Pennantite is a manganese-rich member of the chlorite group that typically forms as soft, micaceous plates. Collectors prize it for its distinct brown to orange-brown color, which differentiates it from the common green iron-magnesium chlorites. It is most frequently found in metamorphosed manganese deposits where it occurs alongside minerals like hausmannite.
Is this pennantite?
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 pennantite with a known reference. Pennantite sits at Mohs 2-2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brownish yellow.
- 3Read the lusterHold the specimen under a strong light. Luster: pearly on cleavage faces.
- 4Match the color rangeCompare against the expected color range: orange red, reddish brown.
- 5Look at form & habitCrystal system: triclinic. Typical habit: microscopic crystals, rosette.
Often confused with
Pennantite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Pennantite leaves brownish yellow, Clinochlore leaves greenish white to white.
How to tell apart: Streak differs, so Pennantite leaves brownish yellow, Chamosite leaves green to gray.
How to tell apart: Streak differs, so Pennantite leaves brownish yellow, Cookeite leaves white.
Often found alongside pennantite
Minerals reported to co-occur with pennantite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- (Mn,Al)₆(Si,Al)₄O₁₀(OH)₈
- Mohs hardness
- 2-2.5
- Density
- 2.89-3.07 g/cm³
- Colors
- Streak
- Brownish yellow
- Luster
- Pearly on cleavage faces
- Transparency
- Translucent to nearly opaque
- Crystal system
- Triclinic
- Crystal habit
- Microscopic crystals, rosette
- Cleavage
- {001}, perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Formed during hydrothermal alteration of manganese deposits, as crusts on manganese minerals, and in veins cutting ore
Where rockhounds find pennantite
Classic worldwide localities
- Bald Knob, near Sparta, Alleghany Co., North Carolina, and Franklin, Sussex Co., New Jersey, USA
- Benallt mine, Rhiw, Lleyn Peninsula, Wales
- Ushkatyn and Dzhumart deposits, Atasu district, Kazakhstan
- Harstigen mine, near Persberg, Värmland, Sweden
Field-hunting tip
Where it forms: Formed during hydrothermal alteration of manganese deposits, as crusts on manganese minerals, and in veins cutting ore. If you start seeing banalsite, analcime, paragonite in float, you are in the right ground. Typical habit: microscopic crystals, rosette. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify pennantite?+
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Find pennantite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
