Tegengrenite

Chemical formula: (Mn₃+₀.₅Sb₅+₀.₅)Mg₂O₄

Tegengrenite is a very rare mineral of the spinel group, an oxide of magnesium, manganese, and antimony, found in skarn deposits.

## Characteristics Tegengrenite is an extremely rare mineral belonging to the spinel group. It occurs as small, octahedral crystals, usually not exceeding 0.3 mm in size. These crystals are most often embedded in calcite. Due to its size and rarity, its visual characteristics are difficult to assess without specialized equipment. ## Physical Properties This mineral is characterized by a hardness of approximately 6.5 on the Mohs scale. It has a metallic luster and is opaque. Its density, calculated based on the chemical formula and unit cell parameters, is 4.35 g/cm³. ## Colors and Varieties Tegengrenite is black in color. No varieties are known. ## History and Name The mineral was discovered in the Långban mine in Filipstad municipality (Värmland, Sweden), which is its type locality. It was approved by the International Mineralogical Association (IMA) in 1993. The name honors Felix Tegengren (1884–1980), a Finnish-Swedish geologist who made significant contributions to the study of ore deposit geology, including the Långban area.

Properties

Mohs hardness: 6.5
Color: Deep ruby red
Luster: Subadamantine
Streak: Black
Density: 0
Cleavage: None
Fracture: Conchoidal
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of tegengrenite is possible almost exclusively using advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to determine chemical composition and X-ray diffraction (XRD) to confirm crystal structure. In collector conditions, identification relies on a label from a reputable source and knowledge of the locality. ## Distinguishing from Similar Minerals Visually, it is indistinguishable from other black, metallic spinel group minerals found in Långban, such as magnetite, franklinite, or jacobsite. Definitive differentiation requires specialized chemical analysis. ## Crystal Forms Tegengrenite forms small, well-developed crystals with an octahedral habit, which is typical for minerals of the spinel group.

Geological environment

## Genesis Tegengrenite forms in metamorphic rocks – skarns, which developed as a result of regional metamorphism of mineralized iron and manganese deposits. It occurs in calcite veins cutting dolomite. ## Mineral Associations This mineral co-occurs with calcite, katoptrite, magnetite, humite, and other rare minerals typical of the Långban deposit. ## Localities The only confirmed occurrence of tegengrenite in the world is its type locality – the historical Långban mine in the Värmland region of Sweden.

Rarity

Extremely rare

For collectors

## Quality Criteria For collectors specializing in systematic minerals or those from Långban, the main criterion for value is the certainty of specimen identification. The size and quality of microcrystal development, as well as the presence of interesting associated minerals, are also important. Every confirmed specimen is a valuable acquisition. ## Popular Localities All known and prized specimens originate exclusively from the Långban mine in Sweden. This is one of the world's most famous classic mineralogical localities, renowned for the occurrence of many rare and unique mineral species.

Care and storage

## Cleaning Due to the extreme rarity and small size of the crystals, mechanical cleaning is not recommended. Specimens are typically stable within their rock matrix (calcite) and do not require special treatment. If necessary, compressed air can be used to remove dust. ## What to Avoid Avoid contact with acids, which can react with the surrounding calcite. Specimens should also be protected from ultrasound and sudden temperature changes. ## Storage Tegengrenite specimens should be stored in stable conditions, in sealed "micromount" boxes, to protect them from dust and mechanical damage. Due to the small size of the crystals, they are easily lost.

External references

https://www.mindat.org/min-7072.html