Fredrikssonite

Chemical formula: Mg₂Mn³⁺O₂(BO₃)

Fredrikssonite is a rare, reddish-brown magnesium and manganese borate, occurring in radiating aggregates.

## Characteristics Fredrikssonite is a rare mineral from the ludwigite group, chemically classified as a magnesium and manganese borate. It forms aggregates of small, acicular or bladed crystals, which often arrange themselves into radiating aggregates. Its typical appearance is compact, fibrous masses with a characteristic reddish-brown color. ## Physical Properties This mineral is relatively hard, reaching 6 on the Mohs scale. It has a density of 3.84 g/cm³, which is quite high for a borate. It is translucent to opaque, and its luster is described as vitreous to silky, especially noticeable in fibrous aggregates. ## Colors and Varieties Fredrikssonite occurs in a uniform, reddish-brown color. No color or commercial varieties are distinguished. ## History and Name The mineral was discovered and described in 1983. Its name honors Kurt A. Fredriksson (1926-1991), a geochemist and curator of meteorites at the Smithsonian Institution in Washington, D.C., who first found this mineral in samples from the Långban mine. ## Uses Fredrikssonite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or minerals from Långban skarns.

Properties

Mohs hardness: 6
Luster: Vitreous, Silky
Streak: Pale brown to yellow
Density: 3.84
Cleavage: One, poor, and a second, very poor
Fracture: Irregular/Uneven
Transparency: Translucent,Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features of fredrikssonite include its reddish-brown color, radiating or fibrous aggregates, and high hardness (6 on the Mohs scale). A pale yellow to brown streak is also helpful in identification. Its occurrence in association with other manganese and magnesium minerals in skarns is a strong indicator. ## Distinguishing from Similar Minerals Fredrikssonite can be confused with other minerals from the ludwigite group, such as ludwigite itself or vonsenite, which have similar structures and forms of occurrence. Differentiation usually requires advanced chemical analyses (EDS/WDS) to determine the exact composition, especially the ratio of magnesium to iron and manganese. Pinakiolite also forms similar aggregates but usually has a darker, almost black color. ## Crystal Forms Fredrikssonite crystallizes in the orthorhombic system. It most often forms small, acicular or bladed crystals that coalesce into radiating, stellate, or fibrous aggregates. Single, well-formed crystals are rarely observed.

Geological environment

## Genesis Fredrikssonite is a mineral of metamorphic origin. It forms in skarns, which are carbonate rocks (limestones and dolomites) that have been altered at high temperatures under the influence of magmatic intrusions. Its formation is associated with metasomatism, a process of chemical exchange of components between the rock and hot solutions rich in boron, manganese, and magnesium. ## Mineral Associations This mineral co-occurs with other minerals typical of manganese skarns. In its type locality (Långban), it was found in association with calcite, hausmannite, magnussonite, tephroite, katoptrite, and manganoclinohumite. ## Localities The only confirmed and well-documented locality for fredrikssonite in the world is its type locality – the historic Långban mine in the Långban ore district, Värmland region, Sweden. This is one of the most famous mineralogical sites in the world, known for the occurrence of hundreds of unique minerals.

Rarity

Very rare

For collectors

## Quality Criteria The most prized fredrikssonite specimens are those that exhibit distinct, well-formed radiating aggregates with an intense, reddish-brown color. Contrast with the surrounding rock (matrix), such as light calcite, is also important. The size of the aggregates matters, and specimens with visible, larger clusters of acicular crystals are more sought after. The purity of the specimen and the absence of mechanical damage are crucial. ## Popular Localities The only source of collectible specimens is the Långban mine in Sweden. Since the mine has been closed for a long time, no new specimens are being acquired. All available on the market come from old collections, which significantly affects their value and rarity.

Care and storage

## Cleaning Fredrikssonite specimens should be cleaned very carefully, preferably using compressed air to remove dust. If wet cleaning is necessary, it is recommended to use a soft brush and distilled water, followed by thorough drying of the specimen. ## What to Avoid Avoid contact with acids and strong chemicals, which can damage the mineral. Due to its brittleness, it should be protected from impacts and vibrations. There are no specific recommendations regarding light or temperature, but it is generally advisable to avoid extreme conditions. ## Storage Fredrikssonite specimens, especially those with a delicate, fibrous structure, should be stored in closed display boxes to protect them from dust and mechanical damage. Due to its rarity, each specimen should be properly labeled with its name and location.

External references

https://www.mindat.org/min-1603.html