Gillulyite

Chemical formula: Tl₂As₇.₅Sb₀.₃S₁₃

Gillulyite is a rare, dark red sulfosalt mineral of arsenic and thallium, occurring as fine, acicular crystals.

## Characteristics Gillulyite is a very rare mineral from the sulfosalt group, chemically a complex sulfosalt of thallium, arsenic, and antimony. It occurs as very fine, acicular or hair-like crystals, which often form felted, dull aggregates. Individual crystals are flexible and dark red in color, but due to their small size, in mass, the aggregates appear black or dark gray with a red tint. ## Physical Properties Gillulyite crystals exhibit a metallic to submetallic luster. The mineral is opaque. Due to its microcrystalline nature, most physical properties, such as hardness or density, are difficult to measure precisely on natural specimens. ## Colors and Varieties This mineral is monochromatic – it occurs in shades from dark red to reddish-black. No color varieties or commercial varieties are distinguished. ## History and Name Gillulyite was discovered in the Mercur Mine (also known as Barrick Mercur Mine) in Tooele County, Utah, USA. It was described and recognized as a new mineral species in 1994. The name honors James C. Gilluly (1899-1980), a prominent geologist with the United States Geological Survey, known for his research on the geology and ore deposits in Utah. ## Uses Gillulyite has no industrial application. Its significance is purely scientific and collectible, being an object of interest for specialized collectors of rare minerals and microminerals.

Properties

Mohs hardness: 2-2.5
Color: Deep red
Luster: Metallic
Streak: Brick red
Density: 4.02
Cleavage: One perfect cleavage and possbly two poor cleavages (based on pitting during polishing).
Fracture: Splintery
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of gillulyite is based on its characteristic appearance – felted aggregates of fine, hair-like, dark red crystals. The key is the occurrence location (type locality) and co-occurrence with other rare thallium sulfosalts. ## Distinguishing from similar minerals Gillulyite can be confused with other sulfosalts of similar appearance, such as lorandite, raguinite, or imhofite, which can also form red, acicular crystals and occur in the same environment. Definitive distinction requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal forms This mineral forms very fine, elongated, acicular or hair-like crystals, reaching up to 2 mm in length, but usually much smaller. These crystals combine into dense, felted or radial aggregates that grow on the host rock.

Geological environment

## Genesis Gillulyite is a low-temperature hydrothermal mineral. It forms in Carlin-type gold deposits, which are rich in arsenic, antimony, mercury, and thallium. It crystallizes in the late stage of mineralization processes, depositing in voids and fractures in the host rock, most often in jasper. ## Mineral associations This mineral co-occurs with other rare thallium and arsenic sulfosalts, such as lorandite, raguinite, imhofite, weissbergite, as well as with realgar, orpiment, pyrite, marcasite, getchellite, and native gold. ## Localities The only confirmed and detailed described locality of gillulyite in the world is its discovery site – the Mercur Mine in the Oquirrh Mountains, Tooele County, Utah, USA.

Rarity

Extremely rare

For collectors

## Quality criteria The most valuable gillulyite specimens are those that have rich, dense coverings of intensely red, acicular crystals on a contrasting matrix. The size and quality of the crystals, although always microscopic, are important. Color contrast with the substrate (e.g., light jasper) and the presence of rare associated minerals are also significant. ## Popular localities The only source of collectible specimens is historical finds from the Mercur Mine in Utah, which is now closed and reclaimed. This means that new specimens do not enter the market, and only those from old collections are available.

Care and storage

## Cleaning Specimens should be cleaned exclusively with compressed air to remove dust. Avoid contact with water and any chemical agents. Due to the fragility and delicacy of the acicular crystals, mechanical cleaning (e.g., with a brush) is highly inadvisable. ## What to avoid Avoid ultrasonic cleaners, water, solvents, and acids. The mineral is sensitive to shocks and mechanical damage, which can destroy delicate crystals. As a mineral containing thallium and arsenic, it is toxic – avoid inhaling dust and wash hands after any contact. ## Storage Gillulyite specimens should be stored in stable conditions, in sealed "micromount" containers that protect them from dust, moisture, and mechanical damage. They should be kept away from direct sunlight and heat sources.

External references

https://www.mindat.org/min-1694.html