Angelellite

Chemical formula: Fe₃+₄O₃(As₅+O₄)₂

Angelellite is a rare iron arsenate, forming tabular, almost black crystals with a metallic luster.

## Characteristics Angelellite is a very rare iron arsenate, occurring as small, tabular or bladed crystals. It typically forms rosette-like or radial aggregates, and individual crystals rarely exceed 1-2 mm. Its color ranges from dark brown to black, and in strong light, it may exhibit reddish-brown internal reflections. ## Physical Properties This mineral is characterized by high hardness, measuring 5.5 on the Mohs scale. It has a strong, submetallic to metallic luster. It is opaque, becoming translucent only on thin edges. Its density is significant, ranging from 4.89-4.95 g/cm³. ## History and Name Angelellite was first described in 1950 by P. Ramdohr, M.T. de Muñoz, and J.M. de Abeledo. The mineral's name honors Juan Angelelli (1908-1975), an Argentine geologist and director of the Argentine Geological Survey, for his contributions to the study of mineral deposits in Argentina. ## Uses Due to its extreme rarity, angelellite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientific institutions.

Properties

Mohs hardness: 5.5
Luster: Submetallic
Streak: Reddish-brown
Density: 4.89 - 4.95
Cleavage: None
Fracture: Uneven
Transparency: Opaque, translucent on thin edges
Crystal system: Triclinic

Diagnostic features

## Identification Angelellite can be identified by its characteristic black, tabular crystals forming rosette-like aggregates. Key features include a high, submetallic luster, a reddish-brown streak, and its occurrence in a specific geological environment, often in association with hematite. ## Distinguishing from Similar Minerals It can be confused with hematite, goethite, or other iron oxides. It differs from hematite by a slightly different crystal habit (more bladed) and often radial arrangement of aggregates. However, definitive confirmation requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are tabular, flattened parallel to {001}, often with a hexagonal or rhombic outline. They typically occur as radial or rosette-like aggregates, as well as thin crusts and coatings.

Geological environment

## Genesis Angelellite is a hydrothermal mineral. It forms in the oxidation zones of ore deposits rich in arsenic and iron. It develops in fissures and rock cavities, often in association with iron oxides. ## Mineral Associations It most commonly co-occurs with hematite, which is its constant companion at the type locality. In other locations, its occurrence has been noted with cassiterite, quartz, and other arsenates. ## Localities The most important and type locality is the Cerro Pululus mine in Jujuy Province, Argentina. It is also known from several other places worldwide, including the Clara mine in the Black Forest (Germany) and the Baitian mine in Hunan Province (China), confirming its extreme rarity.

Rarity

Very rare

For collectors

## Quality Criteria The most prized angelellite specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates. Contrast with a light matrix rock, most often hematite or quartz, significantly enhances visual and collector value. The size of the aggregates is also important, although even millimeter-sized specimens are considered valuable. ## Popular Localities Specimens from the type locality – the Cerro Pululus mine in Argentina – are by far the most sought after by collectors. They are considered classic and exemplary for this mineral.

Care and storage

## Cleaning Utmost caution is recommended. The safest method is to use compressed air to remove dust. Any wet cleaning is discouraged due to the risk of damaging delicate crystals and potential reaction with water. ## What to Avoid Contact with water, acids, and other chemicals must be strictly avoided. The mineral is brittle, so it should be protected from impacts and vibrations. It should not be heated or exposed to prolonged strong light. ## Storage Angelellite specimens should be stored in stable conditions, preferably in a sealed "micromount" box, away from dust, humidity, and sudden temperature changes. Due to the small size of the crystals, observation under a microscope is the best way to view them.

Sources

IMA Mineral List (CNMNC)