Erniggliite is a highly rare thallium-tin-arsenic sulfosalt found almost exclusively in the famous Lengenbach Quarry of the Binn Valley. It typically occurs as tiny, metallic black tabular crystals embedded within dolomite or associated with other rare sulfosalt species. Due to its extreme rarity and toxicity, it is sought after primarily by advanced systematic mineral collectors.
Is this erniggliite?
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 erniggliite with a known reference. Erniggliite sits at Mohs 2-3, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: reddish black.
- 3Read the lusterHold the specimen under a strong light. Luster: “Shiny” on cleavage surfaces.
- 4Match the color rangeCompare against the expected color range: black, gray, black gray.
- 5Look at form & habitCrystal system: trigonal. Typical habit: tabular crystals.
Often confused with
Erniggliite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Streak differs, so Erniggliite leaves reddish black, Sartorite leaves chocolate-brown; luster reads “shiny” on cleavage surfaces on Erniggliite and metallic on Sartorite.
How to tell apart: Streak differs, so Erniggliite leaves reddish black, Hutchinsonite leaves red; luster reads “shiny” on cleavage surfaces on Erniggliite and adamantine to submetallic on Hutchinsonite.
Often found alongside erniggliite
Minerals reported to co-occur with erniggliite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Tl₂SnAs₂S₆
- Mohs hardness
- 2-3
- Density
- 5.24 g/cm³
- Streak
- Reddish black
- Luster
- “Shiny” on cleavage surfaces
- Transparency
- Opaque
- Crystal system
- Trigonal
- Crystal habit
- Tabular crystals
- Cleavage
- On {0001}, perfect micaceous
- Rarity
- Rare
- Uses
- Collector
- Host rock
- In a hydrothermal deposit in dolostone
Where rockhounds find erniggliite
Classic worldwide localities
- Lengenbach quarry, Binntal, Valais, Switzerland (type locality)
Field-hunting tip
Where it forms: In a hydrothermal deposit in dolostone. If you start seeing hutchinsonite, hatchite, wallisite 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 erniggliite?+
Where is erniggliite found?+
Is erniggliite safe to handle?+
What rocks look like erniggliite?+
What minerals are found with erniggliite?+
What kind of rock does erniggliite form in?+
What is erniggliite used for?+
Find erniggliite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
