Armangite

Chemical formula: Mn₂+₂₆As₃+₁₈O₅₀(CO₃)(OH)₄

Armangite is a very rare manganese arsenite, forming small, hexagonal crystals with a characteristic resinous to adamantine luster.

## Characteristics Armangite is a complex manganese arsenite, occurring as small, hexagonal, tabular or prismatic crystals. They typically do not exceed a few millimeters. This mineral is brittle and rarely forms well-developed, single crystals, more often appearing as granular aggregates or intergrown within the surrounding rock. ## Physical Properties Armangite crystals exhibit a hardness of approximately 4 on the Mohs scale. They have a characteristic, strong luster, described as resinous, and on some surfaces, approaching adamantine. It is a relatively dense mineral, which results from the presence of manganese and arsenic in its structure. ## Colors and Varieties Armangite is dark brown to almost black in color. Thin fragments of the mineral may be translucent red. No color or commercial varieties are distinguished. ## History and Name The name "armangite" comes from the abbreviation "ARsen-MANgan-Gruvan" (Arsenic-Manganese-Mine), referring to its chemical composition (arsenic, manganese) and its discovery location – the Långban mine in Sweden. The mineral was first described in 1920 by Gustaf Flink, Aminoff, and Hedelius. ## Uses Armangite has no industrial applications. Its significance is purely scientific and as a collector's item due to its extreme rarity and occurrence in a classic mineralogical locality.

Properties

Mohs hardness: 4
Luster: Adamantine, Resinous
Streak: Brown
Density: 4.43
Cleavage: None
Fracture: Conchoidal
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Key diagnostic features of armangite include its hexagonal crystal habit, dark brown to black color, resinous or adamantine luster, and high density. Red internal reflections in strong light are also very characteristic. Its occurrence in association with other rare manganese minerals in Långban is a strong indicator. ## Distinguishing from Similar Minerals Armangite can be confused with other dark minerals from Långban, such as some garnets (andradite), magnetite, or hausmannite. However, it is distinguished by its lower hardness (approx. 4), characteristic luster, and hexagonal crystal form. Unlike magnetite, it is not magnetic. ## Crystal Forms Crystals are typically tabular or short prismatic with a hexagonal outline. They occur as single, intergrown crystals or in small, granular aggregates.

Geological environment

## Genesis Armangite is a metamorphic mineral. It forms in iron and manganese ore deposits that have undergone contact metamorphism. It occurs in skarns and carbonate rocks (marbles) rich in manganese. ## Mineral Associations This mineral co-occurs with other, often rare, manganese minerals. Typical associated minerals in Långban include baryte, calcite, hausmannite, magnetite, berzeliite, caryinite, phlogopite, and various varieties of garnet. ## Localities The only confirmed and classic locality for armangite in the world is the Långban mine in the Värmland region of Sweden. This is the type locality for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The most prized armangite specimens are those with sharp, well-formed, undamaged crystals, clearly contrasting with the matrix rock. Luster, crystal size (even if only millimeters), and the richness of association with other rare minerals from Långban significantly increase the collector's value of the specimen. ## Popular Localities The only source of armangite specimens is the historic Långban mine in Sweden. All samples available on the collector's market come from this single location, making them extremely desirable.

Care and storage

## Cleaning Armangite specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to its arsenic content and brittleness, avoid washing with water and using any chemical agents. Contact with water can initiate oxidation processes. ## What to Avoid Absolutely avoid contact with water, acids, detergents, and other chemicals. The mineral is sensitive to shocks and impacts. It should not be heated or subjected to ultrasound. It is recommended to wash hands after any contact with the specimen due to its arsenic content. ## Storage Armangite should be stored in stable, dry conditions, away from direct sunlight. It is best kept in a closed, padded display box to protect it from mechanical damage and dust.

Sources

IMA Mineral List (CNMNC)