Pierrotite
Chemical formula: Tl<sup>1+</sup><sub>2</sub>(Sb<sup>3+</sup>,As<sup>3+</sup>)<sub>10</sub>S<sup>2-</sup><sub>16</sub>
Pierrotite is a rare thallium, antimony, and arsenic sulfosalt, forming prismatic, almost black crystals with a metallic luster.
Properties
- Mohs hardness
- 3-3.5
- Luster
- Metallic
- Streak
- Reddish-brown
- Density
- 5.26-5.3
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Pierrotite can be identified by its dark gray to black color, metallic luster, and characteristic prismatic crystals forming acicular or radial aggregates. The reddish-brown streak is an important diagnostic feature. High density can also be helpful in identification. ## Distinguishing from Similar Minerals It can be confused with other rare thallium sulfosalts, such as lorandite or raguinite. However, lorandite has an intense red color and streak. Raguinite has a similar appearance but crystallizes in the monoclinic system and has slightly different physical properties. Final differentiation from other dark sulfosalts often requires advanced chemical analysis (EDS/WDS). ## Crystal Forms Pierrotite crystals are prismatic, elongated along one axis, and often exhibit striations on the faces parallel to the elongation direction. They usually occur as tangled, fibrous, or radial aggregates.
Geological environment
## Genesis Pierrotite is a low-temperature hydrothermal mineral. It forms in ore deposits where solutions rich in thallium, arsenic, and antimony are present. Its formation is associated with volcanic activity and the circulation of thermal waters. ## Mineral Associations This mineral co-occurs with other sulfosalts and sulfides. In its type locality (Jas Roux in France), it was found in association with stibnite, pyrite, sphalerite, realgar, orpiment, and other rare thallium sulfosalts such as raguinite, parapierrotite, and chabournéite. ## Localities The most important and type locality for pierrotite is the Jas Roux deposit in the Hautes-Alpes department of France. This is the only locality from which well-formed, identifiable specimens originate. Outside of France, its presence has been noted in the Allchar deposit in North Macedonia and in the Urup mine in Russia (North Caucasus).
Rarity
Very rare
For collectors
## Quality Criteria The most prized pierrotite specimens are those with well-formed, sharp crystals with a distinct metallic luster, forming aesthetic, radial aggregates. Contrast with a light matrix (most often dolomite or quartz) significantly enhances visual appeal. Crystal size is crucial – specimens with needles exceeding several millimeters are exceptionally desirable. ## Popular Localities The only source of collectible pierrotite specimens is its type locality – Jas Roux in France. Specimens from this location are considered the best in the world and set the standard for this mineral.
Care and storage
## Cleaning Pierrotite specimens should be cleaned only with extreme caution, using a soft brush to remove dust. Avoid contact with water and any chemical agents. Due to the presence of toxic thallium and arsenic, all activities should be performed with gloves, and inhalation of dust should be avoided. ## What to Avoid Absolutely avoid contact with water, acids, and other chemicals. The mineral is sensitive to changes in temperature and humidity. Do not heat it or expose it to direct sunlight. Skin contact should be minimized. ## Storage Pierrotite should be stored in a dry, stable environment, preferably in a closed, airtight display box, away from other minerals that it could scratch. It is necessary to clearly label the specimen as toxic (contains thallium and arsenic). After each contact with the mineral, hands should be thoroughly washed.