Magnussonite

Chemical formula: Mn₂+₅As₃+₃O₉(OH,Cl)

Magnussonite is a very rare manganese arsenite, forming small, regular crystals of an intense, reddish-brown color.

## Characteristics Magnussonite is an extremely rare mineral from the arsenite group, chemically classified as a basic manganese arsenite with a possible admixture of chlorine. It occurs as very small, sharply defined, regular crystals, whose size rarely exceeds 0.5 mm. Most often, it forms granular aggregates or appears as single, scattered crystals within the host rock. Its characteristic features are an intense, reddish-brown to brownish-black color and a strong, almost metallic luster. ## Physical Properties This mineral is characterized by a hardness of approximately 5 on the Mohs scale. It has a high refractive index, which, combined with an almost adamantine luster, gives it great brilliance despite the small size of its crystals. It is a brittle mineral, with a density of about 4.55 g/cm³. ## Colors and Varieties Magnussonite occurs in uniform colors, from reddish-brown, through dark brown, to brownish-black. In transmitted light under a microscope, it reveals a deep red color. No named varieties of this mineral are distinguished. ## History and Name The mineral was discovered in the historic Swedish mining district of Långban in Värmland. It was first described in 1956 by Olof H. Ödman and Gabriella Donnay. The name "magnussonite" was given in honor of Nils Magnus H. Magnusson (1890-1976), a Swedish geologist and professor of geology at the Royal Institute of Technology in Stockholm, who was a prominent expert on the geology of ore deposits in Sweden, including the Långban deposit.

Properties

Mohs hardness: 5
Luster: Adamantine
Streak: Red-brown
Density: 4.55
Cleavage: None
Fracture: Conchoidal
Transparency: Translucent in thin fragments, Opaque
Crystal system: Cubic

Diagnostic features

## Identification Magnussonite is identified based on its unique occurrence environment (metamorphosed manganese and iron deposits), regular crystal form, reddish-brown color, adamantine luster, and association with other rare Långban minerals. Due to the small size of the crystals, final identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Differentiation from Similar Minerals It can be confused with other rare, reddish-brown minerals from manganese deposits, such as hausmannite or braunite. It is distinguished from them by its regular (cubic) crystallographic system and significantly higher luster. Unlike hausmannite, magnussonite does not exhibit such distinct cleavage. ## Crystal Forms It crystallizes in the isometric system, forming very small, well-formed crystals with a habit of cubes, octahedra, and their combinations. It occurs as single, overgrown crystals or in granular aggregates.

Geological environment

## Genesis Magnussonite is a mineral of metamorphic origin. It forms as a result of contact or regional metamorphism of iron and manganese ore deposits. Its formation is associated with high-temperature processes occurring in carbonate rocks (dolomitic and calcitic marbles) that have been enriched in arsenic. ## Mineral Associations This mineral co-occurs with a number of other, often rare, minerals typical of the Långban deposit. The most common associations include: hausmannite, braunite, dolomite, calcite, lead magnussonite, tegengrenite, berzeliite, and manganberzeliite. ## Localities Magnussonite is an extremely rare mineral, and its occurrence has been confirmed in only a few places worldwide. The type locality and main source of the best specimens is the Långban mine in Värmland, Sweden. Outside of Sweden, its presence has also been reported in the Kombat mine in Namibia.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a magnussonite specimen is primarily determined by the size and development of its crystals. The most valued specimens are those with sharp, lustrous crystals approaching 0.5 mm in size, clearly standing out from the host rock. Rich aggregates of small crystals are also desirable. Color contrast with surrounding minerals, e.g., white calcite, significantly enhances the aesthetic value of the specimen. ## Popular Localities The only source of collectible magnussonite specimens is the historic Långban mine in Sweden. Specimens from this locality are considered classics and are a great rarity on the collector's market.

Care and storage

## Cleaning Due to its extreme rarity and small crystal size, mechanical cleaning of magnussonite is highly inadvisable. If absolutely necessary, compressed air can be used to remove loose dust particles. Contact with water and any chemicals should be avoided. ## What to Avoid Ultrasonics, steam cleaners, acids, solvents, and detergents must be strictly avoided. The mineral is potentially sensitive to rapid temperature changes. As an arsenite, it may react with some chemicals, so it should be protected from all contaminants. ## Storage Magnussonite specimens should be stored under stable conditions, in sealed "micromount" containers, to protect them from dust, mechanical damage, and moisture. They should be kept away from direct sunlight and heat sources.

Sources

IMA Mineral List (CNMNC)