Ominelite

Chemical formula: Fe<sup>2+</sup>Al<sub>3</sub>O<sub>2</sub>(BO<sub>3</sub>)(SiO<sub>4</sub>)

Ominelite is a rare iron and aluminum borosilicate, forming dark, tabular crystals in association with tourmalines.

## Characteristics Ominelite is a very rare mineral from the borosilicate group, chemically classified as an iron and aluminum borosilicate. It occurs as small, tabular or platy crystals, often with a hexagonal outline, reaching sizes up to 1 mm. It typically forms aggregates with other minerals. Its color ranges from dark brown to black, and in thin fragments, it can be translucent brown. ## Physical Properties This mineral is characterized by a hardness of approximately 6.5 on the Mohs scale. It has a vitreous to metallic luster. It is a relatively dense mineral, with a calculated density of 3.86 g/cm³. The crystals are brittle and exhibit an uneven fracture. ## History and Name The name ominelite comes from its discovery locality – Ōmi-jima island (formerly Omi-shima) in Niigata Prefecture, Japan. The mineral was first described in 1998 by Japanese mineralogists and approved by the International Mineralogical Association (IMA) under number IMA1997-010.

Properties

Mohs hardness
6.5
Luster
Vitreous to metallic
Streak
Brown
Density
3.86
Cleavage
Imperfect on {001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Ominelite is identified based on its unique occurrence environment (contact metamorphic rocks), association with tourmaline, crystal habit (small, black, tabular), and chemical composition. Identification requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS). ## Differentiation from similar minerals It can be confused with other dark platy minerals, such as biotite, hematite, or some tourmalines (schorl). It is distinguished by the hexagonal outline of its tabular crystals, higher hardness than biotite, and specific chemical composition (presence of boron, silicon, iron, and aluminum). ## Crystal forms Crystals are thin, tabular, with a hexagonal outline. They occur as single plates or aggregates thereof, growing on other minerals, mainly tourmaline.

Geological environment

## Genesis Ominelite forms under conditions of contact metamorphism. At its type locality (Ōmi-jima), it was found in calc-silicate rocks that were subjected to granitic intrusion. Its formation is associated with metasomatic interaction of boron-rich fluids. ## Mineral associations This mineral occurs in close association with tourmaline (often as epitaxial overgrowths on its crystals), as well as with diopside, grossular (garnet), wollastonite, vesuvianite, titanite, and scapolite. ## Localities The only confirmed and described locality of ominelite in the world is its type locality: Suishohama coast on Ōmi-jima island, in Itoigawa City, Niigata Prefecture, Japan.

Rarity

Extremely rare

For collectors

## Quality criteria As a mineral primarily of scientific and micromount significance, specimens with well-formed, sharp crystals are most valued, even if they are small. A well-defined association with other minerals, especially tourmaline, on which it forms characteristic overgrowths, is also crucial. Crystal sizes exceeding 1 mm are considered exceptional. ## Popular localities The only source of specimens is Ōmi-jima in Japan. Material from this locality is extremely difficult to obtain and practically unavailable on the commercial market. Most material is held in institutional and scientific collections.

Care and storage

## Cleaning Due to the rarity and small size of the crystals, mechanical cleaning is not recommended. If necessary, compressed air can be used to remove dust. Contact with water or chemicals is not advised unless absolutely necessary. ## What to avoid Avoid contact with acids and other aggressive chemicals. The crystals are brittle, so they must be protected from impacts, scratching, and vibrations. Specimens should not be subjected to ultrasonic cleaning. ## Storage Ominelite specimens, being extremely rare and usually microscopic, should be stored in specialized "micromount" boxes or in sealed containers that protect against dust and mechanical damage. Exposure to extreme conditions should be avoided.

Sources

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