Picotpaulite
Chemical formula: Tl<sup>1+</sup>Fe<sup>2+</sup>Fe<sup>3+</sup>S<sup>2-</sup><sub>3</sub>
A metallic, black iron-thallium sulfide, occurring in low-temperature hydrothermal veins.
Properties
- Luster
- Metallic
- Streak
- Black
- Density
- 6.13
- Cleavage
- Good on {010} and {100}
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identification of picotpaulite in the field or based on visual characteristics is impossible. The mineral is black, metallic, and occurs as fine grains, similar to many common sulfides. Positive identification requires advanced laboratory techniques, such as X-ray microanalysis (EDS/WDS) to confirm the presence of thallium, iron, and sulfur, and X-ray diffraction (XRD) to determine the crystal structure. ## Distinguishing from Similar Minerals It can be confused with many other black, metallic sulfides, such as pyrite, marcasite, chalcopyrite, or sphalerite. The key distinguishing feature is its chemical composition, particularly the presence of thallium, which is a very rare element. The geological context and co-occurrence with other thallium minerals (e.g., vrbaite, lorandite) can be an important clue. ## Crystal Forms Picotpaulite crystals are extremely rare and microscopic in size. It typically forms irregular, anhedral grains and their aggregates within the rock mass.
Geological environment
## Genesis Picotpaulite is a hydrothermal mineral, crystallizing under low-temperature conditions. It forms in ore veins associated with thallium mineralization. ## Mineral Associations This mineral co-occurs with other, often equally rare, thallium minerals and sulfides. In its type locality (Allchar), it was found in association with vrbaite, raguinite, orpiment, realgar, and marcasite. In other localities, it is accompanied by pierrotite, sphalerite, and pyrite, among others. ## Localities The most important and classic occurrence of picotpaulite is its type locality – the Allchar deposit (Crven Dol mine) in North Macedonia. This is a historic mining region known for its unique thallium mineralization. Additionally, its presence has been reported in the Lanmuchang mercury deposit in Guizhou Province, China.
Rarity
Very rare
For collectors
## Quality Criteria For specialized collectors of rare minerals (so-called "micromounts"), the value of a picotpaulite specimen depends on several factors. The most desirable samples are those where the mineral grains, though small, are clearly visible and well-defined. Specimens rich in associated minerals, especially other rare thallium sulfosalts, forming interesting parageneses, are also highly valued. The quality of a specimen is inextricably linked to precise documentation and analytical confirmation of its identity. ## Popular Localities The only source of collector-grade specimens is the historical type locality: Allchar in North Macedonia. Specimens from this location are classics for systematic collections of rare minerals.
Care and storage
## Cleaning Picotpaulite specimens are typically microscopic grains embedded in the host rock, which generally do not require and should not be cleaned. Any dust can be removed very gently with a soft brush or by blowing it off with compressed air from a safe distance. ## What to Avoid **Toxicity!** The mineral contains thallium – a highly toxic heavy metal. Direct skin contact, inhalation of dust, and any form of ingestion must be strictly avoided. Hands should be thoroughly washed after any contact with the specimen. The mineral should not be heated or exposed to acids. ## Storage Specimens should be stored in tightly sealed, durable containers (e.g., plastic “micromount” boxes), clearly labeled as toxic material. Keep away from children, pets, and food.