Fangite

Chemical formula: Tl¹⁺₃As⁵⁺S²⁻₄

Fangite is a very rare thallium sulfarsenate, distinguished by its deep red color and occurrence as small, anhedral grains.

## Characteristics Fangite is a very rare mineral from the sulfosalt group, chemically classified as a thallium sulfarsenate. It typically occurs as very fine, irregular (anhedral) grains, rarely exceeding 100 micrometers. Its most characteristic visual feature is an intense, deep red color, transitioning into maroon shades. Due to its extreme rarity and microscopic crystal sizes, it is a mineral known primarily within the scientific community. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2-2.5, meaning it can be scratched with a fingernail. It is translucent and characterized by an orange streak. Its luster has not been precisely determined, but for minerals of similar composition and appearance, it is usually resinous to adamantine. The fracture is conchoidal. ## Colors and Varieties Fangite occurs in uniform, intense colors from deep red to maroon. No named color varieties or commercial varieties are distinguished. ## History and Name Fangite was discovered in 1992 and described by J.A. Mandarino, A.C. Roberts, M.C.F. Harris, A.J. Criddle, and J.D. Grice. The mineral is named in honor of Professor Jen-Ho Fang (1929-1991), a crystallographer from the University of Alabama, for her contributions to mineralogy and crystallography. ## Uses Due to its extreme rarity, microscopic size, and toxic composition (thallium and arsenic content), fangite has no industrial or commercial applications. It is of purely scientific significance and is an object of interest for specialized collectors of rare minerals.

Properties

Mohs hardness: 2-2.5
Luster: Resinous
Streak: Orange
Density: 0
Fracture: Conchoidal
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Amateur identification of fangite is practically impossible due to its microscopic size. It requires advanced laboratory techniques, such as chemical composition analysis (EDS) and X-ray diffraction (XRD). A preliminary indication may be its deep red color, orange streak, and co-occurrence with other rare thallium sulfosalts. ## Distinguishing from Similar Minerals Fangite can be confused with other red sulfosalts, such as cinnabar, realgar, or lorandite. Cinnabar and realgar are usually brighter and form larger crystals. Lorandite, although also a thallium sulfosalt, crystallizes in the monoclinic system and has perfect cleavage. Definitive differentiation requires chemical and crystallographic analysis. ## Crystal Forms Fangite forms exclusively anhedral (irregular, lacking proper faces) grains, usually smaller than 0.1 mm, occurring as disseminated particles in the host rock.

Geological environment

## Genesis Fangite forms as a result of low-temperature hydrothermal processes. It occurs in calcite veins cutting through mudstones, within Carlin-type gold deposits. ## Mineral Associations This mineral co-occurs with other rare thallium and arsenic sulfosalts. In its type locality, it was found in association with lorandite, raguinite, pierrotite, realgar, orpiment, pyrite, marcasite, native arsenic, and calcite. ## Localities The only confirmed and described occurrence of fangite in the world is its type locality: the Mercur gold mine (Marion Hill open pit) in the Mercur mining district, Tooele County, Utah, USA.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of fangite specimens is primarily assessed based on the abundance and concentration of grains on the rock matrix. Since this mineral does not form well-developed crystals, their form is not a criterion for evaluation. The intensity of the red color can enhance visual appeal. The most desirable specimens are those with clearly visible, numerous fangite grains, preferably in association with other rare minerals from this locality. ## Popular Localities The only source of fangite specimens is the Mercur mine in Utah, USA. This is a historic locality, and material is only available on the secondary market from old collections.

Care and storage

## Cleaning Fangite specimens, due to their microscopic size and embedding in rock matrix, generally do not require cleaning. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Fangite is a very soft and brittle mineral, susceptible to mechanical damage. Contact with acids and other chemicals must be strictly avoided. Due to its thallium and arsenic content, it is a toxic mineral – avoid inhaling dust and wash hands after any contact with the specimen. ## Storage Specimens should be stored under stable conditions, in a closed, airtight “micromount” box to prevent accidental dusting and contact. Store away from moisture, heat sources, and direct sunlight. It is essential to clearly label the specimen as toxic.