Dalnegroite
Chemical formula: Tl¹⁺₄Pb²⁺₂(As³⁺,Sb³⁺)₂₀S²⁻₃₄
Dalnegroite is a very rare lead-thallium sulfosalt with a metallic luster, found in hydrothermal deposits.
Properties
- Mohs hardness
- 3-3.5
- Color
- Red
- Luster
- Metallic
- Streak
- Brownish-red
- Density
- 4.82
- Cleavage
- None
- Fracture
- Irregular/Uneven
- Transparency
- Opaque
- Crystal system
- Triclinic
Diagnostic features
## Identification Identification of dalnegroite is possible only through advanced laboratory methods, such as chemical composition analysis using an electron microprobe (EDS/WDS) and X-ray diffraction (XRD). Visually, it is a black, metallic mineral occurring as microscopic inclusions in realgar. A characteristic feature in studies is its high content of thallium, lead, arsenic, and antimony. ## Distinguishing from Similar Minerals It can be confused with many other black and metallic sulfosalts that often co-occur with it, such as pierrotite, parapierrotite, or other Tl-As-Sb sulfosalts. Definitive differentiation is only possible based on chemical analysis. ## Crystal Forms Dalnegroite forms exclusively anhedral, i.e., irregular, anhedral grains not exceeding 0.2 mm in size. They occur as inclusions in other minerals.
Geological environment
## Genesis Dalnegroite forms as a result of low-temperature hydrothermal processes. It crystallizes in the final stages of ore vein formation, in an environment rich in thallium, arsenic, antimony, lead, and sulfur. Its formation at the Vorontsovskoye deposit is associated with gold and mercury mineralization. ## Mineral Associations This mineral occurs in close association with realgar (in which it forms inclusions), as well as with orpiment, parapierrotite, pierrotite, routhierite, wakabayashilite, stibnite, pyrite, marcasite, getchellite, and quartz. ## Localities The only confirmed occurrence (type locality) of dalnegroite in the world is the Vorontsovskoye gold deposit, located near Krasnoturinsk in the Northern Urals, Russia.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of a dalnegroite specimen is assessed solely based on the quality of the host mineral in which it occurs – most often realgar. Since dalnegroite is a microscopic mineral, its own visual characteristics are not significant. Its scientific and collecting value lies in the mere confirmation of the presence of this extremely rare mineral on the specimen, which must be supported by chemical analysis. ## Popular Localities The only source of specimens is the type locality – the Vorontsovskoye deposit in Russia. These specimens are practically unavailable on the commercial market and are mainly found in institutional collections and among specialized micromineral collectors.
Care and storage
## Cleaning Specimens containing dalnegroite, which are typically microscopic inclusions in other minerals (mainly realgar), should not be cleaned mechanically or chemically. Any attempt at cleaning may damage the delicate host mineral and the microscopic dalnegroite grains themselves. ## What to Avoid Exposure to sunlight should be strictly avoided, as the co-occurring mineral – realgar – is unstable and transforms into pararealgar under light. Moisture, chemicals, and sudden temperature changes should also be avoided. ## Storage Specimens should be stored in complete darkness, under stable temperature and humidity conditions. The best solution is a sealed, opaque "perky box" placed in a dry location.