Capgaronnite is an extremely rare mercury-silver halide mineral named after its type locality at the Cap Garonne mine in France. It typically forms small, clear, tabular crystals in supergene environments where lead and copper-rich ores have undergone alteration.
Is this capgaronnite?
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 capgaronnite with a known reference. Capgaronnite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: gray-black.
- 3Read the lusterHold the specimen under a strong light. Luster: subadamantine to submetallic.
- 4Match the color rangeCompare against the expected color range: black, brownish black.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: tabular crystals.
Often confused with
Capgaronnite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Capgaronnite leaves gray-black, Halite leaves white; luster reads subadamantine to submetallic on Capgaronnite and vitreous on Halite.
How to tell apart: Streak differs, so Capgaronnite leaves gray-black, Chlorargyrite leaves white; luster reads subadamantine to submetallic on Capgaronnite and resinous to adamantine, waxy on Chlorargyrite.
How to tell apart: Streak differs, so Capgaronnite leaves gray-black, Boleite leaves blue, with greenish tint; luster reads subadamantine to submetallic on Capgaronnite and weakly vitreous; pearly on cleavage surfaces on Boleite.
Often found alongside capgaronnite
Minerals reported to co-occur with capgaronnite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- AgHgClS
- Mohs hardness
- 2.5
- Density
- 6.19 g/cm³
- Colors
- Streak
- Gray-black
- Luster
- Subadamantine to submetallic
- Transparency
- Translucent to opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Tabular crystals
- Cleavage
- {010}, perfect
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Probably formed from oxidation of Hg–Ag-rich tennantite exposed to seawater (Cap Garonne mine, France)
Where rockhounds find capgaronnite
Classic worldwide localities
- Cap Garonne mine, near le Pradet, Var, France
- Chañarcillo, south of Copiapó, Atacama, Chile
- Broken Hill Proprietary mine, New South Wales, Australia
Field-hunting tip
Where it forms: Probably formed from oxidation of Hg–Ag-rich tennantite exposed to seawater (Cap Garonne mine, France). If you start seeing olivenite, cyanotrichite, brochantite in float, you are in the right ground. Typical habit: tabular crystals. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify capgaronnite?+
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What rocks look like capgaronnite?+
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Find capgaronnite on the map
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