Mangani-eckermannite

Chemical formula: NaNa₂(Mg₄Mn₃+)Si₈O₂₂(OH)₂

A very rare silicate from the amphibole group, distinguished by the presence of manganese and forming dark, prismatic crystals.

## Characteristics Mangani-eckermannite is a silicate mineral belonging to the sodium amphibole group. It is a member of the eckermannite series, in which trivalent manganese (Mn³⁺) dominates a specific site in the crystal structure. It forms elongated, prismatic crystals that can occur as granular aggregates or radial clusters. It is usually strongly intergrown with other minerals of the host rock. Its color ranges from dark reddish-brown to almost black. ## Physical Properties This mineral has a hardness of approximately 6 on the Mohs scale. It has a vitreous luster, and on fracture surfaces, it can be dull. It is translucent in thin fragments and opaque in larger pieces. Its calculated density is about 3.18 g/cm³. ## Colors and Varieties Mangani-eckermannite is a mineral of uniform coloration, ranging from dark reddish-brown to black. No color or commercial varieties are distinguished for it. ## History and Name The mineral's name refers to its chemical composition – the dominance of manganese ("mangani") – and its structural relationship to eckermannite. It was formally defined as a distinct mineral species as part of the amphibole group nomenclature revision published in 2012. The type material comes from the historic Långban mine in Sweden. ## Uses Due to its extreme rarity, mangani-eckermannite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for advanced systematic and regional collections.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Pale brown
Density: 3.18
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying mangani-eckermannite based on visual characteristics is practically impossible. It resembles many other dark amphiboles, such as arfvedsonite or some tourmalines. A key clue is its origin from the Långban mine, known for its unique manganese mineralogy. ## Distinguishing from Similar Minerals It can be confused with other manganese-rich amphiboles (e.g., mangano-richterite) or iron-rich ones (arfvedsonite). Definitive differentiation requires advanced analytical techniques, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS), which allow for the determination of precise composition and unit cell parameters. ## Crystal Forms Crystals have a prismatic habit, often elongated and acicular. They usually form disordered aggregates, granular clusters, or radial fans embedded in the host rock (most often calcitic marble).

Geological environment

## Genesis This mineral forms under conditions of contact and regional metamorphism. Its formation is associated with metasomatic processes within manganese-rich, metamorphosed carbonate rocks (dolomitic marbles). ## Mineral Associations In its type locality (Långban), it coexists with minerals such as calcite, phlogopite, richterite, tephroite, and other rare manganese silicates and oxides. ## Localities The only confirmed and significant occurrence of mangani-eckermannite in the world is the Långban mine in Filipstad Municipality, Värmland region, Sweden. This is the type locality for this mineral.

Rarity

Very rare

For collectors

## Quality Criteria The quality of collector specimens primarily depends on the presence of well-formed, even microscopic, crystals. The most valued samples are those in which prismatic mangani-eckermannite crystals form contrasting clusters against a light background, for example, on white calcite. Association with other rare minerals from Långban also increases its value. ## Popular Localities The only source of specimens for collectors is the historic Långban mine in Sweden. This mineral is typically available only as micromounts or small cabinet specimens.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. If necessary, distilled water can be used. Ultrasonic cleaners should be avoided, as they can damage delicate crystals or cause them to detach from the matrix. ## What to Avoid The mineral is sensitive to strong acids. Contact with chemicals and sudden temperature changes should be avoided. It does not require special protection from light. ## Storage It is recommended to store specimens in closed boxes or display cases to protect them from dust and mechanical damage. It is stable under standard room conditions.

Sources

IMA Mineral List (CNMNC)