Billingsleyite

Chemical formula: Ag<sub>7</sub>AsS<sub>6</sub>

Billingsleyite is a rare silver arsenic sulfosalt, forming tiny, metallic-lustered grains intergrown with other minerals.

## Characteristics Billingsleyite is a rare mineral from the sulfosalt group, being a complex silver arsenic sulfide. It occurs as very small, anhedral (irregularly formed) grains, usually not exceeding 0.2 mm in size. It is most often observed as inclusions in other minerals, mainly acanthite and pearceite. Due to its small size and mode of occurrence, its macroscopic features are difficult to observe without specialized equipment. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. Its Mohs hardness is approximately 2.5, classifying it as a soft mineral. The density calculated based on the chemical formula and unit cell parameters is 6.35 g/cm³. ## Colors and Varieties Billingsleyite has a steel-gray color, which in reflected light (under a ore microscope) takes on a shade from gray to creamy white. No varieties have been distinguished. ## History and Name The mineral was discovered in the North Lily mine in the East Tintic district, Utah County, USA. It was described and named in 1968 by C. Frondel and R.M. Honea in honor of Paul Billingsley (1887–1962), a prominent American mining geologist who made significant contributions to understanding the geology of the region. ## Uses Billingsleyite has no industrial application. Its significance is purely scientific and collectible, as a rare component of ore parageneses.

Properties

Mohs hardness
2.5
Luster
Metallic
Streak
Black
Density
6.35
Cleavage
None
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of billingsleyite is only possible using advanced laboratory methods, such as ore microscopy (reflected light analysis) and chemical analysis (e.g., electron microprobe - EDS/WDS) to confirm the Ag-As-S composition. Macroscopic identification is impossible. ## Distinguishing from Similar Minerals Under the microscope, it can be confused with other silver sulfosalts, such as pearceite, proustite, or polybasite. Final distinction requires quantitative analysis of chemical composition and crystallographic data obtained by X-ray diffraction (XRD). ## Crystal Forms Billingsleyite crystallizes in the orthorhombic system. It forms exclusively anhedral, irregular grains and aggregates, often as inclusions in other minerals. No well-formed crystals have been observed.

Geological environment

## Genesis It is a hydrothermal mineral, forming in the final stages of crystallization in ore veins rich in silver and arsenic. It forms under low to medium temperature conditions. ## Mineral Associations It most often co-occurs with acanthite, pearceite, polybasite, proustite, native silver, galena, sphalerite, chalcopyrite, quartz, and barite. ## Localities The most important and best-documented localities worldwide are: - United States: North Lily mine in the East Tintic district, Utah (type locality). - Mexico: Real de Ángeles mine in Zacatecas. - Chile: Chañarcillo district in the Atacama region.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a billingsleyite specimen is determined not by the mineral itself (which is microscopic), but by the richness of the association and the quality of the accompanying minerals on the rock matrix. Most desirable are samples with analytically confirmed presence of billingsleyite, co-occurring with well-formed crystals of other silver sulfosalts, originating from classic localities. ## Popular Localities Specimens from the type locality in Utah and from the historic Chañarcillo district in Chile are most valued by specialized collectors of rare and systematic minerals.

Care and storage

## Cleaning Due to its extreme rarity and occurrence as microscopic inclusions, specimens with billingsleyite are practically not subject to cleaning. Any mechanical or chemical intervention could irreversibly destroy the mineral. In case of dust on the rock matrix, compressed air can be used. ## What to Avoid Avoid contact with any chemicals, acids, and bases. The mineral is soft and susceptible to scratches. It should be protected from moisture and sudden temperature changes. ## Storage Specimens containing billingsleyite should be stored under stable conditions, preferably in closed, padded "micromount" boxes, to protect them from mechanical damage, dust, and environmental factors.

Sources

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