Takéuchiite
Chemical formula: Mg₂Mn³⁺O₂BO₃
Takéuchiite is a rare, black magnesium borate-manganate, found in manganese skarns in Långban, Sweden.
Properties
- Mohs hardness
- 6
- Luster
- Metallic
- Streak
- Brown
- Density
- 0
- Fracture
- Irregular/Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Key diagnostic features include its black color, metallic luster, prismatic crystal form often in radial aggregates, and brown streak. Hardness (6 on the Mohs scale) is also helpful. However, final identification requires advanced analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Takéuchiite can be confused with other black, prismatic manganese minerals from Långban, such as pinakiolite, orthopinakiolite, or ludwigite. Distinguishing them based on visual characteristics is extremely difficult and often impossible without specialized studies. The streak is a helpful clue – many manganese oxides have a black streak, while takéuchiite has a brown one. ## Crystal Forms Crystals are prismatic, elongated along the c-axis. They usually form radial or randomly arranged aggregates. They are less commonly found as single, well-formed crystals.
Geological environment
## Genesis Takéuchiite forms as a result of metamorphic processes within metamorphosed iron and manganese ore deposits, known as skarns. It is a product of high-temperature reactions in a boron-rich environment. ## Mineral Associations This mineral co-occurs with other minerals typical of skarns in Långban, such as calcite, hausmannite, magnetite, hematite, pinakiolite, orthopinakiolite, and rare borates. ## Localities The only confirmed and described occurrence of takéuchiite in the world is its type locality – the Långban mine in the Långban Ore District, Värmland region, Sweden. This is a classic locality for many rare manganese and iron minerals.
Rarity
Very rare
For collectors
## Quality Criteria The most valuable takéuchiite specimens are those with well-formed, sharp crystals forming aesthetic, radial aggregates. Contrast with a light matrix, such as white calcite, significantly enhances visual appeal. Crystal size and the richness of aggregates are also key factors influencing value. ## Popular Localities As this mineral is known only from one place in the world, all collector specimens come from the Långban mine in Sweden. This is a legendary locality, and specimens from there are highly prized by collectors specializing in rare or systematic minerals.
Care and storage
## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. Due to its rarity and potential instability, contact with water and any chemical agents should be avoided. ## What to Avoid Avoid ultrasonic cleaners, chemical agents (acids, bases, solvents), and sudden temperature changes. The mineral should be protected from moisture. ## Storage It is recommended to store specimens in stable conditions, in closed collector boxes or display cases, away from dust, moisture, and direct sunlight. Avoid storing in areas with high humidity.