Hausmannite leaves dark reddish brown; luster reads dull on Woodruffite and submetallic on Hausmannite.
How to tell apart: Woodruffite is noticeably harder (Mohs 4.5 vs. 2.5-3); streak differs, so Woodruffite leaves brown, Lithiophorite leaves blackish gray to greenish black.
Often found alongside woodruffite
Minerals reported to co-occur with woodruffite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Zn,Mn²⁺)Mn⁴⁺₃O₇·1-2H₂O
- Mohs hardness
- 4.5
- Density
- 3.71-4.01 g/cm³
- Streak
- Brown
- Luster
- Dull
- Transparency
- Opaque
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, pulverulent
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific research
- Host rock
- In the oxidized portion of a metamorphosed stratiform zinc orebody (Sterling Hill, New Jersey, USA); in massive manganese ores (Sandur, India); in the oxidized zone of a Ag-Pb-Zn ore deposit (Lardeau, Canada)
Where rockhounds find woodruffite
Classic worldwide localities
- Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA (type locality)
- Willett mine, Lardeau, British Columbia, Canada
- Lecht mine, near Tomintoul, Aberdeenshire, Scotland
- Mont Chemin, Valais, Switzerland
- Maria Weitscach, near Hiittenberg, Carinthia, Austria
- Juliushiitte, Astfeld, Harz Mountains, in slag, Germany
Field-hunting tip
Where it forms: In the oxidized portion of a metamorphosed stratiform zinc orebody (Sterling Hill, New Jersey, USA); in massive manganese ores (Sandur, India); in the oxidized zone of a Ag-Pb-Zn ore deposit (Lardeau, Canada). If you start seeing franklinite, chalcophanite, pyrolusite in float, you are in the right ground. Typical habit: botryoidal, pulverulent. Train your eye for that shape before scanning the outcrop.
Common questions
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