Ellisite
Chemical formula: Tl<sup>1+</sup><sub>3</sub>As<sup>3+</sup>S<sup>2-</sup><sub>3</sub>
Ellisite is a rare thallium arsenic sulfosalt, forming small, metallic-lustered, dark gray crystals.
Properties
- Mohs hardness
- 2
- Luster
- Metallic
- Streak
- Red-brown
- Density
- 7.09
- Cleavage
- Good on {1011}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Amateur identification of ellisite is practically impossible due to its small size and similarity to other sulfosalts. Key features include very high density, low hardness, metallic luster, and occurrence in association with realgar, lorandite, and other thallium minerals. Definitive identification requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Ellisite can be confused with other rare thallium sulfosalts, such as lorandite, weissbergite, or carlinite. Lorandite has a similar composition but crystallizes in the monoclinic system and has a carmine-red color. Weissbergite also crystallizes in the triclinic system but contains antimony instead of arsenic. Differentiation without specialized equipment is impossible. ## Crystal Forms Ellisite crystals are typically very small, tabular or short-prismatic. They often occur as irregular grains or aggregates of grains intergrown with realgar.
Geological environment
## Genesis Ellisite is a low-temperature hydrothermal mineral. It forms by precipitation from solutions rich in thallium, arsenic, and sulfur. Its typical environment of occurrence is Carlin-type gold deposits, where it is associated with decarbonatization and silicification of sedimentary rocks. ## Mineral Associations This mineral co-occurs with other thallium minerals and arsenic sulfides. It is most commonly accompanied by realgar, orpiment, lorandite, weissbergite, carlinite, getchellite, as well as pyrite, marcasite, quartz, and calcite. ## Localities The most important and type locality for ellisite is the Carlin gold deposit in Eureka County, Nevada, USA. It has also been identified in the Allchar deposit in North Macedonia, which is known for the occurrence of many rare thallium minerals.
Rarity
Very rare
For collectors
## Quality Criteria The quality of ellisite specimens is primarily assessed based on the presence of well-formed, even if microscopic, crystals. Due to its extreme rarity, any analytically confirmed specimen is valuable. The most desirable specimens are those where the crystals are clearly visible, have good luster, and are set on a contrasting matrix, for example, in association with red realgar. ## Popular Localities The vast majority of collector specimens come from the type locality – the Carlin deposit in Nevada, USA. Specimens from Allchar in North Macedonia are even rarer on the market.
Care and storage
## Cleaning Ellisite specimens are extremely delicate and small. Mechanical cleaning is not recommended. If absolutely necessary, compressed air (from a safe distance) can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Avoid contact with acids and other chemicals that may react with sulfides. The mineral is susceptible to mechanical damage due to its low hardness. The thallium content is toxic – avoid inhaling dust and wash hands after each contact with the specimen. ## Storage Specimens should be stored under stable conditions, in sealed containers (e.g., micromount boxes), to protect them from dust, moisture, and mechanical damage. Store away from direct sunlight and heat sources.