Hephaistosite is an extremely rare thallium-mercury chloride mineral found exclusively in the ancient metallurgical slags of Laurion, Greece. It typically occurs as minute, vibrant yellow-green tabular crystals or crusts coating other rare secondary minerals in the slag environment.

Hardness

1.5-2

Mohs

Transparency

Transparent

mineralrare

Hephaistosite

Hardness: 1.5-2
Transparency: Transparent

Is this hephaistosite?

3-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch hephaistosite with a known reference. Hephaistosite sits at Mohs 1.5-2, softer than the next harder reference, harder than the previous one.
2
Match the color range
Compare against the expected color range: pale yellow.
3
Look at form & habit
Crystal system: monoclinic. Typical habit: aggregates.

Often confused with

Hephaistosite vs. its common look-alikes, and how to tell them apart in the field.

How to tell apart: Laurionite is the harder of the two (Mohs 3-3.5 vs. 1.5-2).

Compare →

Often found alongside hephaistosite

Minerals reported to co-occur with hephaistosite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: TlPb₂Cl₅
Mohs hardness: 1.5-2
Density: 5.932 g/cm³
Colors: Pale yellow
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Aggregates
Rarity: Rare
Uses: Collector
Host rock: On fragments of altered pyroclastic breccia in a high temperature (400° C) fumarole, most likely formed as a sublimate

Where rockhounds find hephaistosite

Classic worldwide localities

La Fossa crater, Vulcano, Aeolian Islands, Italy

Field-hunting tip

Where it forms: On fragments of altered pyroclastic breccia in a high temperature (400° C) fumarole, most likely formed as a sublimate. If you start seeing bismuthinite, cotunnite, challacolloite in float, you are in the right ground. Typical habit: aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hephaistosite?+

Mohs hardness is 1.5-2. Common colors include pale yellow.

Where is hephaistosite found?+

Notable localities include La Fossa crater, Vulcano, Aeolian Islands, Italy.

Is hephaistosite safe to handle?+

Hephaistosite contains lead and thallium and calls for real care. Contains thallium and mercury, both of which are highly toxic; handle with extreme caution, wear gloves, and wash hands thoroughly after contact.

What rocks look like hephaistosite?+

Hephaistosite is most often confused with Calomel, Laurionite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with hephaistosite?+

Hephaistosite commonly co-occurs with Bismuthinite, Cotunnite, Challacolloite, Pseudocotunnite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hephaistosite form in?+

Where it forms: On fragments of altered pyroclastic breccia in a high temperature (400° C) fumarole, most likely formed as a sublimate. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hephaistosite used for?+

Hephaistosite is used in collector.

Find hephaistosite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.