Osumilite-(Mg)

Chemical formula: KMg₂(Al₅Si₁₀)O₃₀

A rare cyclosilicate of complex composition, forming blue or black, tabular crystals with a vitreous luster.

## Characteristics Osumilite-(Mg) is a complex cyclosilicate belonging to the osumilite group. This mineral most often forms small, hexagonal, tabular or short-prismatic crystals. It also occurs as granular aggregates embedded in rock. Its crystals rarely exceed a few millimeters in size. ## Physical Properties Osumilite-(Mg) is characterized by a hardness of 5-6 on the Mohs scale and a density of approximately 2.64 g/cm³. It has a vitreous luster and is a brittle mineral, with an uneven or conchoidal fracture. It does not exhibit cleavage. It is transparent to translucent. ## Colors and Varieties Typical colors of osumilite-(Mg) include shades of blue (from pale to indigo), pink, brown, gray, and black. The mineral often exhibits pleochroism, which is a change in color depending on the viewing direction, usually from colorless or pale pink to pale blue. ## History and Name The mineral was first described in 1953 by Akiho Miyashiro based on samples from Sakkabira, Kagoshima Prefecture (formerly Ōsumi Province) in Japan. The name refers to the discovery locality. The suffix -(Mg) was added in 2011 by the IMA to distinguish it from other members of the osumilite group in which the dominant cation is iron (osumilite-(Fe)) or manganese (osumilite-(Mn)). ## Uses Osumilite-(Mg) has no industrial applications. It is valued solely as a collector's mineral due to its rarity and attractive crystals.

Properties

Mohs hardness: 5-6.5
Color: Brown, blue
Luster: Vitreous
Streak: White
Density: 2.59
Cleavage: None
Fracture: Uneven to conchoidal
Transparency: Transparent
Crystal system: Hexagonal

Diagnostic features

## Identification Key diagnostic features include the hexagonal habit of crystals, vitreous luster, characteristic blue, pink, or black color, and lack of cleavage. Pleochroism (color change when rotating the crystal) is also an important clue. ## Distinguishing from Similar Minerals Osumilite-(Mg) is sometimes confused with cordierite, which can also be blue and occurs in similar geological conditions. However, cordierite has a different crystal habit (usually pseudohexagonal) and often exhibits stronger pleochroism. It is distinguished from beryl by its lower hardness and specific occurrence environment. Final distinction from other members of the osumilite group requires advanced chemical analysis. ## Crystal Forms Crystals are typically short, prismatic, or tabular with a hexagonal outline. It also occurs as rounded grains embedded in the parent rock.

Geological environment

## Genesis Osumilite-(Mg) is a high-temperature mineral, forming under conditions of contact metamorphism and pyrometamorphism. It forms in aluminosilicate-rich rocks that have been subjected to very high temperatures, such as buchites, xenoliths in volcanic lavas, and some gneisses and granulites. ## Mineral Associations It often co-occurs with minerals such as cordierite, tridymite, hematite, biotite, hypersthene, spinels, plagioclase, and volcanic glass. ## Localities The most important localities worldwide include the type locality in Japan (Sakkabira, Kagoshima), the Eifel Mountains in Germany (where it occurs in volcanic xenoliths), the Lac-Saint-Jean region in Quebec (Canada), the Napier Mountains in Antarctica, and the Vulsini volcanic area in Italy.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, hexagonal crystals of intense blue color. Transparency and lack of damage are also important. Contrasting associations with accompanying minerals, such as hematite, can enhance the specimen's appeal. ## Popular Localities The most famous collector's specimens, known for their sharp and well-formed crystals, come from volcanic ejecta in the Eifel Mountains in Germany.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm distilled water and a very soft brush or paintbrush. Ultrasonic cleaners should be avoided, as they can cause cracks in brittle crystals. ## What to Avoid Avoid contact with acids and strong chemical agents. The mineral is relatively hard but brittle, so it should be protected from impacts and sudden temperature changes. ## Storage Osumilite-(Mg) specimens are best stored in separate, padded collector's boxes to prevent scratches and mechanical damage. It is not sensitive to sunlight.

External references

https://www.mindat.org/min-3038.html