Arsenogoldfieldite

Chemical formula: Cu₁+₆Cu₁+₆(As₃+₂Te₄+₂)S₂-₁₃

Arsenogoldfieldite is a rare sulfide mineral, an arsenic analogue of goldfieldite, found as tiny, metallic grains.

## Characteristics Arsenogoldfieldite is a rare sulfide mineral belonging to the tetrahedrite group. It is an arsenic analogue of goldfieldite, in which arsenic predominates over antimony. It forms very small, individual grains, rarely exceeding 0.1 mm, usually occurring as inclusions within other minerals. It is typically opaque and has a metallic luster. ## Physical Properties This mineral is characterized by a metallic luster. Its hardness and density have not been precisely measured due to the small size of the grains, but they can be estimated by analogy with other minerals from the tetrahedrite group. It is brittle. ## Colors and Varieties Arsenogoldfieldite has a gray color with a slight, greenish-yellow tint. No varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of arsenic – and its structural relationship to goldfieldite. It was first described by S.A. Kirillov in 1973 based on material from the Kockbulak gold deposit in Uzbekistan. ## Uses Due to its rarity and microscopic size, arsenogoldfieldite has no industrial applications. It is of purely scientific significance and is of interest to specialized collectors of rare minerals.

Properties

Luster: Metallic
Streak: Black
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Cubic

Diagnostic features

## Identification Identifying arsenogoldfieldite is extremely difficult and impossible without advanced laboratory methods. Due to the microscopic size of the grains, identification relies on chemical analysis (e.g., using an electron microprobe - EDS/WDS) and crystallographic studies (X-ray diffraction - XRD). Under a reflected light ore microscope, it has a characteristic gray color with a greenish-yellow tint. ## Distinguishing from Similar Minerals It can be confused with other minerals from the tetrahedrite group, such as tetrahedrite, tennantite, or goldfieldite. Differentiation is possible only through precise chemical analysis, which allows for determining the ratio of arsenic to antimony and tellurium. ## Crystal Forms Arsenogoldfieldite occurs as anhedral (irregular) grains or, less commonly, as poorly formed, tiny crystals with a tetrahedral habit, typical of minerals in its group.

Geological environment

## Genesis It is a hydrothermal mineral, forming in ore veins associated with volcanism. It forms under low to medium temperature conditions. It occurs in epithermal gold and tellurium deposits. ## Mineral Associations It most commonly co-occurs with other ore minerals. In its type locality (Kockbulak, Uzbekistan), it was found in association with native gold, pyrite, chalcopyrite, sphalerite, galena, as well as rarer tellurides such as altaite, calaverite, krennerite, and hessite. It also occurs with quartz and barite. ## Localities Confirmed occurrences of arsenogoldfieldite are very few. Besides the type locality in the Kockbulak deposit in Uzbekistan, it has been identified in the Kawazu mine in Shizuoka, Japan, and in the Săcărâmb mine (formerly Nagyág) in Romania, which is also the type locality for goldfieldite.

Rarity

Very rare

For collectors

## Quality Criteria The quality of arsenogoldfieldite specimens is assessed differently than for typical collector minerals. Since it occurs as microscopic grains, the most important criterion is the certainty of identification using analytical methods. A specimen is valuable if it contains rich aggregates of arsenogoldfieldite grains that are visible under a microscope and whose identity has been confirmed. Co-occurrence with other rare minerals, especially tellurides and gold, increases its value.

Care and storage

## Cleaning Specimens containing arsenogoldfieldite are generally not cleaned due to the microscopic nature of its occurrences and their embedding in the rock matrix. Any attempts at mechanical cleaning may destroy the tiny grains. If dust removal is necessary, compressed air can be used from a safe distance. ## What to Avoid Avoid contact with chemicals, especially acids, which can react with sulfides. Also avoid ultrasonic cleaning and steam cleaning. The mineral is brittle, so it should be protected from impacts and scratching. ## Storage Specimens should be stored under stable conditions, away from moisture and chemical contaminants. It is best to keep them in sealed collector boxes to protect them from dust and mechanical damage.

Sources

IMA Mineral List (CNMNC)