Franckeite is a complex lead-tin sulfide mineral often found in distinctive thin, flexible, tabular, or fan-shaped crystals. It is primarily sourced from tin-bearing hydrothermal deposits in Bolivia and is highly prized by collectors for its unique layered crystal structure.
Is this franckeite?
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 franckeite with a known reference. Franckeite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: grayish black.
- 3Read the lusterHold the specimen under a strong light. Luster: metallic.
- 4Match the color rangeCompare against the expected color range: grayish black, blackish gray.
- 5Look at form & habitCrystal system: triclinic. Typical habit: massive, radial, tabular.
Often confused with
Franckeite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Franckeite leaves grayish black, Cylindrite leaves black.
How to tell apart: Streak differs, so Franckeite leaves grayish black, Galena leaves lead-gray.
How to tell apart: Franckeite is noticeably harder (Mohs 2.5-3 vs. 1.5); streak differs, so Franckeite leaves grayish black, Teallite leaves black.
Often found alongside franckeite
Minerals reported to co-occur with franckeite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Pb₂₁.₇₄Sn₉.₃₄Fe₃.₃₄Fe₀.₆₆Sb₈.₀₈S₅₆.₈₇
- Mohs hardness
- 2.5-3
- Density
- 5.5-5.9 g/cm³
- Colors
- Grayish blackBlackish gray
- Streak
- Grayish black
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Triclinic
- Crystal habit
- Massive, radial, tabular
- Cleavage
- {010}, perfect
- Rarity
- Rare
- Uses
- Collector, Geological study
- Host rock
- In hydrothermal Ag-Sn deposits (Bolivia); in a limestone contact metamorphic deposit (Kalkar quarry, California, USA)
Where rockhounds find franckeite
Classic worldwide localities
- Thompson mine, Darwin district, Inyo Co., and the Kalkar quarry, Santa Cruz, Santa Cruz Co., California, USA
- Chocaya (type locality), Poopó, Oruro, Llallagua, Huanuni, Colquechaca, and Cerro Rico, Potosí, Bolivia
- Pirquitas mine, Jujuy Province, Argentina
- Headwaters of the east branch of the Coal River, Yukon Territory, Canada
- Vens Haut, Cantal, France
- Sinantscha zinc deposit, Sichota-Alin, and from Smirnowsk, Transbaikalia, Russia
Field-hunting tip
Where it forms: In hydrothermal Ag-Sn deposits (Bolivia); in a limestone contact metamorphic deposit (Kalkar quarry, California, USA). If you start seeing cylindrite, teallite, plagionite in float, you are in the right ground. Typical habit: massive, radial, tabular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find franckeite on the map
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