Osumilite
Chemical formula: KFe<sup>2+</sup><sub>2</sub>(Al<sub>5</sub>Si<sub>10</sub>)O<sub>30</sub>
Osumilite is a rare ring silicate with a complex composition, most often forming blue or black, tabular crystals with a vitreous luster.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous
- Streak
- White
- Density
- 2.64
- Cleavage
- None
- Fracture
- Uneven to conchoidal
- Transparency
- Transparent to translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Osumilite is primarily identified by its characteristic hexagonal crystal habit, vitreous luster, and color (most often blue, gray, or black). Its occurrence in specific, high-temperature metamorphic rocks (granulite facies) is a key indicator. A hardness in the 5-6 range helps distinguish it from softer minerals. ## Distinguishing from Similar Minerals Osumilite is sometimes confused with cordierite, sapphire, or tourmaline (schorl). - It differs from **cordierite** in habit (cordierite more often forms pseudohexagonal prismatic crystals) and often more intense blue color. Cordierite also exhibits strong pleochroism, which is weaker in osumilite. - It is significantly softer than **sapphire** (corundum) (sapphire's hardness is 9). - It is distinguished from black **tourmaline** (schorl) by its lower hardness (tourmaline 7-7.5) and often more tabular habit. ## Crystal Forms Osumilite crystals are hexagonal, most often short, prismatic or tabular, with a hexagonal outline. It also occurs as granular aggregates, irregular grains, or as a component of rock mass.
Geological environment
## Genesis Osumilite is a mineral typical of rocks subjected to regional metamorphism under conditions of high temperature and relatively low pressure, characteristic of the granulite and pyroxene-hornblende facies. It also forms in volcanic products, such as rhyolitic lavas and tuffs, where it crystallizes from magma in the final stages of cooling. ## Mineral Associations This mineral co-occurs with minerals such as quartz, potassium feldspars (sanidine, orthoclase), plagioclase, biotite, cordierite, hypersthene, magnetite, ilmenite, sapphirine, and tridymite. ## Localities The most important osumilite localities worldwide include: - Japan (Sakkabira, Kagoshima Prefecture - type locality) - Italy (Vulsini volcanic area, Lazio) - Germany (Eifel volcanic region) - USA (Obsidian Cliffs, Wyoming; Jemez Mountains, New Mexico) - Antarctica (Enderby Land) - Tajikistan (Pamir Mountains)
Rarity
Rare
For collectors
## Quality Criteria Specimens with sharply terminated, well-formed, hexagonal crystals of intense blue or indigo color are most valued by collectors. Transparent crystals, which are rare, are also highly prized. Contrasting placement on a light rock matrix (e.g., in rhyolite) significantly enhances visual appeal and specimen value. ## Popular Localities Collector-quality specimens, although usually small, come mainly from volcanic localities. Blue crystals from the Eifel volcanic complex in Germany and from Italian Lazio are particularly sought after. Japanese specimens from the type locality are mainly of historical significance.
Care and storage
## Cleaning Osumilite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, avoiding a strong stream. Ultrasonic cleaners should be avoided, as they can cause cracks in brittle crystals. ## What to Avoid The mineral is sensitive to sudden temperature changes. It should be protected from contact with acids and strong chemicals. Prolonged exposure to direct sunlight is not recommended, although there is no evidence of fading. ## Storage Osumilite specimens, especially those with well-formed crystals, should be stored in separate, padded boxes to prevent abrasions and mechanical damage. It is best to keep them in stable room conditions, away from sources of moisture and dust.