Ángelaite

Chemical formula: Cu₁+₂Ag₁+Pb₂+Bi₃+S₂-₄

Ángelaite is a rare, lead-gray sulfide mineral containing copper, silver, lead, and bismuth, discovered in Argentina.

## Characteristics Ángelaite is a rare mineral from the sulfide group. It occurs as small, anhedral (irregular) grains, typically not exceeding 0.2 mm in size. Its appearance is typical for ore minerals – it has a metallic luster and a lead-gray color. Due to the small size of the grains, its macroscopic features are difficult to observe without magnification equipment. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. Its hardness and density have not been precisely measured due to its rarity and small sample sizes. In reflected light under a polarizing microscope, it exhibits weak pleochroism from gray to creamy-gray. ## Colors and Varieties Ángelaite has a uniform, lead-gray color. No color varieties or commercial forms are known. ## History and Name The mineral's name comes from its discovery location – the Ángela mine in the Gastre Department, Chubut Province, Argentina. It was first described in 1983 by Werner H. Paar, William G. Panczner, and Thomas T. Chen. The International Mineralogical Association (IMA) approved it as a new mineral species in the same year (IMA1983-038).

Properties

Luster: Metallic
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Identification of Ángelaite is impossible with the naked eye and requires advanced laboratory techniques. Recognition is based on chemical analysis (e.g., using an electron microprobe – EDS/WDS) to confirm the presence and proportions of copper, silver, lead, bismuth, and sulfur. X-ray diffraction (XRD) analysis is also crucial for determining the crystal structure. ## Distinguishing from Similar Minerals Macroscopically, it is indistinguishable from many other gray sulfides and sulfosalts with a metallic luster, such as galena, as well as other rare minerals from the aikinite group with which it co-occurs (e.g., benjaminite, berryite). Definitive differentiation is only possible based on chemical and structural analysis. ## Crystal Forms Ángelaite forms exclusively anhedral, i.e., irregular and shapeless, grains intergrown with other minerals. No euhedral (well-formed) crystals have been observed.

Geological environment

## Genesis Ángelaite is a hydrothermal mineral. It formed in quartz veins cutting through volcanic rocks (rhyolites). Its crystallization occurred under low-temperature conditions, during a late stage of deposit formation. ## Mineral Associations This mineral occurs in close association with other sulfides and sulfosalts. It most commonly co-occurs with galena, sphalerite, pyrite, chalcopyrite, aikinite, benjaminite, berryite, matildite, and native bismuth. All these minerals are embedded in a quartz matrix. ## Localities The only confirmed occurrence of Ángelaite in the world is its type locality – the Ángela mine, located in the Gastre Department, Chubut Province, Argentina.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, Ángelaite is not evaluated based on typical aesthetic criteria such as color or form. The collector and scientific value of a specimen lies in the confirmed presence of this extremely rare mineral itself. The most valuable samples are those where Ángelaite occurs in rich associations with other rare sulfosalts, and its presence has been confirmed by laboratory analyses. ## Popular Localities The only source of specimens is the Ángela mine in Argentina. This mineral is practically unavailable on the commercial market and is found mainly in the collections of scientific institutions and museums that acquired the material for research.

Care and storage

## Cleaning Due to its extreme rarity and small grain sizes, Ángelaite specimens are usually found in a rock matrix and should not be mechanically or chemically cleaned by collectors. Any intervention risks irreversible damage or loss of the mineral. Specimens should only be analyzed and conserved under laboratory conditions. ## What to Avoid Avoid contact with chemicals, ultrasound, and any attempts at polishing or grinding, which would destroy the natural, microscopic grains. The specimen should be protected from dust and contamination. ## Storage Specimens containing Ángelaite should be stored under stable conditions, in sealed containers (e.g., "micromount" type), to protect them from mechanical damage and contamination. It is best to keep them in a dry place, away from direct light and temperature fluctuations.

Sources

IMA Mineral List (CNMNC)