Fencooperite is a rare iron-bearing silicate mineral typically found as tabular, hexagonal, brown crystals. It is most famously associated with the hyper-alkaline environments of the Khibiny Massif in Russia. Collectors prize it for its unique chemical structure and rarity within syenite pegmatites.
Is this fencooperite?
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 fencooperite with a known reference. Fencooperite sits at Mohs 4.5-5, 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: vitreous to adamantine.
- 4Match the color rangeCompare against the expected color range: jet black, dirty gray brown.
- 5Look at form & habitCrystal system: trigonal. Typical habit: aggregates, anhedral grains, platy.
Often confused with
Fencooperite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Fencooperite leaves grayish black, Ekanite leaves white.
How to tell apart: Fencooperite is noticeably harder (Mohs 4.5-5 vs. 2-3); streak differs, so Fencooperite leaves grayish black, Barytolamprophyllite leaves light brown.
Often found alongside fencooperite
Minerals reported to co-occur with fencooperite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ba₆Fe₃Si₈O₂₃(CO₃)₂Cl₃·H₂O
- Mohs hardness
- 4.5-5
- Density
- 4.338 g/cm³
- Colors
- Jet blackDirty gray brown
- Streak
- Grayish black
- Luster
- Vitreous to adamantine
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Aggregates, anhedral grains, platy
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In gillespite-rich zones in sanbornite-quartz lenses in low-grade metasedimentary rocks
Where rockhounds find fencooperite
Classic worldwide localities
- Trumbull Peak, near El Portal, Mariposa County and at the Esquire no. 7 claim, Fresno County, California, USA
Field-hunting tip
Where it forms: In gillespite-rich zones in sanbornite-quartz lenses in low-grade metasedimentary rocks. If you start seeing titantaramellite, anandite, kinoshitalite in float, you are in the right ground. Typical habit: aggregates, anhedral grains, platy. Train your eye for that shape before scanning the outcrop.
Common questions
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Find fencooperite on the map
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